Chapter 2 Using the COSPEC in the Field

Chapter 2: G. Williams-Jones, J. Stix & P.A. Nadeau / Using the COSPEC in the Field 63

Williams-Jones, G., Stix, J. & Nadeau, P.A. (2008) Using the COSPEC in the Field. In: Williams-Jones, G., Stix, J. & Hickson, C. (eds.) The COSPEC Cookbook: Making SO2 Measurements at Active Volcanoes. IAVCEI, Methods in Volcanology, 1, 63-119.

Chapter 2

Using the COSPEC in the Field

Glyn Williams-Jones1, John Stix2 and Patricia A. Nadeau1

1Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada, V5A 1S6. email: [email protected]

2Department of Earth & Planetary Sciences, McGill University, Montreal, Quebec,

Canada, H3A 2A7. email: [email protected]

1. INTRODUCTION This chapter is aimed at both the first time and experienced user and details the possible

methods by which COSPEC measurements are made in the field. Whether you are a graduate student going abroad for fieldwork or a Volcano observatory staff member performing background measurements, it is crucial to understand the strengths and weaknesses of COSPEC measurements before commencing a survey. It is also important to tailor the most appropriate method with the activity level of the volcano. For example, what is a sufficient number of traverses for a background or reconnaissance survey compared to monitoring of an imminent eruption?

We therefore look first at the importance of meteorological conditions, notably determining the wind speed, and then discuss the various techniques (e.g., ground-based mobile, airborne, stationary, etc.), along with their advantages and disadvantages, used for COSPEC measurements. The different methods of recording the COSPEC data (e.g., chart-recorder, datalogger, computer, etc.) are then examined, followed by a discussion of potential problems that may arise while making measurements and the best way to remedy the situation. Data reduction is also examined, with a discussion of the formulae used to calculate SO2 fluxes and the various methods of treating analogue and digital data. Once the measurements have been made and the data analysed, one must then decide upon the most appropriate and useful way to present the data in order to facilitate interpretation.

2. METEOROLOGICAL CONDITIONS

2.1. Wind SO2 flux calculations are dependent upon knowledge of the plume direction and speed (i.e.,

velocity), however, direct measurement of these components is often exceedingly difficult. We therefore measure wind speed and wind direction, which we assume to represent the plume velocity. Ideally, this information is obtained from instruments in the downwind plume of the volcano (e.g., using a radiosonde). When making airborne measurements, these data can be obtained by flying at the level of the plume (Doukas, 2002). However, for ground measurements,

Chapter 3: J. Stix, G. Williams-Jones & C. Hickson / Applying the COSPEC at Active Volcanoes 64

this data is not always available, and thus other methods are required. Should the volcano be near an airport or large city, one can obtain the wind speed (generally measured at ~10 m elevation) and wind direction by contacting the local meteorological stations or airports. For example, at Popocatépetl volcano, Mexico, wind data is received from the ground approach radar of aircraft landing in Mexico City (see also Chapter 5).

If wind data is unavailable from the sources discussed above, measurements must be attempted downwind of the volcano. Importantly, any wind measurement not taken within the plume must be considered as a minimum value. In some instances, the wind speed may be too high or too low for COSPEC measurements. For example, at very low speeds, the plume may become diffuse and unfocused and subsequently unmeasurable; the user should wait until the winds increase sufficiently before attempting measurements. On the other hand, excessively high wind speeds that threaten the aircraft would make airborne measurements impossible.

2.1.1. Theodolite, compass or video measurements

One of the most accurate methods of measuring the speed of the plume is by use of a theodolite. An orienteering compass or rangefinding binocular also may be used. The theodolite is placed beneath or adjacent to the plume in order to track optically a packet of gas from the summit to a position downwind. By measuring the time necessary for this “packet” of gas to travel from the source, and knowing the distance travelled (measured from a map), the plume speed may be calculated (Fig. 1). This can also be accomplished with continuous video recording of the plume (Doukas, 2002; Williams-Jones et al., 2006). For measurement of a vertically rising plume, see section 3.10.3.

At least three measurements of a gas parcel should be made with the horizontal angle and time noted for each point (e.g., Points 1, 2, 3; Fig. 1). One measurement should be made at an angle that is exactly perpendicular to the plume. The shortest slant path from the observer to the plume (c) is then calculated, either by knowing the perpendicular horizontal distance between the observer and the plume (b):

(1) c = b/cosA

or by knowing the height of the plume above the observer (a1):

(2) c = a1/sinA

a2 a3

Point 1

Observer

Angle tAngle f

Slanttriangle 2

Slanttriangle 3

Point 2 Point 3

Vertical triangle 2

Observer

Point 2gas plume

angle Ab

ca1

plume

Fig. 1. Determining the speed of a horizontal plume.


Once the value of c is known, the distances between Points 1 and 2 (a2) and between Points 2 and 3 (a3) can be calculated:

(3) a2 = c tanφ

(4) a3 = c tanτ

From this information and the times recorded for each point, the plume speed is calculated. As an example, a parcel of gas is traced at Points 1, 2, and 3, with the angles and times noted (Fig. 1, Table 1). Assume b is 1000 m and angle A is 20°. Then c is 1064 m, φ and τ are both 30°, and a2 and a3 are 614 m each. From the data in Table 1, the parcel has travelled 1228 m over 120 seconds, or at a speed of 10.23 m s-1.

2.1.2. Anemometer measurements

Wind speed measurements also may be made using an anemometer (handheld or mounted). At volcanoes where the plume is close to the ground (e.g., Masaya, Nicaragua; Kilauea, Hawaii), this technique is extremely useful. A fully mechanical anemometer such as the one shown in Figure 2 allows for the integration of fluctuations in wind speed due to gusting. Measurements are typically taken over a one minute period with the result in feet per minute or metres per second (1 ft min-1. = 0.00508 m s-1). Mechanical anemometers may require the application of a correction factor (generally supplied by the manufacturer) to the results. For one set of data, five

to ten measurements are commonly made in order to account for variations in wind speed, with the average being used in the SO2 calculations. Some electronic anemometers will automatically average wind speeds (e.g., Kestral 1000 pocket anemometer) for a given interval, reducing the uncertainty due to wind gusts. Over a day of COSPEC measurements, at least three sets of wind speed measurements should be made at the beginning, middle, and end of the day, in order to characterise the fluctuations in wind speed. A more detailed study of wind speed variation can be made using a continuously recording automated anemometer. In March 1998, February 1999 and March-April 2000, continuous wind speed measurements were made downwind of Masaya volcano at an Instituto Nicaragüense de Estudio Territoriales (INETER) seismic station.

Table 1. Angular measurements from a parcel of horizontally-moving gas

Point Time (hrs) Angle A (°) Distance (m) 1 11:16:38 60 0 2 11:17:38 90 614 3 11:18:38 120 1228

Handheld anemometers

Stationary anemometers

Fig. 2. Examples of handheld and stationary anemometers.


Located on the Llano Pacaya ridge (930 m a.s.l.), the station was at least 300 m above the active vent with the result that wind speed measurements are essentially made almost at the column height.

Care must be taken when deciding upon the placement of ground-based wind speed instruments. According to the World Meteorological Organisation (WMO), “The standard exposure of wind instruments over level, open terrain is 33 feet (10 metres) above the ground” (WMO, 1996). If there are obstacles present, the anemometer is located at a distance downwind from the obstacle that is at least ten times the height of the obstacle (Fig. 3a). If the instrument must be near a tall building, it should be placed upwind at a distance from the building equal to the building’s height, to avoid turbulence caused by the building (Fig. 3a,b). However, if one were to strictly apply this rule to volcanoes, the wind station would have to be quite far from the volcano to avoid the effects of turbulence; for example, one should be five to ten kilometres away from a 1000 m high volcano. Towers such as those used for TV, radio and cellular transmissions are ideal for siting an anemometer. In practice, however, the installation of such stations in proximity to volcanoes is not always possible, leaving handheld anemometers or overflying aircraft as the most likely sources of wind speed data.

When using a handheld anemometer, the effects of ground layer shear may be reduced somewhat by mounting the instrument on a long pole. Measurements are greatly facilitated if the anemometer has a timer and recording capability, such that it can be placed aloft and set to record for a given period of time. With a handheld digital or mechanical anemometer, one can reduce the uncertainties by starting the anemometer at ground level and quickly raising the pole (Fig. 4).

1x Heightabove

1x Height before 5x - 10x Height beyond

Wind

T H10 m

10x T10x H

a

b

Fig. 3. (a) Standard positioning for ground wind instrumentation recommended by the World MeteorologicalOrganisation; (b) suggested siting of anemometers near ground level, downwind of an obstacle (WMO, 1996).


2.1.3. Dual spectrometer method

With the introduction of new small and low-cost spectrometers (e.g., FLYSPEC, see Chapter 6), new techniques have been developed to determine a more accurate representation of volcanic plume velocity. With only two instruments it is possible to derive the apparent speed of a plume. To do so, the location of the most concentrated part of the plume is located by completing a preliminary traverse beneath the plume (see below). If local geography and infrastructure beneath the concentrated segment permits, each of the spectrometers is placed on a tripod and set to collect SO2 path-length concentrations. By placing the time-synchronized instruments along a line parallel to the plume’s propagation (Fig. 5), similar signals are recorded on each of the instruments. The combination of the known distance between the spectrometers (from an integrated GPS units or a tape measure) and the time lag between corresponding points on the data plots allows for the calculation of plume speed by comparing the data signals recorded by each spectrometer, and noting the time lag between them (Fig. 6; Williams-Jones et al., 2006).

Beyond a simple two-spectrometer approach to determining plume speed, a rotating mirror may be integrated into a single spectrometer’s field housing, allowing for scanning of two sectors of the sky during a single traverse. Doing so allows for determination of plume height, and subsequently plume transport speed (McGonigle et al., 2005a). Such a method does become complicated, however, in locales lacking a flat road network on which to traverse, as is the case

Fig. 4. Placement of ananemometer upon a 1-2 m-long pole to reduceboundary-layer effects nearthe ground. If installing acontinuously recording stationary anemometer, itshould be placed as high aspossible (see Fig. 3).

1 - 2 mlong

Continuouslyrecordinganemometer


Fig. 6. a) SO2 pathlength concentration for two FLYSPECs at Masaya, Nicaragua on March 25, 2003. Instrument separation is 40.5 m, determined by tape measure. Inset is a 4-min. window showing an apparent time separation (16 s) between the 2 signals. b) The SO2 signals for the entire 30-min. sampling period are compared to each other for timeshifts between -60 and 60 s at 0.1 s iterations. The maximum correlation coefficient (r2 = 0.959) for the signals occurs at a time difference of 13.1 s, which for a 40.5 m separation, results in a plume speed of 3.1 m s-1. (modified from Williams-Jones et al., 2006)

Fig. 5. Field deployment for determination of plume speed at Masaya, Nicaragua. The FLYSPECs were mounted downwind of the gas source on small, lightweight camera tripods separated by 20 m. Measurements were made for 30 minutes, with a GPS antenna in each configuration to ensure time-synchronization of the datasets.

0

20

40

60

SO p

athl

engt

h co

ncen

tratio

n (p

pm-m

)2

80

100

120

140a)

-40

-20

13:39 13:44 13:49 13:54Local time

13:59 14:04

20

100

13:51

16 s

13:52 13:53

60

UpwindDownwind

FlySPEC

GPSantenna

Notebookcomputer

Spiritlevel

FlySPEC


at many volcanoes. In deriving plume height, the pitch of the instrument at the time of each spectral acquisition must be known. Corrections for pitch compensate for undulating topography and resultant deviations of the instrument’s field of view from the intended azimuth and inclination, but such continuous measurements of pitch may not always be practical or feasible.

As an alternative to the determination of plume speed, additional spectrometers may be added to the two-spectrometer configuration in order to determine plume velocity (McGonigle et al., 2005b; Williams-Jones et al., 2006), much in the manner of the two-spectrometer approach.

2.1.4. Wind direction

The direction of the wind and therefore the gas plume is also important for the calculation of SO2 flux. The wind direction can be obtained from meteorological stations or local airports. However, for mobile COSPEC measurements, the most accurate and reliable method entails determining the direction from the chart record (or digital record) and map of the volcano. The plume azimuth (the angle that the plume direction makes with respect to geographic north/south) is required to correct for the lack of perpendicularity of the traverse segments with respect to the plume direction (see below). Specifically, one determines the azimuth of the plume by locating the ground position on a topographic map at which the maximum SO2 peak on the chart record was detected, and drawing a line on the map from this point to the volcano’s summit (Fig. 7).

1633

1400

12001000

1000

800

600

600400

600

Fig. 7. Determination of the plume azimuth using maximum or median SO2 concentrations from the ground based chart record (inset). Numbered dashes on the road refer to segment markers. Example from Arenalvolcano, Costa Rica. (Williams-Jones et al., 2001)


One also may use the midpoint of the plume from the chart record, rather than the maximum point, in order to determine the plume direction. The midpoint may represent the average plume direction, whereas the maximum SO2 peak may be due, in part, to heterogeneities in the plume. The user is urged to adopt a consistent method, using either the midpoint or the highest peak on the chart record. For stationary measurements, the wind direction can be determined using a theodolite or compass as above (Fig. 1).

2.2. Clouds and condensed water vapour Condensed water vapour in the

plume may increase the path of ultraviolet rays in the plume, resulting in a scattering effect that will overestimate the observed SO2 burden by as much as 10% (Millán, 1980; Stoiber et al., 1983). The position of the plume with respect to any cloud, fog, or haze can also have an affect on measured SO2 concentrations. If the plume is above the cloud, scattering of the ultraviolet rays passing through both the plume and cloud may cause a secondary plume to appear on the chart record, increasing the total measured SO2 emission (Fig. 8). It is therefore preferable that, where possible, measurements be made on clear days or when the cloud is above the plume.

2.3. Online Wind and Meteorological information

Meteorological stations typically are run by federal, provincial/state, and municipal governments as well as by regional airports. These government agencies can therefore be excellent sources of wind speed, pressure, humidity, and temperature data. The principle international meteorological agency is the World Meteorological Organisation (WMO), which has a webpage at www.wmo.ch and maintains a list of International Weather Monitoring stations (and their reference codes) which can be found through at www.wmo.ch/web/www/ois/ois-home.htm. By far the best source for meteorological data and information is through the U.S. National Oceanic and Atmospheric Administration (NOAA) webpages (e.g., Environmental Information Services, www.eis.noaa.gov). A real-time archive of radiosonde data (civilian and some military sites) for most of North, Central America and the Caribbean is available from the US National Weather Service (raob.fsl.noaa.gov), while US and International meteorological and surface wind data can be found at weather.noaa.gov/weather/ccworld.html. The Weather Underground page (www.wunderground.com) is extremely useful as it maintains current and archived surface wind data.

