Portable digital seismological AC station over mobile telephone network and internet

Computers & Geosciences 29 (2003) 685–694

Portable digital seismological AC station over mobiletelephone network and internet

Carlos A. Vargas-Jimeneza,*, Sergio Rinc !on-Boterob

aDepartamento de Geociencias, Universidad Nacional de Colombia, sede Bogot !a, ColombiabFacultad de Ingenier!ıa, Universidad de Manizales, Manizales, Colombia

Received 1 November 2001; received in revised form 31 October 2002; accepted 15 November 2002

Abstract

We have developed a portable station with a low-frequency signal conditioning and acquisition system for seismic

event recording. This station records events in different modes and transmits them in the cellular mobile telephone

network via active pages. Setup of the station is done directly at the station or remotely via a TCP-IP connection to the

site. The designed station has a 101 dB dynamic range and nine channels. Three are for the tri-axial array geophones.

The other six are used for the acquisition of signals with less than 100 samples per second requirements (temperature,

radiometry, inclinometers, battery monitoring, etc.). The station software allows detection of seismic events using a

short-term/long-term coverage standard algorithm, as well as threshold detection, periodic capture and continuous

channel capture. The conditioning and acquisition system was designed as an embedded command-driven system with

its own real-time clock and storage memory.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Seismic station; CDPD; Data acquisition; Data transmission

1. Introduction

Field stations allow the capture of signals such as

those coming from sensors for seismic registration,

water-level measurement in rivers, environment tem-

perature, precipitation, and many other variables, which

are transmitted so that they can be analyzed.

In general, most of the seismic monitoring stations are

based on conventional telemetry systems or autonomous

captures. In the first case, requirements for acceptable

transmission conditions are not always met (no line of

sight, high-retransmission costs, etc.), and in the second

case, qualified personnel are required to collect the

information at the site periodically.

In the present work, a portable station is proposed

with connection to the cellular mobile telephone

network (Fig. 1). The station acquires seismic signals

for post-processing and analysis, including data

storage and generation of events that can be remotely

requested.

2. Embedded acquisition system

The seismic events capture is done using a tri-axial

geophone, that is, sensors which register the earth’s

ground velocity in three directions E,N,Z (East–West,

North–South, Up–Down). The analog seismic signals,

as well as others coming from temperature sensors,

inclination, etc., are captured, conditioned and con-

verted into digital signals that are processed on the

computer, using the embedded acquisition system

(SAE). Data request handling is received via RS232

serial port interface.

The SAE system is composed of the Conditioning and

Signal Capture Unit, Central Unit (microcontroller),

Memory and Storage Unit, Time Base Unit, Serial

Interface and Display (Fig. 2).

ARTICLE IN PRESS

*Corresponding author.

E-mail address: [email protected]

(C.A. Vargas-Jimenez).

0098-3004/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0098-3004(03)00041-4

2.1. Control process unit

The system requires a speed controller, with low

power consumption, ports for I/O peripherals, external

interruptions, timers and UART for serial communica-

tion, so that the control and acquisition integration,

storage, display, time base and communication can

be accomplished. The ATMEL AT89C55 micro-

controller was chosen in the system development due

to its characteristics: Intel 805X Instruction set compat-

ibility allowing the use of C compilers, 20K Flash

program memory, low cost and availability. The

microcontroller address bus allows the assignment of

memory blocks to different devices, as is accomplished

in a PC. The I/O memory block for each device is

designated according to the number of necessary

registers for its operation.

Once the address for each device is assigned, the base

for its selection must be generated. In the design,

programmable logic devices (PLD) carry out the device

selection according to the actual memory block access.

Due to the microcontroller architecture (it uses a shared

byte for the address and data bus), it is necessary to hold

the address bus low byte once the access to the memory

mapped devices is generated.

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Fig. 1. General scheme of system: (A) Analog signal captured that it is sent to PC (portable station); and (B) Connection between

portable station and cellular telephone network.

Fig. 2. Functional diagram of embedded acquisition system

(SAE).

C.A. Vargas-Jimenez, S. Rinc !on-Botero / Computers & Geosciences 29 (2003) 685–694686

2.2. Conditioning and acquisition

The signals with the highest spectral components are

those that come from the geophones (30Hz). In such

situations, the chosen sampling frequency, Fs is 100Hz.

The digital resolution is 16 bits, so we are able to

guarantee the seismic signal acquisition, usually limited

to a 60 dB dynamic range for seismic events (Lee, 1989).

The broad band seismic instruments can measure very

weak events with periods of 1–300 s (Lemaire, 1997)

which require as high as 154 dB dynamic range, but the

implemented card will be able to capture signals with a

100 dB dynamic range as will be further discussed. The

tri-axial acquisition is carried out at the same time for

each one of the three sensors, to insure that there are no

introduced delays between axis signals. The signal

conditioning sigma-delta analog to digital converter

AD7714 was chosen because it has a very low noise level

and good conditions for the input signal (preamplifier,

filtering and digitalization). The acquisition system

inputs were implemented as differential signals, and it

can be switched between unipolar or bipolar format

according to the signal nature.

The AD converter low pass filter nominal frequency

response is (Kester et al., 200l)

jHðf Þj ¼1

N

SenðNpf =fsÞSenðpf =fsÞ

��3

:

The voltage references are necessary to provide each AD

converter with a very stable potential level with a low

temperature dependency and low noise level. Here the

AD780AN is used, because of its temperature stability

(3 ppm/�C), low noise level (100 nV/OHz), low cost, andbecause it has a pin which generates a linear voltage

according to the chip temperature; this pin was used to

compute the card temperature.

2.3. Storage and real-time clock

The data storage must be of the random type, with

nanoseconds access time, with equal or more capacity

than the allowable addresses in the control process unit

data bus, and should be low cost. The BQ4842 IC was

chosen because of the 128K static non-volatile RAM,

very low power consumption, fast access time, em-

bedded real-time clock, internal oscillator and time

calibration registers for lack of adjustment due to

temperature.

2.4. Serial communication

The information captured by the system must be

transmitted to a PC, where the software that processes

and stores the events is located. The communication

speed between the SAE card and the PC is adjusted for

19200 bps so enough time is left between samples for

real-time processing.

2.5. Display

The acquisition system requires displaying the actual

mode status and the capture details, especially when the

autonomous mode (periodic channel capture) takes

place. A 16 character � 4-line liquid crystal display

(LCD) with the Hitachi HD44780 controller was used.

2.6. Implementation

The connection of all system devices used in SAE card

is shown in Fig. 3. A 12V–38Ah rechargeable battery

regulated to 5V by the circuit, which provides enough

charge to allow continuous operation for more than

500 h, feeds the system.

The RMS noise level on the card was obtained from

tests of 500 samples. The input was connected to a 390Oresistance, which is the impedance of the geophone when

its mass is anchored. The minimum step for this

configuration is 78.29 uV, and the obtained noise is

41 uV as is shown in Fig. 4. The effective resolution,

Reseff, is defined (Analog Devices, 1998) as the

maximum input scale (2 � Vref � Gain) to RMS out-

put noise ratio, that is

Reseff ¼ Log22� 2:5� 10:000041

� �¼ 16:89 bits:

The dynamic range is defined as the maximum to

minimum ADC quantifiable input ratio. In this case, the

quantization noise is less than the RMS noise, so the

minimum quantifiable input is 41 uV, and the dynamic

range is

DR ¼ 20 Log5V

0:000041V

� �¼ 101:72 dB:

The noise comes from different sources: the quantization

noise and device noise, and the potential difference

between the system grounds and the noise added by the

digital logic to the bias lines.

The design of the printed circuit board (PCB) is very

important for the acquisition response. It was designed

using independent analog and digital ground planes with

more than 75% board area used for ground purposes

and a low capacitive coupling between them, wide bias

tracks, in a doubled sided Sn/Pb Board (Kester and

Bryant, 2001). Because the geophone’s dynamic range

will not be more than 60 dB for natural seismic events

(Lee, 1989) and the system’s dynamic range is more than

100 dB, it was not necessary to design a more complex

PCB ground system with optical coupling between the

microcontroller digital logic and the converters. The

offset error is 108 uV, which is an error near two times

the LSB step potential (76 mV). It is possible to reduce

ARTICLE IN PRESSC.A. Vargas-Jimenez, S. Rinc !on-Botero / Computers & Geosciences 29 (2003) 685–694 687

the error by programming the converter’s calibration

registers.