Fig. 8. Development of a secondary plume when cloud, haze or fog are between the COSPEC and the plume because ofincreased scattering of UV rays.

Sun(UV radiation)

Cloud/haze/fog Scattering

Plume

Primaryplume

Secondaryplume


Background COSPEC tests

a

UV absorption

High calibration

AGC16

:36

(loca

l tim

e)17

:00

AG

C: 3

.716

:30

A

GC:

3.7

19:3

0

AG

C: 4

.520

:00

AG

C: 4

.3

b

c

19:5

5A

GC:

4.4

Fig. 9. Background COSPEC tests on (a) April 29, (b)May 2, and (c) May 5, 1997 performed in Montreal,Canada. The diagrams show a steady increase in AGCand signal “noise” towards sunset. Note an approximatelyfour-fold increase in background noise from ~16:30 to~20:00 on May 2, 1997. Spikes represent calibrationstaken every 30 min.

2.4. Sun angle and Plume dynamics

2.4.1. Sun angle

The COSPEC is an ultraviolet spectrometer that is dependent on the amount of ultraviolet radiation entering the instrument. In the early morning and late afternoon at mid-latitudes, the lower sun angle results in low amounts of ultraviolet radiation entering the vertically oriented telescope. This leads to higher background noise and reduced precision. Background tests performed in Montreal, Canada (45.52°N, 73.57°W) on April 29, May 2 and 5, 1997, show a gradual increase in the Automatic Gain Control (AGC - represents the amount of ultraviolet radiation reaching the optics), towards nightfall (Fig. 9). More significantly, on May 2, the “noise” of the background SO2 signal increased by up to a factor of four from 14:30 to 20:00 local time. Interestingly, the relative absorption initially decreases from 14:30 to ~17:30. It then

increases dramatically, peaking at ~19:20 before decreasing towards the end of measurements at 20:00 (Fig. 9a, b). This increase and subsequent decrease near sunset is due to the rapidly changing sun angle at this time, resulting in varying levels of ultraviolet radiation entering the COSPEC. Measurements should therefore be made during high sun angle so that sufficient ultraviolet radiation enters the instrument. In general, the measurement period should be approximately symmetrical around solar noon (e.g., 1st July in Montreal: 11:56 and Anchorage: 13:03 local time). However, during solar noon, i.e., when the sun is at its highest “zenith angle”, the ultraviolet radiation entering the COSPEC may cause spikes in the data. This problem is reduced in the COSPEC V by baffles in the spectrometer and the Cassegrain telescope, which buffer some of the ultraviolet radiation entering the instrument. Earlier versions of the COSPEC have baffles only in the telescope and are thus more vulnerable to the ultraviolet radiation of solar noon (R. Dick, Barringer Research, personal communication, 1997). If an older COSPEC is being used or if spikes continue to occur, it is advisable to stop measurements and turn off the COSPEC for approximately half an hour or until the sun passes its apex. During airborne measurements, banking turns may sometimes pick up solar spikes when


travelling east west. An excellent online resource for calculating solar noon, as well as apparent sunrise and sunset can be found at the NOAA Surface Radiation Research Branch webpage (www.srrb.noaa.gov/highlights/sunrise/sunrise.html).

2.4.2. Plume dynamics

Plumes are rarely vertically or laterally homogenous, and thus heterogeneities in the SO2 concentrations of the plume may effect the calculated burden. However, if mobile measurements are being made directly below the plume, the ultraviolet radiation will pass through the entire plume thickness and be absorbed by SO2, minimising scattering effects and allowing for complete measurement of SO2 in the vertical axis. Only at the thin edges of the plume will there be an increased scattering effect (Fig. 10). Spatial heterogeneities in the plume then can be integrated into the measurement by completely transecting the plume during a traverse.

The opacity of the plume also may have an effect on the accuracy of the COSPEC measurements. If the plume is heavily laden with ash, ultraviolet radiation is partially blocked by the ash (rather than being absorbed by the SO2) increasing the apparent measured SO2 by raising the effective background levels. An increase in the AGC would suggest that this was the case. The AGC will tend to increase gradually towards the end of the day as the sun angle decreases and the amount of UV radiation entering the instrument decreases. Aerosols between the plume and the ground also will scatter ultraviolet radiation, further reducing the ground measured SO2 (Millán, 1980). However, laboratory experiments by Andres and Schmid (2001) indicate that the COSPEC reliably measures SO2 burdens within a 10% accuracy for plumes that are up to 50% opaque.

3. USING THE COSPEC In the field, the COSPEC can be connected to any 12 V DC storage battery or other 12-volt

(or converted 115 VAC, e.g., car cigarette lighter) power source. The instrument should be allowed to warm up for at least half an hour prior to use, in order for the electronics to stabilise at a constant operating temperature. The COSPEC is connected to a chart recorder (analogue) and/or a computer or datalogger (digital). The COSPEC generally draws 800 mA (R. Dick, Barringer Research, personal communication, 1997) while a chart recorder typically draws 400 mA for a total of 1.2 Am. One can therefore nominally run both instruments on a 6 Ah (Amp hour) gel cell for 5 hours (6/1.2). In practice, we suggest using two 6 Ah gel cells in parallel or one 12 Ah gel cell to allow for 10 hours of measurement time.

The COSPEC may be used in many different configurations depending upon access to the volcanic plume. Thus, accessibility to the volcano and monitoring strategy must be determined

UV radiation

Increasedscattering

Increasedscattering

Fig. 10. Scattering of ultraviolet radiation at the thin edges of a plume.


prior to setting up the instrument. We first discuss this strategy and other general considerations, then examine the different COSPEC configurations.

3.1. Assessing access to the volcano One of the most important factors in any COSPEC survey is determining the best access to

the volcano and thus the best possible technique for that situation. Use of an airplane is ideal for measurements if the plume is not too close to the ground. If, as is often the case, planes are not available or the plume is too close to the ground, then mobile ground-based measurements should be attempted. One must then determine whether there are roads running approximately normal to the plume direction (such as at Masaya, Nicaragua and Arenal, Costa Rica). If there are no appropriate roads, stationary measurements should then be attempted. If even stationary access is impossible (e.g., White Island, New Zealand; Mt. Spur, Alaska), than airborne measurements (fixed wing or helicopter) may be the only viable option.

3.2. Monitoring strategy - how often to make measurements?

The rate at which repeated surveys are made is greatly influenced by access to the equipment and by safety, budgetary, and time considerations. The presence or absence of volcanological observatories in proximity to the volcano also plays an important role in determining the frequency of surveys. A survey should consist of at least 5-10 traverses or measurements.

In times of relative quiescence or dormancy, monthly surveys are generally sufficient to maintain a baseline flux. Volcanoes such as Galeras and Nevado del Ruiz, Colombia, and Kīlauea, Hawaii, are currently surveyed quite frequently, at least once a week, due to their continued activity. For example, when Popocatépetl volcano (Mexico) was in crisis, ground and airborne survey are made at least three times a week (see Chapter 5).

The type of activity also will affect the decision. If the volcano is constantly emitting gas, fewer measurements may be necessary to characterise the plume (e.g., Masaya, Nicaragua). However, if the volcano is erupting or “puffing” (e.g., Arenal, Costa Rica), more measurements will be necessary to properly represent daily emissions. In the case of increased seismic activity and/or deformation, a large number of COSPEC surveys will prove useful in better understanding the state of the volcano. Post-eruption decay of SO2 emission can generally be characterised by a reduced number of surveys. A return to non-crisis levels would then only necessitate baseline monitoring (e.g., monthly).

3.3. Optimising the distance for measurements

Ideally, measurements should be made as close to the volcano as possible, in order to reduce dispersion and dilution of the plume. However, as one approaches the volcano, the plume can become more heterogeneous. If SO2 concentrations are elevated near the summit to the point where the levels exceed the high COSPEC calibration cells (and a high concentration kit is unavailable), it will be necessary to increase the distance from the crater to allow for some dilution of the plume and a lower SO2 concentration, albeit over a larger width. Thus it is important to choose a distance from the crater where the plume is relatively homogeneous without being so far removed that there is significant attenuation and dispersion of the gas. This can lead to an apparent reduction in SO2 emission making the measurements unrepresentative. While moving downwind is quite feasible during airborne surveys, stationary or ground-based surveys may be seriously limited because of access difficulties.


3.4. Concentrations and calibrations

The COSPEC is equipped with two quartz-glass cells with fixed concentrations of SO2. These cells are used as calibrations before and after measurements are made. A low concentration gas cell is typically on the order of 50-150 ppm·m SO2 while the high concentration gas cell contains between 300-500 ppm·m SO2. Prior to and following measurements beneath a gas plume, both calibration cells should be used in order to determine which is appropriate for the flux calculations. Ideally, the maximum plume peak height should always be less than the high calibration, with the high calibration consequently being used in the flux calculations (Fig. 11a). However, if the maximum peak height of the plume is less than that of the low calibration cell, then the low calibration should be used in calculations (Fig. 11b). If the plume substantially exceeds the high concentration cell (Fig. 11c), measurements should be made farther from the volcano where the plume is more dispersed and dilute. Alternatively, a high SO2 burden correlator disc assembly and high concentration calibration cells may be installed, allowing for the measurement of concentrations ranging from ~1,000-10,000 ppm·m. However, changing a correlator assembly requires great care and should be performed in a clean environment (e.g., laboratory).

In the field, it is advisable to make a note of the typical heights and the peak height ratio of the two calibration peaks as they appear on the chart recorder paper for a given voltage (e.g., 0.5 V). If the ratio of the calibration peaks is different from that normally observed, this might indicate problems with the COSPEC (e.g., leaking calibration cell). Similarly, it is also useful to make a few rapid high-cell calibrations while in the plume to verify that the relative calibration peak height remains more or less constant (Fig. 11d).

Fig. 11. Examples of SO2 emission chart records and the calibration-cell concentration that should be used in final flux calculations. (a) The high calibration heights and concentrationsshould be used when the maximum anomaly peak height isequal to or lower than the high calibration height. (b) When themaximum anomaly peak height is equal to or lower than the low calibration height, the low calibration should be used. (c)When the plume peak height exceeds that of the highconcentration calibration, COSPEC measurements should bemade further downwind if possible or the high concentration kit should be used. (d) During long traverses, proper operation ofthe COSPEC may be verified by performing rapid highcalibrations during the measurement.

a

Use high calibration cell height and concentration in calculations

b

c

d

Use low calibration cell height and concentrationin calculations

Move further downwind to allow dispersion anddilution of plume or put in high concentration kit

Rapid high calibrations during measurements willconfirm proper operation of the instrument


3.5. Mobile ground-based measurements by vehicle

3.5.1. Setting up the instrument

In the case of ground-based mobile COSPEC measurements, the instrument is mounted in a vehicle (car, van, truck, etc.). In some cases where roads were unavailable, the COSPEC has been mounted on horseback or even onto a backpack frame (Stoiber et al., 1983). A minivan is an ideal vehicle as it allows for the easy installation of equipment through sliding side doors. The instrument is normally placed in the passenger seat on a box or crate (plastic Coke or beer crates work well and are often easy to find in the field) and is secured to both the crate and seat with elastic cords or webbing. To minimise vibration to the instrument, foam padding should be placed between the COSPEC and the straps and the boxes. This assembly brings the COSPEC to the level of the window and allows the right angle mirror of the Cassegrain telescope to protrude from the side window. The extension tube is attached to the right angle mirror so that the instrument is effectively “looking” vertically (Fig. 12). There must be sufficient clearance between the doorframe and the Cassegrain telescope so that the telescope is never in danger of hitting the frame of the vehicle. Foam padding should be attached around the tube as a precaution. The right angle mirror should be oriented such that the extension tube is vertical (i.e., the COSPEC may be angled into the seat) to ensure that the field of view is as close to vertical as possible during travel. A 12-volt battery or the vehicle’s battery acts as the power supply for the COSPEC. The COSPEC is connected to the battery by a power cable with alligator clips on one

end, supplied with the instrument. The clips should be secured with electrical tape on the battery to prevent accidental slippage which can result in electrical short-circuits. The instruments may also be connected to the vehicle's 12-volt supply via the cigarette lighter socket using a plug for car accessories. For data collection, the COSPEC is connected to a chart recorder, portable computer, and/or datalogger via data cable (COSPEC end provided, customised end for the output device). The clock of the computer/datalogger should always be synchronised with the operator's watch before starting measurements.

3.5.2. Making the measurements

As roads near volcanoes are rarely straight or perpendicular to the gas plume, an individual traverse below the plume is divided into segments in order to correct for the deviation from perpendicularity of the traverse segment with respect to the column (Fig. 13). The azimuth (angle from geographic north/south) of each segment must be measured from a map so that a cosine correction can be made later (Fig. 14). The

Fig. 12. Proper installation of a COSPEC in avehicle. Straps or webbing can be used to secure theinstrument. Care should be taken that the telescopesare “looking” vertically upwards and unable to beknocked (e.g., against the door) during themeasurements. Plastic cases (coke or beer) can beused to bring the COSPEC up to the level of thewindow.


segment positions are marked on the chart recorder with the event recorder or noted for the computer by recording the time when a segment boundary is reached in the vehicle. This can be greatly facilitated by using a handheld GPS connected via a datalogger or directly to a laptop computer. It is therefore possible, with the aide of fairly simple software, to accurately determine the vehicle’s position every second and knowing the source location, correct for the deviation from perpendicularity of the road.

The instrument is driven along the plume segment at an approximately constant speed. The vehicle speed is by necessity highly dependent upon the width of the plume. On a volcano such as Popocatépetl where the plume is typically very wide (20-50 km, as seen from the road), the vehicle is driven at ~50-60 km hr-1; speeds must be greatly reduced when passing through towns or villages. On the other hand, at volcanoes such as Arenal or Soufrière Hills with narrow plumes (2-3 km), 10-20 km hr-1 is the typical vehicle speed. The quality of the roads where the measurements are made also will affect the optimum vehicle speed; one can drive at a more constant speed on a fully tarred road than on a gravel road full of potholes (where one is swerving from side to side to miss the holes). The time constant of the COSPEC should be set to either 1 or 2 in order to reduce the effect of overhanging trees. However, if distal measurements (e.g., tens of km from the volcano) are being made and/or if the plume has low concentrations of SO2, one should use a higher time constant (4 or 8) in order to improve the signal to noise ratio. Gas cell calibrations are made before and after each traverse in order to calculate the plume

Fig. 13. An example of the segmentation of a complete COSPEC traverse around Masaya volcano, Nicaragua. Thedistance and azimuth for each segment must be determined as well as the position of the plume. Field descriptions of the segment markers greatly facilitate measurements. This becomes unnecessary if GPS data is integrated with SO2data. A map should accompany each survey and volcanological events (e.g., ash fall) should be noted. Inset is an example of how to make the cosine correction for perpendicularity to the plume (also see section 6).


burden later. Both gas cells should be used (see section 3.4). It is also wise to perform quick calibrations while in the gas column if conditions (e.g., ultraviolet light level, ash, etc.) are changing rapidly (see section 3.4).

3.5.3. Advantages/disadvantages

Ground-based mobile surveys are often the most practical method for the majority of volcanoes with some road access, as they are relatively easy and inexpensive to perform. In some cases where the plume is hugging the ground, this is the only feasible technique. However, this technique is clearly impossible in areas where there are no roads. When there are roads, they are rarely normal to the plume direction, necessitating the segmentation of the roads and increasing the number of calculations. In many cases, the only accessible roads are far downwind, resulting in possible dilution and dispersion of the plume and thus increased uncertainty. Roads are also frequently bordered by trees which overhang the road and interfere with the measurements by blocking ultraviolet radiation. This effect may be partially overcome by temporarily raising the pen of the chart recorder or briefly pausing the computer when passing under trees. Stopping the chart recorder/computer when passing through towns (with interference from telephone/power poles) is also an option. Although this results in loss of data, one can easily interpolate the missing data if the plume is significantly wider than the portions of road which contain trees/towns.

3.6. Mobile ground-based measurements by boat


During the eruption of Soufrière Hills, Montserrat, ground-based COSPEC measurements were often made by boat. As with mobile land-based measurements, the COSPEC is set up so that the COSPEC is looking vertically. The instrument should be installed in a protected area (e.g., behind driver’s windshield, etc.), in order to prevent contact with salt water, while also maintaining a clear field of view for the COSPEC. To further protect the COSPEC, plastic bags or sheeting can be placed around the instrument, telescope, cables, and power supply. The instrument should be secured in a seat or on a crate with foam padding to absorb any vibrations. If the sea is calm, the COSPEC simply can be placed on the deck of the boat. A 12-volt battery (gel cell or car battery) or the boat’s battery acts as the power supply for the COSPEC. The COSPEC is connected to the battery by a power cable with alligator clips on one end and secured with electrical tape. The battery should be secured to limit any movement (from tossing of the boat) during the survey. For data collection, the COSPEC is connected to a chart recorder, portable computer, and/or datalogger via data cable (COSPEC end provided, customised end for the output device). These should all be protected from water/spray (e.g., plastic covering) and synchronised with the operator's watch prior to starting measurements.


The boat is steered in a straight course as normal to the plume as possible. The position of the boat at any given time may be determined by taking back sights with a compass on two landmarks and tracing lines on a map back from those points. The intersection of the two bearing lines will be the position of the boat (Fig. 14). With points on either side of the plume, the azimuth or bearing of the traverse can be determined; the traverse should be drawn on a map to facilitate calculations (Fig. 14). The azimuth of the traverse is required to correct for the lack of


perpendicularity of the traverse with respect to the plume direction (Fig. 13). The plume azimuth can be determined in the same way as the ground-based vehicular method; assuming that the point of maximum SO2 on the chart record/computer represents the centre of the plume and knowing the speed of the boat, the position can be plotted on the map and the azimuth determined (Fig. 14). The determination of the boat’s position, and consequently the traverse and plume bearings, can be greatly facilitated through the use of onboard or handheld navigation systems (e.g., GPS, Loran, Navstar, etc.). The time constant should be set to 1 or 2. The boat should travel at a speed fast enough to allow for rapid traverses (at approximately constant speed) below the plume and good resolution. High and low calibrations must be made on both sides of the plume.

3.6.3. Advantages / disadvantages

In this method, flux calculations are simplified as the traverses are independent of road systems and thus generally consist of one straight traverse (Fig. 14). Boats also are generally easily available. One must be particularly careful in protecting the instruments from the spray of water thrown up by the boat. If the wind is very strong, roughness of the water may make measurements difficult and increase the uncertainty of the measurements because of excessive rocking of the boat. The traverse speed should therefore be reduced to minimise this rocking.

A CB

353o

327o 9o

93o

66o

C

BA

Fig. 14. Determination of the traverse (navigation positions A and B) and plume azimuth for mobile boat-mounted COSPEC measurements. Bearings should be taken for A and B using back sighting on landmarksprior to entering and on leaving the plume area.


Care must also be taken to note the possible effects of an onshore breeze that may blow the plume back upon itself resulting in unrepresentative measurements.

3.7. Mobile ground-based measurements by foot.


In rare cases where airborne, ground-based vehicular or stationary measurements are not possible, COSPEC surveys may be made on foot. This was necessary at Poás volcano (Costa Rica – February, 2001), because of extremely low fluxes from fumaroles near the crater lake and a poor road network. The COSPEC was attached to a backpack frame (Fig. 15a) and carried beneath the plume. A 12-volt battery (gel cell or car battery) acts as the power supply for the COSPEC. The COSPEC is connected to the battery by a power cable with alligator clips on one end and secured with electrical tape. The small size and light weight of gel cell batteries makes them preferable. For data collection, the COSPEC is connected to a chart recorder and/or datalogger/computer via data cable (COSPEC end provided, customised end for the output device). The clock of the computer/datalogger must be synchronised with the operator's watch (or vise versa) prior to starting measurements. At least three people were required in order to carry the COSPEC, chart recorder, battery and GPS (Fig. 15b).


As with any COSPEC measurement, the orientation and location of the plume must first be determined in order to define the optimum traverse. Calibrations are made before and after traversing the plume. Care must be taken to walk at as constant speed and as straight as possible. Furthermore, because of the limited length of the power/data cables, at least two of the three people (those carrying the COSPEC, battery and chart recorder/datalogger) walk close together

Fig. 15. (a) Proper installation for ground-based COSPEC surveys on foot. Note the COSPEC is attached to a backpack frame and the telescope is oriented upwards. (b) When making a traverse beneath the plume, at least three people are required in order to carry the COSPEC, chart recorder, battery and GPS. Measurements were made in February, 2001 on the crater floor of Poás volcano, Costa Rica.

a Backpack frame

COSPEC

chartrecorder

chartrecorder

GPS

COSPEC

b


and at the same speed to avoid straining the cables. In the case of the Poás survey, a handheld GPS was used to determine the position and direction of the traverse; it also allowed one to determine the speed of the traverse. The direction and azimuth of the plume was estimated visually. The time constant should be set to 1 or 2. Wind speed measurements were made using a handheld anemometer on a “Dome” located immediately above the fumaroles, i.e., the gas source.


This method is generally only advantageous when all other mobile survey options are impossible (e.g., ground-based vehicular, airborne). Nevertheless it is still a viable option in a number of cases (e.g., Poás, Costa Rica, Fuego, Guatemala, Kawah Ijen, Indonesia). In the case of Poás, being so close to the plume resulted in very high concentrations while the traverse over rocky ground resulted in a “noisy” signal (due to rocking of the COSPEC). The fact that surveys are made on foot means that only a small number of measurements can be made in a given period of time; however the small plume width at this proximity to the source meant that a traverse could be made in ~15 minutes. Access into the crater was difficult and time consuming and extra care was necessary to get the instrument safely into the crater. This type of measurement is now significantly easier to perform (by only one person) with the new lightweight UV spectrometer systems (see also Chapter 6).

3.8. Mobile measurements by fixed wing aircraft

Light single-engine planes such as Cessnas, Pipers and Senecas (Fig. 14d) are ideal for airborne measurements, as they are relatively inexpensive, generally quite easy to find, commonly have a removable hatch just behind the wing, and travel slowly enough to allow for a reasonable amount of measurement time beneath the plume (see also Chapter 4). However, virtually any plane can be modified/adjusted to carry a COSPEC (Fig. 16). For some high-altitude volcanoes, some single engine planes are incapable of reaching the higher altitudes, making a larger pressurised plane necessary.


The COSPEC is usually set up with the telescope extension tube oriented vertically upward (Fig. 17) and protruding at right angles out a window or door (i.e., need to remove a window/door) or through an open hatch (Fig. 17). The wing of the aircraft must not obstruct the vertical view of the telescope.

If the plane must be pressurised or open windows/doors are not acceptable, a fused quartz window can be installed in the roof in order to allow ultraviolet radiation to pass through to the COSPEC (Fig. 16e). In some instances, it is possible to purchase windows or doors from crashed aircraft in order to specifically modify them to allow the COSPEC to protrude. The opening around the extension tube may also be sealed with a plastic barrier in order to reduce noise. This plastic “door” should be taped down with duct tape in a crosshatch pattern made up of multiple layers of overlapping duct tape (Fig. 17b). One must ensure that the vertical layering of the tape starts from the door outwards for the windward side, i.e., first strip nearest to opening, second strip overlapping half of first strip, third strip covering half the second, etc. In other words, it is a process of always moving from the back forward to the front of the aircraft. Thus, for the downwind side of the opening, the first strip of tape should start away from the opening, with the


Fig. 16. Examples of various single and multi-engine aircraft and the appropriate COSPEC installation method. Inset photos modified after (a) Dexter Francis, (b) NASA DFRC, c) Mike Reyno, d) Cessna Inc., e) Lockheed Martin Inc.

Emergencywindow

Twin Engine aircraft

Single Engine aircraft Multi-engine aircraft

Window

a

d

High Wing

Pass throug hhatch

Rep laceplex iglass

Remove or ho le in door

cut

Remove or ho le in door

cut

Cargodoor

Cargodoor

Mad e of quartzglass

Upward facingwindow

Low Wing

ExtensionDoor

Door

(e.g., Aerocommander)

(e.g., Piper Navajo) (e.g., Twin Otter)

(e.g., Cessna Skyhawk)(e.g., C-130)

b c

e

a

c

b

d

Fig. 17. Proper installation of a COSPEC in a single engine aircraft. (a) The COSPECs must be securely strapped within the plane. Note that the COSPEC field of view is always unobstructed. (b) The correct way to secure a plastic cover using duct tape. Note that the tape is in a crosshatched pattern. In some instances of extreme cold (e.g., Alaska), this tape can become brittle and crack. (c) A sealed Perspex window or (d) reinforced card can also be used. Photos c and d courtesy of the USGS Volcano Hazards Program.


last strip oriented vertically next to the opening (Fig. 17b). This ensures that wind friction on the leading edge of tape will minimise effects on the taped sheet of plastic when the wind passes over the taped area.

As with ground-based measurements, the COSPEC is connected either to the aircraft power supply (with a 24 to 12 V converter) or to an independent battery (e.g., a 12-volt gel cell battery). It is important that liquid lead-acid not be used. The COSPEC is also connected to a chart recorder and/or portable computer/datalogger, the clock of which must be synchronised with the operator's watch before starting measurements.


Before the survey can start, one must first determine the location and direction of the plume. If the wind direction is not known (see section 2.1.3), a complete traverse around the volcano should be made using the COSPEC as a reconnaissance tool. Once the lateral extent of the plume is determined by COSPEC (several orbits may be necessary), the lower limit of the plume must be found. Commonly, the bottom of the visible plume does not coincide with that of the SO2 plume. Multiple passes at levels below and in the plume using your nose to detect the odour or SO2 will allow you to make measurements hugging the bottom of the plume. The COSPEC is flown normal to the plume and as close to the bottom of the plume as possible to ensure that the plane is in fact below the plume. As with any COSPEC measurement, calibrations are made before and after traversing the plume.

The amount of time in which the instrument and plane are beneath the plume is short, due to the high speed of the aircraft. Note that an altitude correction to the relative airspeed is required: 2% added to airspeed for every 1000 ft (~300 m) of altitude. Thus one should reduce the airspeed in order to maintain a constant ground speed. The chart recorder should be set at a high chart rate (e.g., 60 mm min-1) in order to obtain sufficient chart resolution while the plane is beneath the plume. The time constant of the COSPEC should be set to one second. The traverse segment width and azimuth may be measured by a number of methods:

(a) The aircraft ground speed can be determined using two timed points on the ground (or estimated by a 2% increase per 1000 ft of altitude as above), marked on the chart record, with the distance between the same points measured from a map. This will give the average ground speed of the aircraft. Knowing the chart speed, one can then determine the chart time under the gas plume and multiply this value by the ground speed to

Fig. 18. Determination of traverse segment width and azimuth during airborne measurements, using (a) two landmarks and (b) the proportion of actual distance on a map to the width of the plume on the chart record.

Popocatepétl

A

Plume

X m

Huejotzingo23, 930 m

Distance measured from map

Atlixco

Therefore, 8.1 cm 23,930 m5.8 cm = X m

X = 21,831 m

d = 8.1 cm2

d = 5.8 cm1

B

Atlixco

Huejotzingo

50 10 km

θ


determine the width of the plume (Fig. 18a). The traverse azimuth can be determined by measuring the angle between the traverse line and geographic north/south on a map.

(b) The plume width is also determined by taking the proportion of (1) the chart length of the plume between the two landmarks noted on the chart record and using the (2) distance between the landmarks, measured from a map (Fig. 18b).

(c) Determine the position of the aircraft at a given point in time using a GPS; this is done repeatedly to obtain the aircraft’s ground speed and then doing the calculations as in methods (a) or (b) above. The traverse azimuth also can be determined using the GPS and method (a).

The wind speed can also be determined by flying with and against the wind (along the axis of

the plume) and comparing the true air speed with the true ground speed. For example, if it takes 3 minutes per km flying upwind and 5 minutes per km downwind, then the wind speed is 2 min km-1 divided by 2, i.e., 1 min km-1 or 16.7 m s-1. New methods using GPS technology are now available and greatly facilitate these measurements (Doukas, 2002).


Aircraft are by far the most effective method of making COSPEC measurements, since many measurements can be made over a very short period of time. Traverses are independent of road systems and thus generally consist of one straight traverse, which greatly facilitates flux calculations. The velocity of the plume also may be measured by flying along the axis of the plume at plume height. However, aircraft rental is expensive, and access to a plane is sometimes difficult (although government support is sometimes possible). If the plume hugs the surface of the volcano, airborne measurements are not possible.