To show the filter response for the first notch

(100Hz), we captured a sinusoidal signal with

5–105Hz variable frequency in 5Hz steps keeping the

amplitude constant. Fig. 5 illustrates the gain value in

dB obtained when the RMS voltage value, used as the

reference, is 5V.

Due to the real-time clock precision, it is necessary to

calibrate the system periodically with a reliable time

base. The most common is the UTC (Coordinated

Universal Time) available from several agencies, such as

the National Institute of Standards and Technology

(NIST), the Navy Observatory, and the International

Bureau of Weight and Measures in Paris, among others.

UTC can be obtained through the Global Positioning

System (GPS) or Internet via the daytime (RFC-867),

time (RFC-868) and network time (RFC-1305) proto-

cols. The acquisition system receives periodic time

updates when the user requests them locally or remotely.

As can be seen in Fig. 1, the SAE is connected to a

laptop which has an Internet connection, allowing the

computer’s clock to be updated with the UTC using

programs like the one offered by the NIST Internet

Time Service (NIST, 2001). The acquisition system is

updated with the UTC time when the SAE card’s clock

is synchronized with the computer’s clock. This method

gives a system time drift no greater that 1 s per day

against the UTC because of the network delays and the

execution time for the clock setup commands on the

SAE card. If the system is not going to be used

for interaction between several stations (as for this

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Fig. 3. SAE card schematic diagram.


application), then this method can be valid and system

time drift will be no greater that 0.1 s per day. To

synchronize multiple portable stations, the integration

of the SAE card and the GPS system must occur to

insure that the system’s time is precise within ms. For

this reason, the pins P1.2 and P1.3 were left free so a

GPS could be connected serially to the SAE card using a

second UART Chip.

2.7. SAE card programming

The microcontroller supervises the integration and

operation of all the devices and it waits for a task

request from the computer (Fig. 6). These tasks can be

of several kinds: configuration, information and mode.

For example, it is possible to change or request the gain

and mode (unipolar or bipolar) for every channel, the

system date and hour, card temperature, and also to

begin the capture modes according to the acquisition

nature (geophone capture, periodic channel capture, and

samples continuous capture). A card test may also be

performed to check the memory and the real-time clock

integrity. The C language was chosen for microcon-

troller software development and the ABEL language

was used for the PLDs.

The microcontroller program structure is based on

function pointers. Each function represents a system

configuration, information or mode task. In the serial

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Fig. 5. Converter’s filter frequency response.

Fig. 4. SAE card noise and power spectrum with 0V input.

C.A. Vargas-Jimenez, S. Rinc !on-Botero / Computers & Geosciences 29 (2003) 685–694 689

interrupt the task manager is located, in charge of the

argument parsing, request validation and function

pointer adjustment so the command can be executed.

The short execution time for the command interpreta-

tion compared with the sample period and the applica-

tion nature allows the use of this model without the need

for a preemptive multitasking kernel (Labrosse, 2000),

because only one mode will be executed at the same time

and all tasks have the same priority.

The total number of implemented commands is 16,

which once requested are interpreted and executed; if the

request cannot be executed due to argument errors or

because another task is already executing, then the error

is reported via the serial port whenever it is available.

The data stored on the external memory remains even

if there is no voltage source present; this data includes

the gain and mode configuration information for the

input channels. The data is verified at power up reset,

which can be generated externally using the serial

interface or when the voltage source is plugged in. The

synchronization can be done from the server (console)

or remotely from the user application (client).

3. PC software

The PC handles the integration of the acquisition

system, database, event transmission and remote setup.

A Windows server application was designed; it sends

commands to the acquisition system, processes the user

requests, and stores the events in the database (Fig. 7).