3.9. Mobile measurements by helicopter


While helicopter-borne COSPEC measurements are not normally performed on a regular basis, trials at Mount Etna have resulted in the development of two techniques (Caltabiano et al., 1992). In the first technique, the COSPEC was mounted with the telescope protruding out the doorway of a single rotor (2 blades) Augusta-Bell AB-212 helicopter. The

Fig. 19. Installation of a COSPEC in a helicopter for (a) horizontal (below the plume) and (b) vertical traverses of the plume. The field of view of the COSPEC in (b) lies outside thecircumference of the rotor blades. Modified after Caltabiano etal. (1992). Inset photo courtesy of Michael Doukas.

COSPEC

COSPEC

Traverse under plume

Helicopter flyingtowards you

a

b

Helicopterdescending

Traverse down plume

Plume comingtowards you


extension tube was placed so that it was “looking” vertically up through the rotor blades above it (Fig. 19a). The second method entailed positioning the COSPEC such that the telescope’s field of view is oblique to the plane of the rotor blades without being intersected by the blades (Fig. 19b). The COSPEC was connected to an independent power supply (e.g., 12-volt battery) as well as to a chart recorder and portable computer.


The first measurement technique is similar to that for fixed-wing aircraft, with the helicopter being flown along a traverse beneath the plume. Calibrations must be made before and after traversing the plume. At Mount Etna, the time constant was set to 8 seconds in order to reduce a component of the high-frequency interference pattern from the rotor blades intersecting the field of view of the COSPEC. The optimum time constant setting may vary depending on the type of helicopter, as the rotation frequencies of the rotor blades and the number of blades will differ from one model to another. If the time constant is high, the helicopter should be flown slowly across/beneath the plume in order to allow time for instrumental response. The resulting traverse record (Fig. 20), although maintaining the repetitive interference patterns, may nevertheless allow for reasonable measurements of a given plume. This assumes that the concentration and size of the plume is such that the rotor blade interference patterns are minor thus resulting in a low signal/noise ratio (Fig. 20). As with airplane COSPEC measurements, the wind speed can be determined using a GPS or by plotting the position of two landmarks on a map and measuring the time it takes to fly between both points. Similarly, the plume azimuth is determined by

tracing a line on a map from the volcano summit to the position of maximum SO2 on a map, taken from the chart record, and measuring the line’s angle with respect to geographic north/south. The traverse segment azimuth is estimated in a similar fashion; plot the position of two landmarks on a map and measure the angle of the line from geographic north (Fig. 7).

The second COSPEC technique, which attempts to eliminate the rotor interference, entails a vertical traverse of the plume. Starting at a point above or below the plume, the helicopter is allowed to descend or ascend vertically at a constant velocity. The time that the helicopter is traversing the plume can be determined by:

(5) T = ΔH/Vd

where ΔH is the difference in altitude from the top to bottom of the plume, i.e., the virtual plume thickness, and Vd is the descent velocity of the helicopter (Fig. 21). By noting the altitude on the chart record every 100 m and knowing the chart speed, one may determine the descent velocity

AG

C (V

)

AGC signal

High calibrations

SO signal2

10

8

6

4

2

0

10

8

6

SO A

bsor

ptio

n (V

)2 4

2

0

Fig. 20. Example of a chart record for horizontal traverse helicopter-mounted COSPEC measurements. Note the repetitive interference pattern caused by therotor blades. The time constant is set to 8. Modified after Caltabiano et al. (1992).


of the helicopter. The horizontal distance (Dh) that the helicopter is offset by the force of the prevailing wind may be determined by:

(6) Dh = (Vw + Vh)⋅ΔH/Vd where Vh is the horizontal speed of the helicopter needed to counter the wind speed, Vw. The angle of deviation (θ) of the flight path from normal is then calculated from:

(7) θ = arctan (Dh/ ΔH) Once the angle of deviation is known, the cosine of this angle is used in the flux calculations (see section 6). A typical chart record for this technique is shown in Fig. 21.

In both cases, determining the position and speed of the helicopter, and plume speed is greatly facilitated with a GPS. As with fixed-wing aircraft, the first technique also can take advantage of geographical points of reference to determine the position and ground speed of the aircraft. Both techniques require a horizontal traverse along the axis of the plume in order to determine the plume speed and azimuth (see section 3.8). However, the second technique is more problematic with respect to determination of the aircraft’s position at any given time, unless a GPS is used.

VW

ΔHVD

θ

Dh

VW

AG

C (

V)

10

8

6

4

2

0

SO

Abs

orp

tio

n (V

)2

10

8

6

4

2

0

Fig. 21. Trigonometry of a helicopter-mounted vertical COSPEC traverse and example of the resulting chartrecord. Note the lack of interference patterns on the chart record because the COSPEC field of view is outsidethe area affected by the blades (see Fig. 17). Modified from Caltabiano et al. (1992).


3.9.3. Comparison of helicopter rotor blade RPM with COSPEC time constants & orientations

Setup

To examine the effects of the helicopter's rotor blade and position of the COSPEC, we performed some preliminary tests in August 1997 by varying the orientation and the time constant of a COSPEC V. We used a Bell 206 Jet Ranger helicopter which had a double-blade rotor turning at 330 rpm during normal operation. The tests were made while the helicopter was powered up on the ground. The COSPEC was mounted on the back seat of the helicopter, and the right angle mirror and extension tube were attached to the COSPEC in order to look vertically upward through the rotor blade. The COSPEC was mounted in two orientations. It was initially placed on its side on the helicopter seat so that the entrance slits were perpendicular to the rotor blade (Fig. 22a). The instrument was then placed in a normal position on the seat so that the slits were parallel to the rotor blade (Fig. 22b). For each COSPEC orientation, tests were made at time constants of 1, 4, and 8. The backgrounds, high calibrations, and low calibrations were measured on a chart recorder.

Noise levels

The chart records from the test are shown in Fig. 23. The data reveal at least two frequencies of noise. First, there is high frequency noise on the order of seconds. A lower frequency component also is observed on the order of tens of seconds. This lower frequency component appears to lengthen as the time constant decreases. For a given COSPEC orientation, the high frequency noise amplitude of the signal decreases as the time constant is increased (Fig. 23). For a given time constant, the high frequency noise amplitude is higher when the COSPEC is oriented vertically. This is likely due to the fact that the entrance slits of the COSPEC are perpendicular to the rotor blade, leading to decreased resolution. The signal appears to be least noisy when the COSPEC is oriented horizontally with a time constant of 4. This can be seen both from differences between maximum and minimum values for a given time interval (Table 2) and from average standard deviations for a series of intervals (Table 3). Using the differences data, noise levels are between 192-196 ppm·m for a time constant of 1, and 56-78 ppm·m for time constants of 4 and 8 (Table 2). The difference values should be regarded as maximum noise levels, and they are probably not the best measure of the true noise level. A better way is to examine the average of the standard deviations for the different time intervals (Table 3). Using the 2σ average standard deviation, noise levels for time constants of 4 and 8 are 15-27 ppm·m. While it is difficult to estimate the noise for a time constant of 1 using this method, the noise is probably double that for time constants of 4 and 8, i.e., 30-60 ppm·m.

Accuracy

Backgrounds are generally higher by about three vertical chart units when the COSPEC is in a vertical orientation, compared to a horizontal position. The chart unit values are roughly similar for the high and low calibrations for the different COSPEC orientations and time constants. However, the calibration values are attenuated compared to reference calibration values without the helicopter's rotor turning (background 21.5 units, high calibration 47 units, low calibration 14.5 units, high/low = 47/14.5 = 3.2) (Table 3). The high/low ratios closest to the reference are those for a horizontal COSPEC position with time constants of 1 and 4 (high/low = 3.3), followed closely by the vertical position at time constant of 8 (high/low = 3.4). The elevated


High CalHigh Cal

High Cal

High Cal High Cal

High Cal

Horizontala

Low Cal Low Cal

Low Cal

Low Cal Low Cal

Low Cal

AGCAGC

AGC

AGC AGC

AGC

SO2SO2

SO2

SO2 SO2

SO2

Time Constant = 8

Vertical

Time Constant = 4

Time Constant = 1

b

c

Fig. 23. Examples of chart records from COSPEC experiments in a Bell 206 helicopter, Vancouver Airport, Canada, 25 August 1997. Experimental conditions: helicopter rotor ~330 rpm (single blade); chart speed 60mm min-1; SO2 0.5 V; AGC 5 V; high calibration cell 326 ppm·m; low calibration cell 103 ppm·m. Each vertical division on the chart paper equals 1 cm. (a) Time constant = 1. The vertically-oriented COSPEC produces a signal which is significantly noisier than the horizontally-mounted COSPEC. The horizontal position appears to show both short-period and longer-period noise. (b) Time constant = 4. For both orientations, the noise is substantially lower compared to a time constant of 1. Two timescales of noise areagain observed. Short-period noise is lower for the horizontally-oriented COSPEC compared to vertically-mounted. (c) Time constant = 8. For both orientations, short-period noise is lower than time constants of either 1 or 4. However, longer-period noise remains a problem.

Fig. 22. Examples of a COSPEC (a) vertically-oriented on the helicopter seat with entrance slits normal to the rotor blades and (b) horizontally-oriented on the seat with the entrance slits parallel to the rotor blades.

a b


Table 2. Comparison of COSPEC slit orientation and noise levels due to helicopter blade interference.

COSPEC Orientation Vertical Horizontal Time Constant 1 4 8 1 4 8 Background 1* 25 12 10 21 8 9 High Calibration 30 10 7 21 6 5 Background 2 30 9 5 30 7 9 Low Calibration 23 9 10 22 10 8 Background 3 25 9 7 29 7 7

Average 26.6 ± 3.2 9.8 ± 1.3 7.8 ± 2.2 24.6 ± 4.5 7.6 ± 1.5 7.6 ± 1.7 Noise Level (ppm⋅m)† 196 78 59 192 56 57

*For each background or calibration, the numbers represent the difference between minimum and maximum values from the chart record over a given time interval. †The noise levels are determined by comparing the difference between above values and average peak values of the high calibration (326 ppm⋅m) (see Table 3)

COSPEC Orientation Vertical Horizontal Time Constant 1 4 8 1 4 8 Background 1* ~18 12.3 ± 2.4 13.3 ± 2.2 ~10 8.4 ± 1.0 14.2 ± 1.8 High Calibration ~60 53.0 ± 2.2 58.4 ± 0.9 ~54 53.3 ± 1.0 54.8 ± 1.3 Background 2 ~15 10.9 ± 1.2 15.6 ± 1.3 ~12 9.9 ± 0.8 9.8 ± 1.7 Low Calibration ~25 27.8 ± 1.4 28.1 ± 1.7 ~25 22.4 ± 1.0 22.3 ± 1.7 Background 3 ~14 13.2 ± 1.1 17.0 ± 1.5 ~15 8.5 ± 1.3 10.9 ± 1.3

Average background 15.7 ± 2.1 12.1 ± 1.2 15.3 ± 1.9 12.3 ± 2.5 8.9 ± 0.8 11.6 ± 2.3 Average std. dev. of - 1.7 1.5 - 1.0 1.6 5 values above Noise level (ppm⋅m)†

from 2 sigma avg. std. - 27 23 - 15 24 dev. High cal. - Avg. bkd. 44.3 40.9 43.1 41.7 44.4 43.2 Low cal. - Avg. bkd. 9.3 15.7 12.8 12.7 13.5 10.7 High / Low 4.8 2.6 3.4 3.3 3.3 4.0 *Values for backgrounds and calibrations are in arbitrary units from the chart record †The noise levels are determined by comparing the 2 sigma average standard deviation of the backgrounds and calibrations to the average peak height of the high calibration (326 ppm⋅m) The reference calibration values, when the helicopter was not operating, are 47 chart units for the high calibration (326 ppm⋅m) and 14.5 units for the low calibration (103 ppm⋅m). Thus, the high/low ratio is 47/14.5 = 3.2, which is the same as the concentration ratio (326/103 = 3.2).

Table 3. Averaged chart record values for backgrounds and calibrations during helicopter tests.


value for a horizontal position at time constant of 8 (high/low = 4.0) is due mainly to an increased average background value (11.6) which decreases the low calibration value more than the high calibration. The opposite is true for a vertical orientation at time constant of 4; the high/low ratio of 2.6 results from a comparatively low background which raises the low calibration value more than the high calibration.

Overall, it is recommended that the best choice for this particular type of helicopter is a horizontal COSPEC orientation with time constant of 4. The horizontal position provides better resolution of the SO2 signal, while the time constant of 4 decreases the noise level. This selection appears to improve accuracy, precision, and detection limits when using this helicopter. However, until more experience and data are gathered, these test results should be regarded as preliminary on this helicopter.


Helicopters have the same advantages as airplanes: rapid measurement periods at variable elevations, straight traverses, and the ability to accurately determine the plume speed. Helicopters also have the ability to access areas that a fast fixed-wing aircraft can not. However, the principle disadvantage of helicopter-borne COSPEC measurements is the interference pattern created by the rotor blades (when using the “through the blades” technique). In some instances, the general plume signal may be visible, as the interference patterns caused by the rotor-blades should be constant. However, unless one is using a computer for data collection and has access to software capable of removing or reducing the interference patterns, calculation from chart recorder data may be difficult. Helicopters can also unstable in the often turbulent and variable winds that are common around many volcanoes. In some cases, helicopters may be incapable of obtaining sufficient lift near high altitude volcanoes. Helicopters are also generally far more expensive than fixed wing aircraft to rent and operate.

3.10. Ground-based stationary measurements Making stationary measurements with a COSPEC is more difficult than mobile

measurements. Since the instrument is rarely looking vertically, atmospheric scattering effects are more pronounced, and the geometric relationship of the instrument to the gas plume requires more consideration. In particular, the main difficulties are in establishing the true width of the plume and its velocity (speed and direction). On the other hand, a greater number of measurements can be made in a stationary mode compared to mobile techniques. On balance, it is our opinion that stationary measurements are inferior to mobile measurements. However, there are many situations when stationary measurements are the only possibility and therefore should be attempted. Such situations include difficult access to the volcano due to lack of roads, lack of vehicles, danger due to heightened activity, semi-continuous monitoring (see Chapter 6), etc. 3.10.1. Setting up the instrument - the tripod

To make stationary measurements, the COSPEC should be mounted on a sturdy, solid tripod. The tripod should be set up in a position that can be easily located on a map, and this position should be known in relation to the gas plume.