This program allows the remote users edition from the

geographic places where the stations will remain. The

captured events are shown with this application, or in

the client software. The server application is pro-

grammed in Microsoft Visual Basic and the database

link was done with Microsoft Access using the Jet

Engine (Fig. 8).

The server application has four capture modes: Mode

0 is optimized for seismic event detection using the

short-term average–long-term average STA–LTA algo-

rithm (Basseville and Benbeniste, 1986); three geophones

are connected to the SAE card for this configuration:

two horizontal for the E–N plane and one vertical for

the Z-axis.

The STA–LTA algorithm is a low-pass filter that

averages two windows (short-term and long-term) with

the last captured data and computes the ratio between

them: this average can be seen as the power density

which will be very close to one when there is no

important change in the sensor signal. When an event is

generated, the short-term window raises its power

density and becomes bigger than the long-term window

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Fig. 6. SAE’s task manager.

Fig. 7. Structure of PC server software.


(Fig. 9). The window’s time values indicate the cutoff

frequency for the normalized filter, which can be

defined as

STA ¼ STA þðabsðfi � 32768Þ � STAÞ

k1;

LTA ¼ LTA þðabsðfi � 32768Þ � LTAÞ

k2;

where fi is the actual read sample, STA is the

approximated short-term window average and LTA is

the approximated long-term window average. The value

32768 is the captured 16-bit sample offset. kl and k2 are

the number of samples of the STA and LTA windows,

respectively. The previous equations can be rearranged

as

yðnÞ ¼ ð1=kÞ xðnÞ þ ððk � 1Þ=kÞ yðn � 1Þ;

where x represents the input, y the computed average,

and k is k1 or k2. The n index represents the actual time

and n�1 is the previous sampled time. The user canconfigure the ratio values that will activate the beginning

of the event (EnThrHld) and the end of the event

(DisThrld), being this less than the activation ratio. A

window with n samples preceding the beginning of the

event (PEM) will be stored along with another window

with n samples after the detection of the end of the event

(PET). Even though the sizes of the LTA and STA

windows depend on the spectral content of the seismic

signal, for most applications these values are generally

set between 30–60 and 0.5–3 s for the LTA and STA

windows, respectively.

Signal processing using the STA–LTA algorithm

allows the decrease of the disk storage requirements.

Although it cannot guarantee that all events are

captured, nor that all the captured events are really

seismic events, it is a good and widely used selection

tool.

The mode 1 capture is used for the acquisition of a

fixed number of samples coming from any channel.

Mode 2 generates an event when the threshold of a

minimum and maximum range of values is detected for a

single channel. Mode 3 executes a periodic capture of six

channels every n seconds according to the gain and

mode previously configured. In addition, it does not

require the laptop connection when it is not necessary to

obtain the data until the capture mode is finished

(Fig. 10).

The active pages were programmed Personal Home

Page (PHP) and they execute the events query and

remote download from the database; the link is

implemented in PHP with ODBC. This model allows

the search of events by capture date, capture mode,

user who requested the capture, and station location.

The download and information requests for events

are received via HTML in a browser and are pro-

cessed so a SQL sentence is generated, which is executed

at the server and the result is sent back to the user

in a table, generating a link for every event found.

This link takes you to a new page with extended

information about the event and allows you to down-

load a file containing the capture in ASCII format for

the headers and binary format for the digitalized signal

(Fig. 11).

4. Client software

The user can verify the events generated by the station

using an HTML browser. The station setup or capture

mode request requires that the user installs the Client

Application (Fig. 12).

The client application was programmed in Microsoft

Visual Basic and its structure is the same as for the

server application, with the difference that the data is

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Fig. 8. SAE server application.

Fig. 9. Pre-event and post-event of a seismic signal.


not obtained from the database but from the down-

loaded files from the active pages using the browser.

To decrease the system cost, the server current

application can be exported to personal digital assistants

(PDAs) and handheld personal computers (H/PC) with

a type II PCMCIA slot so a CDPDWireless modem can

be connected. Such devices can be programmed with the

Visual Basic for Windows CE (Embedded).

ARTICLE IN PRESS

Fig. 11. Event search. Active pages execute events query and remote download from database; link is implemented with ODBC.