The tripod should be able to scan both vertically and horizontally over a large angular range (360° horizontally and 180° vertically). The tripod must have some means to measure the vertical and horizontal angular position of the instrument. Ideally, this consists of angular scales


on the tripod which are adjusted by means of goniometers which can be smoothly turned by a hand crank (Fig. 24a). The goniometers allow the angular position of the COSPEC to be altered at a steady rate. The angular position also can be varied by means of motorised goniometers. While this arrangement permits a constant rate of movement, the motor must be powered by batteries which can fail; this can be overcome through the use of a generator. Many tripods do not have such arrangements, and alternatives can be used. To measure the vertical angle of the COSPEC, a clinometer or plumb bob can be used (Fig. 24b). To measure the horizontal angle, a transparent 360° scale can be mounted where the tripod rotates, and a pointing device can be taped near the scale to serve as an angular marker (Fig. 24c).


To measure a gas plume, the objective is to scan the instrument through the plume in a direction that is normal to the plume's movement. Thus, one scans horizontally for a vertical plume, and vertically for a horizontal plume. It is essential to scan at a rate which is as constant as possible, either by means of hand-cranked goniometers, by motor, or by simple grips on the tripod. The scan of the plume should also be made at high angles in order to reduce, as much as possible, ultraviolet scattering (Fig. 25).

Generally, measurements require two people, one person to move the angular position of the COSPEC and the other to record data on the chart paper. One person can accomplish the measurement if a motorised goniometer is used. The chart speed should be set fast (60-80 mm min-1) to record sufficient detail on the paper. As the first person adjusts the tripod at a slow and constant rate, he or she should call out each 5° or 10° increment to the second person. The second person then records the increment using the event marker on the chart recorder, noting the time to the nearest second. In this manner, the scan rate can be established, and the number of degrees traversed through the plume can therefore be calculated (Fig. 26). Motorised goniometers can be set at a given scan rate. In this example with a chart speed of 60 mm min-1, the scan rate is 10° vertically

Fig. 24. Examples of tripod-mounted COSPECs for stationary measurements. Photos b and c from USGS/Cascades Volcano Observatory.

a

b

c

(b)

(a)

(c)


COSPEC

a

COSPEC

bUV scattering

Fig. 25. COSPEC orientation for stationary plumes. The plume should be scanned at high angles in order to reduce ultraviolet scattering.

Scanning verticallydownward

Chart speed60 mm/min

Scan rate0.5 /so

First calibrationat 0 from vertical11:15 hrs

o Second calibrationat 60 from vertical,11:18 hrs

o

0o 10o 20o 30o 40o 50o 60o

Gas plume

11:15:20 11:15:40 11:16:00 11:16:20 11:16:40 11:17:00 11:17:20

Fig. 26. Example of a chart record for a traverse through a plume using the stationary ground technique. Notethat the degree of inclination and time should be recorded on the chart record during and after the measurement. In this example, 0º refers to an arbitrary starting point with the field of view of the COSPECoutside the plume and is thus independent of the actual angle between the COSPEC and the ground.


every 20 seconds, or 0.5° s-1. Although recording the time is good practice, it is not strictly necessary as long as the chart speed is known. For example, if a traverse covers 20° over 40 mm of chart paper with a chart speed of 60 mm min-1, then the duration is 40 seconds for a scan rate of 20°/40 s or 0.5° s-1.

Before and after scanning, both high and low calibrations should be made, with the time and the angular position (both vertical and horizontal) of the COSPEC noted on the chart paper (Fig. 26).

At the same time that the chart recorder is being used, the data also can be recorded digitally on a computer or datalogger. Normally a recording interval of 0.5 s or 1 s is used, with the time recorded on the chart paper. The clock of the computer/datalogger must be synchronised with the operator's watch (or vice versa) prior to starting measurements.

3.10.3. Vertical plumes

In the case of a vertical plume, the COSPEC is generally angled upward and scanned horizontally from side to side. The COSPEC must be looking sufficiently above the crater to avoid any interference, but at the same time low enough that the plume is not too dispersed and wide (Fig. 27a). The entrance slits of the COSPEC should be oriented vertically, parallel to the plume, in order to achieve the best possible spatial resolution (Fig. 27b). This is particularly important in the case of a thin or distant plume. In this case, only the Cassegrain telescope is used (without the corner mirror), with the quartz plate cover as protection.

As mentioned above, the main difficulties are the plume width and the ascent speed of the plume. We will address each of these parameters in turn. To a first approximation, the width of the plume as it leaves the crater will be no larger than the width of the crater itself. However, the plume could be significantly thinner if the source of the gas is from individual fumaroles. As the plume rises above the crater, it widens or coalesces. Therefore, it is important to calculate the visual width of the plume directly from the COSPEC measurements.

A schematic diagram of a COSPEC measuring a vertical plume is shown in Fig. 28. Since the COSPEC is frequently positioned below the crater, the instrument is tilted at an angle A on its tripod. Normally, angle A is measured, and the

a

b

Entrance slit(s)oriented vertically

COSPEC scannedfrom side to side

Fig. 27. COSPEC orientation for vertically rising plumes. Note that the entrance slits are oriented vertically and that the COSPEC is scanned horizontally across the plume. The COSPEC position should be cross wind to the volcano such that the plume travels.


horizontal distance between the COSPEC and the vent (b) is measured from a map. The height of the gas column above the COSPEC (a) and the width of the plume (d) are unknown. Our objective is to solve for d. First we calculate c (Fig. 28a):

(8) c = b/cosA

To solve for d, we need to know the angle β of the oblique slice through the gas plume. β is the number of degrees through which the COSPEC intersects the gas plume during a horizontal scan (Fig. 28a, b). This value can be determined from the start and end of the plume on the chart record. Once β is known, we use its half value to solve for d (Fig. 28c):

(9) tan(β/2) = (d/2)/c

(10) d/2 = c tan(β/2)

(11) d = 2c tan(β/2) Since the COSPEC cuts through the plume at an oblique angle, a correction must be made for

perpendicularity. This is simply the cosine of angle A (Fig. 28d).

Rise velocity of a vertical plume

The remaining unknown parameter is the rise speed of the plume. To determine the speed, an observer tracks an individual parcel of gas during its upward movement. Recording the parcel on videotape and scaling it with a time stamp may also facilitate rise speed determination. For the parcel, periodically record its angle (A) with a clinometer and the time (Fig. 29). By knowing this

COSPEC

COSPEC

angle A

angle β

β/2

β/2

β

a

A B

b

c

d

c

C D

COSPEC

β/2d/2

c

angle A

Vent

Verticalplume

COSPEClook

direction

d/2

d/2

Fig.28. Determining the width of a vertically rising plume.


angle and the horizontal distance between the observer and the vent (b), the height of the parcel (a) can be determined:

(12) a = b tanA

An example is given in Table 4 with b equal to 300 m. The height difference between 10:15:20 and 10:15:40 hrs is 163 m, and between 10:15:40 and 10:16:00 hrs 162 m. Thus, the rise rate is 162.5 m every 20 seconds, or 8.1 m s-1.

The availability of new low-cost hand-held digital cameras with video recording features

(~30 s) gives a measurement option that does not require repeated angular measurements (Fig. 29). Rather, it is possible to determine the vertical “Field of View” (vFOV) for a given focal length lens and distance, b, from the volcano. As such, it becomes possible to determine the area of plume seen in the camera image (Fig. 30) and deduce the image resolution (pixel per metre) of the vertically moving puffs of gas.

In order to determine the vFOV, one must relate the frame or sensor size of the camera (e.g., 35 mm) to the focal length. As digitial cameras do not use film, the frame size is directly related to the size of the CCD (Charge-Coupled Device) and can be determined from the specifications of the camera (e.g., Image Format, vertical/horizontal, of 22.2 x 14.8 mm or 3456 x 2304 pixels). Most digital cameras also have a focal length multiplier or conversion factor to relate the lens focal length to that of 35 mm film cameras. It should be noted that with “point and click” digital

Point Time (hrs) Angle A (°) Height (m) 1 10:15:20 33 195 2 10:15:40 50 358 3 10:16:00 60 520

Horizontal distance b = 300 m

Observer

Point 1

A1

Point 2

Point 3

A2

A3

a1

b

a2

a3

Fig. 29. Determining the rise rateof a vertical plume. Points 1, 2,and 3 are arbitrarily chosen.

Table 4. Angular measurements from a parcel of upward-moving gas


cameras where there is minimal control on the focal length used, the minimum or maximum (e.g., 6.0 - 72.0 mm for 0 to 12x optical zoom) should be used such that the plume immediately above the crater can be seen at sufficient resolution to observe puffs.

Thus, for a tripod-mounted digital SLR camera with a 55 mm lens (with multiplier of 1.6x) focused at infinity, the angular vFOV can be determined as follows:

(13) angular vFOV = 2 * arctan (frame size / (focal length * multiplier * 2)) = 2 * arctan (22.2 mm / (55 mm * 1.6 *2)) = 14.4º

Using Equation 12, for a horizontal distance, b, of 300 m and angular vFOV, A, of 14.4º, the vFOV in metres, a, equals 77 m, which for a 3456 x 2304 pixel image. This results in a resolution of 45 pixels per metre. Thus, using simple video processing software, it becomes possible to track a puff of gas from the first video frame (or time-stamped photo) as it moves vertically through the image (and vFOV) until it passes out of the image or last video frame. Thus, if a puff was observed to pass from pixel 50 through to pixel 3000 in 20 s of video, the vertical plume rise speed, V, would be:

(14) V = (2950 pixels / 45 pixel m-1) / 20 s = 3.3 m s-1

3.10.4. Horizontal plumes

For horizontal plumes, two general aspects are important. First, the operator should orient the COSPEC approximately perpendicular to the plume. Second, since the entrance slits of the COSPEC need to be parallel with the direction of the plume for greatest spatial resolution, the COSPEC is oriented to scan with the slits horizontal (Fig. 31). For this orientation, the right-angle mirror and extension tubes are required, in addition to the Cassegrain telescope. To scan

Fig. 30. Determining the rise rate of a vertical plume with a digital camera or video.

Tripod-mounteddigital camera

A

a

b

Time

AngularvFOV

vFOV(m)

Digital image / video

0 s

5 s

10 s

pixel / m

pixe

l / m


Entrance slit(s)oriented horizontally

plume

COSPEC rotatedclockwise or counterclockwiseabout its long axis

Fig. 31. COSPEC orientation for horizontal plumes. Note that the entrance slits are oriented horizontally and that the COSPEC is scanned vertically across the plume.

COSPEC

Cross-sectionof cylindricalplume

angle A

angle β

d/2

d/2

angle /2β

a

c

b

Fig. 32. Determining the width of a horizontal cylindrical plume.


across the plume, the COSPEC is rotated back and forth on the tripod keeping the COSPEC axis parallel to the plume (Fig. 31).

Once again, the two parameters that are difficult to measure are the plume width and plume velocity. Plume dynamics modify the shape of the plume constantly, and this factor complicates the width measurement. For simplicity, we assume two models, (1) a cylindrical plume and (2) a diffuse plume. The plume spreads out, diffusing laterally with distance from the volcano and also under low-wind conditions. The true shape of the plume probably falls somewhere between these two cases.

We first examine a cylindrical plume, since this configuration is geometrically the most simple. A schematic diagram is shown in Fig. 32. As with vertical plumes, we assume that the horizontal distance between the COSPEC and the plume centre (b) is known from maps. The angle from horizontal to the plume centre (A) is determined by summing (1) the angle from horizontal to the plume base, determined from the chart record, and (2) half the number of degrees subtended by the plume (angle β/2), also determined from the chart record. Using b and A, we calculate c, which is the slant distance from the COSPEC to the plume centre:

(15) c = b/cosA

Normally, the diameter of the plume is small relative to b. However, if the plume diameter is large, then d is large relative to b, and the determination of b will be subject to some uncertainty (Fig. 32). As an alternative for calculating c, b is not known but the height of the plume (a) is from Equation 12. Thus:

(16) c = a/sinA

Once c is known, we can calculate the plume diameter (d):

(17) d/2 = c sin(β/2)

(18) d = 2c sin(β/2)

COSPEC

a

angle A

angle β

ac

d/2

d/2

b COSPEC

angle β

angle /2β

c

d/2d/2

b

Fig. 33. Determining the width of a horizontal cylindrical plume: (a) nearly overhead; (b) directly overhead.


In the case of a plume nearly overhead (Fig. 33a), the calculations are the same. In the case of a plume directly overhead (Fig. 33b), we need to know the distance from the ground to the plume centre (c). Here the top of the volcano could be used as a proxy for the centre of the plume. As before, the plume diameter is:

(19) d = 2c sin(β/2) Again, there is a potentially large error measuring c if the plume is either thick or close to the ground. In both cases, it will be difficult to distinguish the centre of the plume from its base.

Under certain conditions, plumes can spread, becoming wider in the process (Fig. 34). Dispersed plumes can be distinguished from more cylindrical plumes by their greater angular spread, longer chart records, lower concentrations, and asymmetric traces on the chart record (Fig. 34). A dispersed plume also should be suspected if the COSPEC detects its far edge near the horizon.

The geometry of dispersed plumes is difficult to treat, and we initially adopt an approximate technique by measuring the width of the plume from one side (Fig. 35). We require prior knowledge of the height of the plume above the COSPEC (a), which can be difficult to estimate. One approach is to assume that the height is equal to the height of the volcano above the COSPEC. Another way to determine a is by estimating, if possible, the distance between the COSPEC and the near edge of the plume (b1):

(20) a = b1 tanθ

COSPEC

A

COSPEC

Chart Record Scanning vertically down

lateral spreading of plume

narrow plume

B

Fig. 34. Distinguishing (a) cylindrical and (b) flattened horizontal plumes.


Fig. 36. Configuration of horizontal flattened

COSPEC

A Stubby plume

b1

a

Plume

a

b2

b overestimated2

COSPEC

B Tapered plume

b1

a

Plume

a

b2

b not overestimated2

COSPECα

θ

b1

a

Plume

a

b2

Fig. 35. Determining the width of a horizontal flattened plume. plumes: (a) stubby; (b) tapered.


Once a is established, the width of the plume is then calculated by subtracting the distance

between the COSPEC and the near edge of the plume from the distance between the COSPEC and the far edge (Fig. 35):

(21) width = (b2-b1) = (a/tanα) - (a/tanθ) This equation is very sensitive to the value of α at angles less than 10°. Correspondingly, the COSPEC should be placed as near to the plume as possible, in order to increase α. Another problem is that the value of b2 is overestimated if the plume edges are stubby (Fig. 36a). The problem is lessened if the edges are thinned and tapered (Fig. 36b).