Fig. 10. Mode 1 capture for Z vertical component with gain 8.


5. Data transmission

For the real-time capture systems, where the transmis-

sion is done late and there is a low capacity connection

required, the cellular mobile telephone network can be

used.

The field transmission used the AMPS cellular mobile

telephone network infrastructure, via cellular digital

packet data (CDPD) in wide area networks, with a

19200 bps transmission speed, full duplex in asynchro-

nous mode, over the TCP/IP protocol package. For the

current assembly 600mW CDPD modems, with RSA

128-bit RC4 symmetric stream cipher air interface data

encryption, were used. The CDPD service provider is in

charge of the fixed IP Assignment; this IP address can be

seen within the provider network and the Internet, so

the setup and event acquisition can take place in the

provider area and world wide via an Internet access

point.

6. Prototype

In Fig. 13, the final SAE Card prototype is shown.

This prototype is used for the Observatorio Vulcanol-!ogico y Sismol !ogico de Manizales—INGEOMINAS in

temporary campaigns to evaluate the seismological

activity of the Nevado del Ruiz Volcano, Colombia.

The development of this prototype was US$500, reason-

ably low cost if we compare it with stations produced in

mass by companies that manufacture digital seismolo-

gical stations. However, due to high costs of the cellular

telephony in this region, it is not practical to operate it

for long periods of time.

7. Conclusion

A portable station was designed and implemented.

This station consists of a conditioning and acquisition

system used primarily for seismic capture purposes but

can be extended to other low-frequency signal applica-

tions.

The designed station possesses a 101 dB dynamic

range, nine channels: three for the tri-axial array

geophones and six others for the acquisition of signals

with less than 100 samples per second requirements

(temperature, radiometry, inclinometers, battery mon-

itoring, etc.).

This station generates events in different modes and

publishes them in the cellular mobile telephone network

using active pages. The station’s software allows seismic

event detection using the STA–LTA algorithm, and also

periodic and continuous capture of the channels.

ARTICLE IN PRESS

Fig. 12. Remote setup with SAE client application.


The conditioning and acquisition system was designed

as an embedded command-driven system with its own

real-time clock and storage memory.

References

Basseville, M., Benbeniste, A., 1986. Detection of Abrupt

Changes in Signals and Dynamical Systems. Springer,

Berlin, Germany, 373pp.

Labrosse, J., 2000. Embedded Systems Building Blocks, 2nd

edition. CMP Books, Kansas City, Missouri, 640pp.

Lee, W.H.K., 1989. Toolbox for seismic data acquisition,

processing, and analysis. International Association of

Seismology and Physics of the Earth’s Interior, El Cerrito,

CA, Vol. 1, pp. 21–46.

Analog Devices, 1998. 3V/5V, CMOS, 500 uA Signal Con-

ditioning ADC, AD7714, http://www.analog.com/pdf/

AD7714 c.pdf.

Kester, W., Bryant, J., 2001. Grounding in high speed systems.

Hardware design techniques, application note, Analog

Devices, http://www.analog.com/support/standard linear/

seminar material/.

Kester, W., Bryant, J., Buxton, J., 2001. High resolution signal

conditioning ADCs. Analog Devices, Section 3, Analog

devices application note, http://www.analog.com/support/

standard linear/seminar material/.

Lemaire, C., 1997. Low cost, iMEMS accelerometers sense

strong motion seismic activity for hazard mitigation

applications. Analog Devices, Inc. www.analog.com/

publications/whitepapers/products/Seismart.html.

National Institute of Standards and Technology, NIST, 2001.

Internet time service, http://www.bldrdoc.gov/timefreq/

service/its.htm.

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Fig. 13. SAE card final prototype.


http://www.analog.com/pdf/AD7714_c.pdf



http://www.analog.com/support/standard_linear/seminar_material/








http://www.analog.com/publications/whitepapers/products/Seismart.html

http://www.analog.com/publications/whitepapers/products/Seismart.html

http://www.bldrdoc.gov/timefreq/service/its.htm

http://www.bldrdoc.gov/timefreq/service/its.htm

Documents

Portable digital seismological AC station over mobile telephone network and internet