These complications can be overcome if the measurements are made beneath the plume, rather than from the side (Fig. 37). This is our preferred orientation for a flattened horizontal plume. In this case, the two component lengths, b1 and b2, are summed:

(22) width = (b1 + b2) = (a/tanθ) + (a/tanα) Once the width is determined, the plume speed can be measured as shown in section 2.1. 3.10.5. Advantages/disadvantages

This is the least reliable of techniques, as significant error is incurred due to the large and highly variable path-lengths from the plume, resulting in signal attenuation from scattering. Varying plume geometries also add to the error in the flux measurements. However, in many cases this is the only possible technique due to the volcano’s inaccessibility either by air or ground.

4. RECORDING THE DATA It is important to record as much information as possible (e.g., date, time, wind speed and

direction, volcanic activity, etc.) before and during a measurement; at the end of the day, the data processing and interpretation will be significantly easier. Data recording from the COSPEC usually consists of an analogue paper chart recorder, a portable computer, and/or a datalogger. We now discuss these three options in turn.

4.1. Chart recorder

4.1.1. Useful chart recorder characteristics

1) 1-2 channels

COSPECαθ

b1

a

Plume

a

b2

(b + b ) = 1 2 = plume widthtan θa

+ tan αa

Fig. 37. Determining the width of a horizontal flattened plume from beneath the plume.


2) internal rechargeable battery if possible 3) flatbed so easier to write on – paper lies flat 4) millimetre paper with squares on it 5) spring loaded pens and quick drying ink 6) small enough to hold in your lap 7) variable paper speeds and attenuation voltages

4.1.2. How to do it

Data from the COSPEC is in analogue form (i.e., a voltage), and thus a chart recorder has been the traditional method of data collection. The chart recorder is connected to the COSPEC data outlet and may be powered either by internal batteries or a 12-volt power source such as a storage battery, gel-cell battery, or the vehicle power supply. Prior to commencing measurements, recharge the batteries and keep spares on hand. The chart recorder should be set to an appropriate chart speed in order to give good spatial resolution. For example, over a medium duration measurement on the ground (e.g., 30 min), the chart may be set to 20 mm min-

1, while for longer measurements (e.g., 1-2 hrs), the speed may be set to 160 or 180 mm hr-1 (2.7-3.0 mm min-1). By contrast, aircraft measurements should use fast chart speeds (40 mm min-1 or higher), since the time spent beneath the plume is short. Airborne measurements at Colima volcano, Mexico, in January 1997 (traverse time ~5 minutes) used chart speeds of 50-60 mm min-1, while car-based measurements at Arenal volcano in March 1996 (traverse time ~20 minutes), had chart speeds of 20-30 mm min-1. Chart speeds as slow as 180 mm hr-1 were used at Popocatépetl in April 1997, where a typical ground traverse would take approximately one hour.

The attenuation voltage of the recorder can be changed in order to enhance the signal amplitude coming from the COSPEC. For example, if the concentration in the plume is low and the resulting peak heights are small, a decrease in the voltage of the chart recorder (e.g., from 1 V to 0.5 V) will increase the apparent peak and calibration height, increasing resolution and

A

B

1 VHigh ppm-m cellcalibration

Low ppm-m cellcalibration

1 V

0.5 V

0.5 V

Fig. 38. The results of (a) reducing the chart recorder voltage from 1 to 0.5 V to increase resolution and (b) increasing from 0.5 to 1 V to fit the plume trace to the chart record. At least one reconnaissance orbit or traverse is needed to properly set the voltage.


facilitating subsequent calculations and reducing potential reading error (Fig. 38a). The inverse is also true; should the incoming signal be too high for the chart paper, an increase in the chart recorder voltage will decrease the apparent peak height (Fig. 38b). As a rough guideline, one wants to maximise the height of the high (or low) calibration peak on the chart paper, assuming that the maximum peak height of the plume fits. In practice, at least one complete reconnaissance measurement is normally required to determine optimum chart recorder settings.

When passing under UV obstacles or trees (ground-based mobile), the pens of the chart recorder can be lifted off the paper in order to reduce the interference. Once the signal has returned to the previous level, the pen may be placed back on the paper (Fig. 39a). Should stretches of road have significant amounts of tree coverage, it is advisable to stop briefly in clear treeless areas every so often in order to get at least some measurements for that section of the traverse. One also may leave the pen recording continuously, despite the interference (as is always the case when using a datalogger/computer), and attempt a best fit to the curve in order to aid in calculations (Fig. 39b). In this case, a time constant of 1 on the COSPEC is essential so that the recording pen will quickly return to its correct position once the trees have been passed. The operator should also decide how he/she prefers to read the chart record, i.e., from left to right or right to left. This can be easily adjusted by changing the polarity (i.e., switch the wires at the back of the chart recorder) of the input signal. A copy of the chart record should always be made as a backup.

4.1.3. What needs to be recorded?

The more information that is recorded before and during a given measurement, the easier it is to interpret when in the lab. Prior to making any measurements, the following details should be written on the chart record and/or notebook:

Date Voltage attenuation (e.g., 0.5 V) Volcano Pen colour defined (e.g., red = SO2, blue = AGC) Survey type (e.g., air, stationary, car) COSPEC model Chart speed (e.g., 20 mm min-1) AGC (on ground and at takeoff if doing airborne

measurements) Time Constant on COSPEC

Segmentevent mark

Pen up

35

36 37

38

APen up

Segmentevent mark

Interpolate lineto facilitatemeasurements

Interference Interference

B

High ppm-m cellcalibration

Low ppm-m cellcalibration

Fig. 39. (a) Example of a chart record with the pen lifted off the paper at intervals in order to avoid spikes caused by interference of overhanging trees. (b) If the interference patterns are left on the chart record, an interpolated line may be drawn through the interference in order to facilitate calculations.


During measurements:

Note time at event or segment marker Direction of measurement (e.g., Eastwards = traverse #1)

Use codes for events (e.g., T = tree, St. = stop)

A two-channel chart recorder is often used, allowing for the simultaneous measurement of SO2 and AGC. Monitoring both the AGC and SO2 signal is useful but not essential; in some instances SO2 and CO2 (via LIDAR) are measured simultaneously. The traces made by the chart recorder pen represent the variation in UV absorption by SO2, with the actual SO2 concentration (ppm·m) being determined by comparison to the calibration peaks. When passing segment boundaries on the road or landmarks when airborne, the position and local time are marked on the chart paper using the event marker (Fig. 39). Noting the position and/or time of the segments on the chart record and in a notebook is necessary for the subsequent flux calculations of the individual segments, depending on the method used.


Chart recorders have the advantage of being very simple to use. They allow for immediate visual determination of the plume position and give the operator a permanent backup paper copy of the data. The chart recorder is also ideal for areas where there is intermittent interference (e.g., when passing below overhanging trees during ground measurements), as by lifting the pen from the paper, one effectively removes the interference patterns that otherwise make interpretation of the data difficult (Fig. 39b). It has the disadvantage of requiring time consuming manual calculation after measurements have been made. The chart recorder pens will tend to dry up and on uneven roads or in turbulent conditions (e.g., aircraft, boat), it is often difficult to write clearly on the record. Furthermore, in contrast to a computer or digital datalogger, a person is required to watch it.

4.2. Computer and analogue/digital converter (ADC)

4.2.1. How to do it

The data coming from the COSPEC are initially in analogue (voltage) format, and thus an analogue-digital converter (ADC) of some sort is required to digitise the information. The digital data (Fig. 40a) are collected on a portable computer or datalogger, with measurements being taken every second. For traverses lasting up to an hour, the measurement interval is usually one second (e.g., 3600 data points over an hour). For longer traverses, an interval of 2, 5, or 10 seconds can be used, depending on the length of the traverse. The data can then be processed and graphed using commercial spreadsheet software. From the graph, the beginning and end of the plume traverse may be located from event marker times or notes taken, consequently the flux may be calculated (Fig. 40b).

Many different ADCs exist on the market today, and they are continually evolving. For this reason, we do not want to recommend a specific make or model. Rather, we would like the user to consider the following points:


• the ADC ideally should not require a separate power supply, instead being powered by the computer or independently;

• the resolution should be sufficient to record short plumes; usually 8-bit resolution (1 part in 256) suffices;

• the sampling rate should be variable, from at least 1 sample s-1 to 1 sample per 10 s;

• multi-channel ADCs allow one to record SO2 and the AGC (and perhaps temperature, air pressure, CO2, GPS altitude & position, etc.);

• the computer software should be simple to use in the field;

• use of an ADC requires a portable laptop computer which needs to be powered for a sufficient length of time (typically several hours).

4.2.2. What needs to be recorded?

As with the chart recorder, the UV absorption signal should be measured. It is also necessary to record the exact time at each segment marker in order to determine the position of each segment in the digital record. Thus, one must synchronise the operator’s watch with the internal computer clock (or vice versa) prior to start of the measurements. This should be done at the beginning of each day’s measurement, as computer clocks are often not very accurate. It should also be recorded in field notes and on the chart paper at the start, at several places during the traverse and at the end of the measurement.


Computer recording has the advantage of allowing rapid and precise data collection. Recent advances in palm top computing greatly enhance the portability of these instruments. However, in order for extended use, the portable computer should be capable of being connected to a constant power supply (e.g., 12 V car battery using a cigarette lighter adapter), as computer batteries are notorious for their short life spans. Software that graphs the data in real time is desired. A chart recorder may also be connected in series to record hard copy data simultaneously. Spikes in the data, caused by interference from overhanging trees, radio interference, solar spikes and unknown transgressions, must be removed from the data set before making calculations; unless specialised software is used, this can be very time consuming. Because the COSPEC output is in analogue form, an analogue/digital converter is required. This small converter is often battery powered, and thus data will be lost if the battery fails.

Fig. 40. Example of (a) digital data received from the COSPEC and (b) the resulting digital graph. Note the grey area which represents the digital data for the high calibration peak and that the background is for a clear sky background.

Time stamp(1 second interval)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

13:52:08 13:53:10 13:54:13 13:55:16 13:56:18 13:57:21 13:58:23 13:59:26 14:00:29 14:01:31 14:02:34 14:03:36

Dig

ital u

nits

Time

ARelative SOabsorption signal = "0"

2background

High calibrationpeak

B

13:52:54 0.4502413:52:55 0.4520413:52:56 0.454113:52:57 0.4533713:52:58 0.4539313:52:59 0.4566213:53:00 0.4543213:53:01 0.4480913:53:02 0.450813:53:03 0.458813:53:04 0.4529613:53:05 0.5277513:53:06 0.7173313:53:07 0.767613:53:08 0.785713:53:09 0.7836313:53:10 0.7904213:53:11 0.7952713:53:12 0.7994913:53:13 0.7993913:53:14 0.797713:53:15 0.7965613:53:16 0.7966613:53:17 0.7942513:53:18 0.7971513:53:19 0.7951313:53:20 0.7884713:53:21 0.5568613:53:22 0.4831113:53:23 0.46448


4.3. Dataloggers

Instruments exist that allow for the digital collection of data from the COSPEC without using a portable computer. Dataloggers are typically small enough to be handheld, while some of the newer varieties are sufficiently small to be attached directly to the COSPEC and/or chart recorder (Fig. 41). Before purchasing a datalogger, one must determine the features required for efficient use with a COSPEC:

• is the memory size sufficient to store a day’s worth of points (e.g., >20,000 points) or merely an hour (e.g., 3,600 points) at a sampling rate of 1 sample s-1?

• is the power supply sufficient for long periods of measurement, or must one change batteries frequently?

• how easy is data transmission and conversion; can one easily transfer the data to a computer after the measurements?

• what is the range of sampling rates (e.g., 1 sample s-1, 1 sample hr-1, etc.)?

• is there an internal clock to synchronise with the computer, operator’s watch or GPS receiver?

• what is the resolution of the instrument, i.e., 8-bit (1 part in 256), 12-bit (1 part in 4096) or 16-bit ?

• does the datalogger have more than one channel, i.e., can one record both SO2, AGC, CO2, GPS, temperatures, etc. at the same time?

• is the instrument sufficiently small to make it more practical than a portable computer?


The main advantage over portable notebook computers for digital recording is the relatively small size and sealed electronics of the datalogger. Dataloggers often have shock-resistant, sealed cases for protection from the elements. However, the need to transfer and convert the data from the datalogger to the computer and the limited memory capacity of some dataloggers can make this method less attractive for data collection.

Of all the recording techniques mentioned above, the computer/datalogger is by far the most efficient means of recording data, and subsequently calculating SO2 flux. The paper chart recorder and field notebook can nevertheless be used as redundant, backup systems.

5. TROUBLESHOOTING

A properly tuned and aligned COSPEC can be expected to perform reliably for extended periods of time. However, it is nevertheless advisable that one test the instrument (turn on the COSPEC and record a section of chart with low and high calibrations) at regular intervals

Fig. 41. Example of a computer / ADC connected to a chart recorder and COSPEC.

Laptop/ADC

Chartrecorder

COSPECsIV & V


depending on the usage (monthly to daily during a crisis). Keeping a record of all maintenance in a logbook is also advisable, especially if the COSPEC eventually has to be returned to the manufacturer for repair.

5.1. Tuning the micrometer

Once the micrometer has initially been set by the manufacturer, it should not be changed. However, if for some reason the micrometer position changes, it will be necessary to readjust it to the original position (see the manufacture’s guide for the specific grating angle for your instrument). This is done by first connecting the COSPEC to a power supply and turning it on. The telescope is connected to an artificial light source (the “telescope lamp”), supplied by the manufacturer. One must remember to turn on the lamp. The COSPEC is then connected to a two-channel chart recorder to measure SO2 and AGC so that the variation of the grating angle can be determined (Fig. 42). Note that the chart speed should be slow and with an appropriate attenuation voltage set (see section 4.1). The outer protective casing of the micrometer should be removed (if one exists) and the locking screw, at the base of the micrometer, should be released. With the high calibration SO2 cell in the light path of the instrument, the grating angle is decreased by manually turning the micrometer screw at a constant and smooth rate. Starting at short wavelengths (higher AGC readings), turn the micrometer towards longer wavelengths (decreasing AGC values). On the chart record, the pen will draw a series of positive and negative peaks (from “zero”) for the SO2 signal (Fig. 42). Continue slowly turning the micrometer screw until these peaks lose amplitude. From the chart record, the second peak to the left of the highest

peak corresponds to the proper grating angle for the instrument (Fig. 42). One should then return to the shorter wavelengths and again start turning the micrometer screw slowly towards longer wavelengths while watching the chart record. One continues to turn the micrometer screw until the chart recorder pen reaches the top of second peak to the left of the highest peak. The locking screw should then be tightened and the protective micrometer case should be replaced. This process should be repeated a number of times until the operator becomes familiar with it.

5.2. Changing the printed circuit boards

Dust and ash can get inside the COSPEC and environmental conditions can corrode the contacts on the boards. Contacts can be cleaned with a pencil eraser. A spare set of circuit boards always should accompany the COSPEC when in the field. The circuit boards come as part

SO Signal2

Posit

ive

Sign

al

8.0

8.50

** 8.65

9.4

AGC

AG

CIn

crea

sing

volta

ge re

adin

g (d

ecre

asin

g lig

ht)

Fig. 42. Typical chart record of the variation of the SO2and AGC signals as the micrometer is tuned. The optimalmicrometer position (wavelength) is at the top of thepeak which is located two peaks to the left of the highestpeak (in this case, 8.50). Data from Barringer ResearchLtd.


of a matched set and thus should always be replaced together. One must never mix boards from different versions of the COSPEC, e.g., COSPEC IV to V.

5.3. Electrical problems/power sources

5.3.1. Power supply

If there are problems with the COSPEC power, the first thing to verify is that everything plugged in and that any batteries being used are fully charged.

5.3.2. Wiring problems

If the COSPEC is not functioning properly, verify that the connections are good and that there are no loose wires. If a contact has broken, it should be resoldered or screwed back in place. Spare power and data cables, fuses, and duct tape should always accompany the instrument.

5.3.3. Electrical fields

Electrical fields such as those generated by ground-based radio antennae and pilot transmission/communication may affect COSPEC measurements by causing spikes or noise on the chart record. If possible, radio communication should be limited to periods before or after the traverse. These electrical interferences are often difficult to avoid and must be eliminated from the chart record or digital record before doing the flux calculations. With a digital record (computer or datalogger), the spikes may be removed using commercial spreadsheet software. In the case of analogue chart recorders, these points may be ignored by simply plotting a line through the interference pattern (Fig. 39) or by digitising the chart.

5.3.4. What happens when you hook things up backwards?

If the analogue wires (the SO2 and ground) are connected to the chart recorder backwards, the relative SO2 signal and calibrations will appear inverted (Fig. 43). If the COSPEC output is switched to the analogue/digital converter (ADC) and to the computer, the ADC may stop or the computer program recording the data may crash. If the program is relatively flexible, the data simply may be recorded as inverted values, with the resulting data plot resembling that of the inverted signal on a correct chart record.

5.3.5. Chart recorder/computer problems

Dust and ash may cause serious problems with the instruments (COSPEC, chart recorder, computer, datalogger, etc.) if allowed to accumulate. One should therefore clean the instruments after an ashy day or after having worked in a dusty environment (dirt/gravel roads, etc.). Cleaning the COSPEC glass and mirrors with high purity alcohol and swabs will help maintain

High calibration

Low calibration

InvertedSO signal2

Fig. 43. Example of an inverted SO2 signal, which is the result of connecting the SO2 and ground wires backwards.


high signal quality. By making COSPEC measurements with both chart recorder and computer/datalogger connected, one reduces the possibility of total data loss should one of the data recorders fail.

6. REDUCING THE DATA In all COSPEC measurements, the SO2 flux is determined by calculating the flux for each

segment, which is then summed to determine the total SO2 flux for a given traverse. In the case of airborne or boat-mounted measurements, only one flux calculation per traverse is generally performed, as it is rarely necessary to divide the traverse into multiple segments. The SO2 flux (F) in metric tonnes per day is calculated using the following equation:

(23) F = [SO2]pl cosθ dseg νwind CF where [SO2]pl is the average path-length concentration of SO2 (ppm·m) in the plume for the segment, θ (º) is the deviation from perpendicularity of the segment of road with respect to the gas plume (Fig. 13) and dseg is the length (m) of a particular segment. For ground-based mobile measurements, the segment width is determined from a map or GPS. The term νwind is the average wind speed (m s-1) and Cf is a conversion factor changing ppm·m3 s-1 into metric tonnes per day (t d-1). One ppm·m of SO2 is one cubic centimetre of SO2 gas uniformly mixed in one million cubic centimetres of air and viewed by the COSPEC over an optical path of one metre at a pressure of 101.325 kPa and a temperature of 20°C. Thus, the conversion factor (CF) is derived as follows:

(24) CF = ρSO2 CfSTP 0.001 10-6 86,400 = 0.00023

where ρSO2 is the density of SO2 gas at standard temperature and pressure STP (2.8579691 kg m-

3 at 0°C and 101.325 kPa), CfSTP is a correction factor (273.15/293.15 = 0.9317755) to change the SO2 gas density from 0°C to 20°C (273.15 K to 293.15 K), 0.001 converts the kg term in the density to metric tonnes, 10-6 converts ppm to mass units, and 86,400 are the number of seconds in one day. Thus, CF is equal to 0.00023008194, with units of t s m-3 d-1 ppm-1.

The average concentration of SO2 for the segment ([SO2]seg) is calculated from:

(25) [ ] [ ]cal2cal

segseg2 SO

PP

SO =

where [SO2]cal is the concentration of the appropriate calibration gas cell in ppm·m, Pcal is the peak height of the appropriate calibration gas cell in arbitrary units and Pseg is the average peak height for the segment calculated from:

(26) idthsegment w

areasegment Pseg =

If the average segment peak height is lower than the low calibration peak height, then the low calibration cell should be used in the above calculation. If the average segment height falls between the high and low calibration, then use the average of the two. If average segment height is greater than the high calibration peak height, use the high calibration.

When making stationary COSPEC measurements, the flux is calculated from:

(27) E = cosA d νrise CF [SO2]


where A is the angle is the COSPEC inclination (see section 3.10; Fig. 28d), d is the plume width (m), νrise is the plume rise speed (m s-1); CF is the conversion factor (t s m-3 d-1 ppm-1; Eqn. 24) and [SO2] is the average concentration (ppm·m; Eqn. 25).

An example of a typical COSPEC flux calculation follows (Eqn 23; Table 5). For a traverse between segment markers L2 and L3 at Masaya volcano, Nicaragua, the segment width (dcol) and average peak height (Pseg) are 3,400 m and 2.8 cm, respectively. With a high calibration peak height (Pcal) of 5.85 cm and concentration ([SO2]cal) of 326 ppm·m, the segment concentration ([SO2]seg) is calculated to be 135.9 ppm·m (Eqn. 25). The deviation from perpendicularity of the segment of road with respect to the gas plume (θ) is 81º and the measured wind speed (νwind) is 10 m s-1. Thus, with a conversion factor (CF) of 0.00023008194 (Eqn. 24), the SO2 flux E for segment L2-L3 is calculated to be 1050 tonnes per day. (Eqn. 23). The total SO2 flux (1270 t d-1) for the traverse is thus the sum of segments L1.5-L2, L2-L3 and L3-L4 (Table 5).

6.1. Chart recorder

6.1.1. Area calculations – determining Pseg

While there are a number of different ways of determining Pseg, it is advisable to have several methods available as backups in case of technical problems. Box counting

There are a number of different methods for calculating chart recorder data. The most basic method is the “box counting” method. The chart record first should be broken into segments (if necessary, e.g., a ground survey) so that individual calculations and corrections can be made (a) Box counting method of calculating the average peak height for a given segment; (b) simple triangle method - half base x height = area; (c) reduction of the segment width to exclude the part outside the plume (Fig. 44). The width (of each segment) of the anomaly is divided into equally spaced (e.g., 1 mm) divisions. One then determines the baseline or background signal, which

Table 5. Example of a spreadsheet for COSPEC calculations of ground-based mobile measurements at Masaya volcano, Nicaragua

Instrument: COSPEC V High cal (ppm-m): 326

Route: Las Palmas to El Crucero Low cal (ppm-m): 103

Segment No.

Length of segment

(m)

Azimuth of segment (degrees)

Azimuth of plume

(degrees)

Difference of segment & plume: Δθ (degrees)

Abs. value of Δθ

(degrees)

Sin of Δθ

Abs. value of Sin Δθ

Wind speed(m/s)

Cal cell conc.

(ppm-m)

Cal cell Peak

height

Segment Peak

height

Segmentconc.

(ppm-m)

SO2 flux (metric

tons/day)

L-2-L-1 3350 285 256 29 29 0.4399 0.4399 10 284 5.85 0.00 0.00 L-1-L0 650 248 256 -8 8 0.1253 0.1253 10 284 5.85 0.00 0.00 L0-L1 5000 350 256 94 94 0.9955 0.9955 10 284 5.85 0.00 0.00 L1-L1.5 4000 309 256 53 53 0.7396 0.7396 10 284 5.85 0.00 0.00 L1.5-L2 3300 315 256 59 59 0.7996 0.7996 10 284 5.85 0.52 25.24 164.24 L2-L3 3400 337 256 81 81 0.9557 0.9557 10 284 5.85 2.8 135.93 1049.90 L3-L4 1400 0 256 -256 256 -0.7705 0.7705 10 284 5.85 0.36 17.48 54.60 L4-L4.5 3600 325 256 69 69 0.8838 0.8838 10 284 5.85 0.00 0.00 L4.5-L5 1700 28 256 -228 228 -0.4258 0.4258 10 284 5.85 0.00 0.00 L5-L6 2800 9 256 -247 247 -0.6730 0.6730 10 284 5.85 0.00 0.00 L6-L7 5500 343 256 87 87 0.9792 0.9792 10 284 5.85 0.00 0.00 L7-L8 3000 15 256 -241 241 -0.6004 0.6004 10 284 5.85 0.00 0.00 L8-L9 1800 14 256 -242 242 -0.6129 0.6129 10 284 5.85 0.00 0.00 Total: 1268.74 ↓ Total SO2 Flux: 1270 t d-1


should be a nearly horizontal line above which is the plume signal. The baseline is then set to zero and values assigned (relative to this baseline) to the mid-point of each division (Fig. 44a). For example, if the chart recorder uses metric paper, one divides the plot into millimetre-width columns and then assigns a value to each division relative to the zero baseline. This is repeated throughout the plot. Each data point, with its assigned y-value, is then summed for the segment in question, and then an average value determined. This average value thus represents the average concentration of the plume for that segment in units of millimetres. The same is done with respect to the calibration peaks. The average segment height in mm is then related to the calibration peak height in mm so that the plume-segment concentration in ppm·m may be determined using Equations 25 and 26. In cases where the segment is not entirely in the plume, reduce the segment width to exclude the part outside the plume (Fig. 44c).

Simple Triangle

A very simple and quick method of determining the area beneath a chart record is to calculate the best fit triangle to the plume anomaly; the area is equal to ½ Base times Height (Fig. 44b). While this is a very quick method, it is much less accurate than box counting and should only be used for rapid first order estimates.

Planimeter

The second method for calculating chart data uses a mechanical or digital planimeter to measure the area beneath the segment of the plume. The “bull’s-eye” or pointer of the planimeter is placed on the chart recorder strip and zeroed. Moving in a clockwise direction, one then uses the “bull’s-eye” to trace the SO2 curve for the segment in question. One must start and end at the same segment point, i.e., follow the curve to the end of the segment and return along the baseline to the starting position. Typically, two to three planimeter measurements are made for each segment to check precision and accuracy. One then divides the area by the width of the chart segment to obtain the average anomaly height for the segment (Eqn. 26). The average anomaly

zerobaseline

A

1

1002 1 0.4 0 0 0

2.33.5

7.3

12

17 .3

20 .3

24

24 .9

24 .3 23 .5

19

12.37. 3

5.5

2

Segments

Average of all boxes= 9.9

Plum

e sig

nal v

alues

2 3 4

C

1

Segments

2 3 3a 4

distance 3-4partia l distance

6

10

14

18

22

26

B

1 Segments

Height

Base

2 3 4

Fig. 44. (a) Box counting method of calculating the average peak height for a given segment; (b) simple triangle method - half base x height = area; (c) reduction of the segment width to exclude thepart outside the plume.


height is then compared to the height of the calibration peak(s) to obtain the concentration of SO2 for the segment (Eqn. 25). A drawn area of 1 square cm or 20 mm on a side should be used to practice using the planimeter prior to starting measurements of the anomaly. This method has the advantage of being somewhat faster than the box counting method for multiple iterations on the same segment and has a higher precision and accuracy in the case of very steep slopes on the segment curve. These mechanical techniques (box counting, simple triangle and planimeter) are simple and relatively fast ways of making area measurements.

6.2. Digital data

6.2.1. Area calculations

The basic digital method is exactly the same as the box counting method, except there is no need to give each data point a value. Using any commercial spreadsheet program, one separates the data into segments (if necessary), based on the times for each segment division noted on the chart paper during measurement. One then takes the average of the data points for the desired segment and the average of the data points for the appropriate calibration, in order to calculate concentrations in ppm·m, as shown above (Eqn. 25). As the software will average all data points for a given segment, one must remove all interference spikes due to trees, radio transmission, etc., prior to averaging (Fig. 45). These data spikes may be removed manually by using a spreadsheet program and selectively erasing data, or by creating a macro to remove all data points greater than a given limit (i.e., smoothing). A backup must be made before beginning in order to avoid erasing useful data.

A more efficient method graphically analyses the data and determines the area beneath the curve for the desired segment. This may be done with scanned (auto-digitised) chart recorder data and digital data recorded by computer or datalogger. Many of these software also have the ability to filter out repetitive interference patterns (e.g., trees, helicopter blades, etc.). However, in order to make this technique truly efficient, a software package written specifically for COSPECs is required, i.e., one that automatically records the data digitally, presents it graphically, and analyses the area beneath the graphed curve.

6.3. The use of spreadsheets in data reduction Even if one is manually calculating concentrations from chart recorder strips, the use of

computer spreadsheets greatly facilitates data reduction and flux calculations. As discussed above, spreadsheets can treat the initial raw digital data, letting the operator selectively remove interference spikes. Once the constant values (e.g., segment width, segment azimuth) and flux

Dig

ital u

nits

Time

Time stamp

ARelative SOabsorptionsignal

2

Interference from trees, street lamps, power lines

B

-1

-0.5

0

0.5

1

9:50:24 9:53:17 9:56:10 9:59:02 10:01:55 10:04:48 10:07:41

10:03:08 0.1649410:03:10 0.1939710:03:12 0.1469210:03:14 0.1244110:03:16 0.0544710:03:18 0.0300710:03:20 0.0493710:03:22 0.080510:03:24 -0.4564510:03:26 -0.8875310:03:28 0.0622210:03:30 -0.6980410:03:32 -0.7242210:03:34 0.0279510:03:36 -0.1831210:03:38 -0.178710:03:40 -1.0157610:03:42 0.0983710:03:44 0.0016610:03:46 -0.8058910:03:49 -0.5489510:03:51 -0.5483110:03:53 -0.52391

Fig. 45. Example of (a) digital data received from the COSPEC, and (b) the resulting digital graph, showing noise caused by interference from overhanging trees by the road.


calculation are entered, one need only enter the variable data (e.g., wind speed, column azimuth, segment concentration) to determine the total SO2 flux for a given traverse (Table 5).

6.4. Uncertainty calculations Constraining the uncertainties in COSPEC flux calculations can be problematical. The most

important uncertainty is clearly the accurate determination of plume height wind speeds (see section 2.1), however, calibration, environmental and instrumental uncertainties must also be accounted for. A total “uncertainty” for a give flux can be calculated using the square root of the sum of the squares (Table 6):

(28) Total 223

22

21yUncertaint nUUUU ++++= K

It is up to the user to decide whether to present these uncertainties as “error bars” or merely state the average uncertainty for a measurement.

6.4.1. Calibration uncertainties

If there appears to be significant variations between calibrations before and after a given measurement (e.g., due to variable cloud cover), the most “representative” calibration or calibration closest to the plume should be used in the calculations. Multiple traverses on a given day will allow the user to determine what is a truly representative calibration peak. The user may also consider performing two or three flux calculations, (i.e., a low-, high- and average-calibration calculation) and thus presenting minimum, average and maximum values.

6.4.2. Background uncertainties

Variations in calculated SO2 fluxes between individual traverses may be due, in part, to fluctuations in wind speed and direction, changes in cloud cover, and change in sun angle, resulting in variable amounts of solar ultraviolet radiation. The opacity of an eruptive plume also varies due to changes in ash content that will increase the absorption of ultraviolet radiation (Andres and Schmid, 2001). Instrumental uncertainties include instrument calibration (± 2%), digital/analogue chart reading uncertainties (± 2%), varying vehicle/aircraft speed (± 5%), and wind speed measurement (± 0-60% or greater) (see below, Table 7; Casadevall et al., 1981; Stoiber et al., 1983). Chart and vehicle speed uncertainties can be reduced somewhat by digital recording the SO2 absorption and vehicle speed (see sections 4.2 and 4.3).

Table 6. “Calculated” uncertainties for SO2 measurements, Arenal volcano, 1995-1996.

Calibration cell concentrations: COSPEC IV: 300 ppm-m 2%COSPEC V: 339.2 ppm-m 2%

Digital record reading uncertainty: 2%Variation in car speed: 5%Windspeed determination:

1995: 0-60% 30%1996: 6-26% 14%

Total uncertainty (square root of the sum of the squares):

1995 Minimum: 6% Maximum: 60% Average: 31%

1996 Minimum: 9% Maximum: 27% Average: 15%


6.4.3. Wind speed uncertainties

At Arenal volcano in 1995 and 1996, the uncertainty in wind speed measurement was due, in large part, to the fact that measurements were made at the base of the volcano (elev.: ~550 m), which did not represent wind speeds at the summit of the volcano (1690 m) where degassing is taking place (Williams-Jones et al., 2001). As there is approximately a 1-1.5 km difference in altitude between the ground and the plume, the plume velocity may be up to 10 times greater than that measured on the ground (Willett and Sanders, 1959). In an extreme example, a radiosonde profile of wind speeds recorded from the Mexico International Airport (2230 m a.s.l) shows that wind speeds were almost 20 times greater at the summit height of nearby Popocatépetl volcano (5430 m a.s.l) than ground level winds at the airport (Williams-Jones et al., 2006).

Wind speed measurements are also affected by instrumentation uncertainties. Wind measurements made in 1995 have an average standard deviation of ~30%, as the digital anemometer that was used gave readings that varied continually with local wind gusts (Table 7). The operator thus was forced to estimate the average range of speeds at the time of measurement. The 1996 wind measurements used a fully mechanical anemometer that allowed for the integration of wind speed over an interval of one minute. Consequently, the standard deviation was ~14%, or about half that of the previous year (Table 6). Total average uncertainties of ~31% and ~15% for the SO2 flux were calculated for 1995 and 1996, respectively (Table 6). However, these uncertainties are probably significantly underestimated and thus, where possible, plume height wind speed measurements should be made (e.g., with the dual spectrometer method, see section 2.1.3).

Table 7. Wind speed measurements 4 km west (elev. ~550 m) of the summit of Arenal volcano, Costa Rica, 1995-1996.

1995 1996 Date Speed (m s-1) Avg.

(m s-1) StdDev % Dev Date Speed (m s-1) Avg.

(m s-1) StdDev % Dev.

28/02/95 2.5 2.5 2.5 2.5 28/02/96 2.23 2.13 2.39 2.3 0.13 5.8 04/03/95 4.1 4.1 01/03/96 3.29 3.84 2.25 3.1 0.81 25.8 05/03/95 3.9 3.6 3.8 0.21 5.7 03/03/96 2.63 2.6 0.00 08/03/95 2 1 1.5 0.71 47.1 05/03/96 2.77 4.09 4.09 3.7 0.76 20.9 22/03/95 2.6 2.6 06/03/96 2.73 3.83 3.6 3.4 0.58 17.1 23/03/95 2 1.8 1.9 0.18 9.4 08/03/96 2.82 3.33 3.1 0.36 11.7 24/03/95 1.2 1.2 1 3 1.6 0.95 59.6 Average: 13.6 27/03/95 2.2 1.3 3.5 2.3 1.13 48.8 30/03/95 4 3.5 3.8 0.35 9.4 31/03/95 2.25 2.3 01/04/95 4 1.8 2.9 1.59 55.3

Average 33.6


6.4.4. Wind / plume direction uncertainties

Uncertainties in plume azimuth have a comparatively minor impact on the final SO2 flux calculation. A difference of one degree, for example, gives a deviation of only 0.3%, while a difference of ten degrees results in deviation of 4.6%. Thus, while the user should attempt to determine the plume direction as accurately as possible, small uncertainties in direction can be neglected.

7. PRESENTING THE DATA Once the SO2 flux data have been collected and reduced, the data must be presented in a

manner which facilitates interpretation. Firstly, because of the difficulties in accurately measuring wind speed as well as other uncertainties, the actual flux for a survey (e.g., of 10 measurements) should be rounded to the nearest 10 t d-1, i.e., 2640 ± 90 t d-1 instead of 2639 ± 93 t d-1. Critically, the wind speed and direction should always be presented in conjunction with SO2 flux data and distance from the volcano. The averaged SO2 fluxes for a day, month or year must also be presented along with values for individual traverses. In order that other users may

Fig. 46. Diagrams for Galeras volcano, Colombia, showing (a) SO2 flux with standard wind speed of 1 m s-1 and SO2 flux with measured wind speed, modified after Zapata et al. (1997); (b) SO2 flux and duration of long period seismic events versus time. Dots and connecting lines refer to SO2 flux, bars are the durations of seismic events. Note the low SO2 flux prior to the eruptions. Modified after Fischer et al. (1994).

BPhase 2

March 93February 93January 93

200

0December 92

duration

Eruption

Eruption

SO2Jan. 11

400

600

SO e

mis

sion

rate

(t d

)2

-1

SO e

mis

sion

rate

(t d

)2

-1

800

1,000

0

50

100

Dur

atio

ns o

f LP

even

ts (s

)

150

200Phase 1

Eruption

Feb. 1

Mar. 12Phase 3

erup

tion

250016 July 1992 eruption

SO data withmeasured windspeed

22000

1500

1000

500

0

erup

tion

May

150016 July 1992 eruptionSO data with standardwindspeed of 1 m s

2-1

June July August

1000

500

0

A(a)

(b)


properly compare data from a given volcano taken at different times, all of the above data should be routinely presented.

A plot of SO2 fluxes calculated using both the measured wind speed and a standard wind speed of 1 m s-1 is a useful method to eliminate some of the wind speed uncertainties (Fig. 46a, Zapata et al., 1997). By standardising to 1 m s-1, one eliminates the wind contribution, facilitating comparison with other flux data standardised in the same manner. The measured wind speed is nevertheless needed in order to determine the calculated fluxes.

Another common method of presenting the data is by plotting SO2 fluxes and other measurements versus time. A line graph is used to show the variation in the SO2 flux with time, while seismicity is shown as a bar chart (Fig. 46b, Fischer et al., 1994). One also can plot SO2 flux versus time as a log normal graph to reduce some of the scatter resulting from large variations in flux (Fig. 47, Connor et al., 1988), although one risks obscuring some detail. Moving statistics (Cleveland and Kleiner, 1975) can be used with high noise to signal ratios in order to search for structure in the data sets. These moving statistics can enhance the scatter plot and bring out structures in otherwise noisy data. With large datasets, filtering methods also may be applied to the data in order to reduce noise. At Etna, for example, a low-pass filter (threshold of 28 days) and a high-pass filter (threshold of 60 days) were applied to the data to minimise noise and allow identification of three distinct degassing intervals (Fig. 48, Caltabiano et al., 1994). Another method for seeing changes in a data set is to plot cumulative SO2 flux versus time (Fig. 49, Caltabiano et al., 1994). This plot also reduces noise and clarifies changes in the degassing behaviour of the volcano, which will be evident from changes in the slope of the cumulative curve (Fig. 49). Cumulative plots also can be used for presenting seismic and deformation data, and thus this approach permits comparison between different datasets.

One also should decide whether all of the data (SO2, seismicity, deformation, etc.) for a given period is presented or whether only the average measurements of a day, month, year, etc., are presented. Using only the average SO2 flux is yet another method of reducing the “noise” of a given data set to bring out any possible correlations. For example, comparison of average monthly SO2 with average monthly micro-gravity change at Masaya volcano, Nicaragua, shows a distinct correlation (Fig. 50; Williams-Jones et al., 2003).

Fig. 47. Diagram for Halema’uma’u crater, Kīlauea volcano, Hawaii, showing ln (SO2) versus time with moving statistics applied to reduce noise. Top curve is the upper semimidmean, middle curve is the midmean, and the lower curve is the lower semimidmean. From Connor et al. (1988).

806040204.0

0

4.5

5.0

5.5

ln [S

O] (

t d)

2-1

Time (days)

+

++

+ +++++ +

+

+++++ ++ ++

++ + ++++ +++ +++ +++

++++++

+++

++++ ++

++++

++

+++

++

+

++++

+ +++

+++++++

+

+++

+++++++

++

+

++++++

++

++ ++

+

++++++++

+++++

++++ +

+++++++++++

++

++++

+

+

+

+

++

+++++++

+

+ + +++++

+++++++

+

++ +++++

+++++

+

+

+++

+++

+

+

+

+++ + +

+ +++

+++++++++

+++++ ++++++

+

+++ ++

++++

+ +++++++

+ ++ ++ +

+++

6.0


Fig. 48. SO2 flux versus time at Mount Etna (a) before filtering and (b) after filtering with cutoff threshold of 28 days and (c) 60 days. Note the smoothing effect of increasing the cutoff threshold. Zones A, B, and C represent three different phases of eruptive activity. From Caltabiano et al. (1994).

(c)

(b)

Jan. 92Jan. 91Jan. 90Time

Jan. 890

Jan. 88

5,000

10,000

15,000

20,000SO

Flu

x (t

d)

2-1

25,000

20,000 CA

5-Dec-919-Feb-9114-Apr-9018-Jun-8922-Aug-88

Time

B

25,000

15,000

10,000

5,000

SO F

lux

(t d

)2

-1

027-Oct-87

20,000 CA

5-Dec-919-Feb-9114-Apr-9018-Jun-8922-Aug-88

Time

B

25,000

15,000

10,000

5,000

027-Oct-87

(a)

Filtered - 60D

Filtered - 28D

SO F

lux

(t d

)2

-1

Unfiltered

CA

5-Dec-919-Feb-9114- Apr- 9018- Jun- 8922- Aug -88

Time

B

8

6

4

2

SO F

lux

(Mt)

2

027- Oct-87

Fig. 49. Cumulative SO2 flux versus time at Mount Etna. Note the change in the slope of the curve for the three intervals of eruptive activity. From Caltabiano et al. (1994).


Acknowledgements

We thank Robert Dick of Barringer Research Ltd. for discussion regarding the inner working of the COSPEC and Robert Andres of the Department of Space Studies, University of North Dakota, for information on the influence of plume opacity on COSPEC measurements. We also thank John MacGregor of Transport Canada for time in the Bell helicopter and Gary Grass, also of Transport Canada, for assistance with the helicopter measurements.

??

?

?

1 Standard deviationof repeat measurements

Error (20 Gal) on1 measurement

μ

Gravitychange

SO flux2

-100

-80

-60

-40

-20

Gra

vity

cha

nge

(G

al)

μ

-120

-140

-1601992

500

1500

2000

2500

3000SO

em

issio

n ra

te (t

d)

2-1

1000

01993 1994 1995 1996 1997 20001998 20011999

0

Fig. 50. Average monthly SO2 fluxes (circles) compared with average monthly gravity change (diamonds) atMasaya volcano, Nicaragua. Grey circles represent average COSPEC measurements while shaded region represents one standard deviation of flux values (~30%). Black diamonds denote repeat gravity measurements; white diamondsdenote single measurements. From Williams-Jones et al. (2003).


REFERENCES

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McGonigle, A.J.S., Hilton, D.R., Fischer, T.P., and Oppenheimer, C. (2005b) Plume velocity determination for volcanic SO2 flux measurements. Geophysical Research Letters, v. 32, L11302, doi:10.1029/2005GL022470.

Willett, H. C., and Sanders, F. (1959) Descriptive Meteorology; Academic Press Inc., New York, 355 p.


Williams-Jones, G., Stix, J., Heiligmann, M., Barquero, J., Fernandez, E., and Duarte Gonzalez, E. (2001) A model of degassing and seismicity at Arenal Volcano, Costa Rica. Journal of Volcanology and Geothermal Research, v. 108, p. 121-141.

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Williams-Jones, G., Horton, K.A., Elias, T., Garbeil, H., Mouginis-Mark, P.J., Sutton, A.J., and Harris, A.J.L. (2006). Accurately measuring volcanic plume velocity with multiple UV spectrometers. Bulletin of Volcanology, v. 68, p. 328-332.

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Zapata, J. A., Calvache V., M. L., Cortés J., G. P., Fischer, T. P., Garzón V., G., Gómez M., D., Narváez M., L., Ordóñez V., M., Ortega E., A., Stix, J., Torres C., R., and Williams, S. N. (1997) SO2 fluxes from Galeras volcano, Colombia, 1989-1995: progressive degassing and conduit obstruction of a Decade Volcano, Journal of Volcanology and Geothermal Research, v. 77, p. 195-208.

Documents

Chapter 2 Using the COSPEC in the Field