1-s2.0-S1355030609001348-main(1)

7/30/2019 1-s2.0-S1355030609001348-main(1)

1/9

Geophysics and the search of freshwater bodies: A review

Rachael Parker a, Alastair Ruffell a,, David Hughes b, Jamie Pringle c

a School of Geography, Archaeology and Palaeoecology, Queen's University, Belfast, BT7 1NN, United Kingdomb School of Planning, Architecture and Civil Engineering, Queen's University, Belfast, BT7 1NN, United Kingdomc School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, United Kingdom

a b s t r a c ta r t i c l e i n f o

Article history:

Received 17 July 2009

Received in revised form 19 September 2009Accepted 23 September 2009

Keywords:

Geophysics

Search

Body recovery

Sunken objects

Geophysics may assist scent dogs and divers in the search of water bodies for human and animal remains,

contraband, weapons and explosives by surveying large areas rapidly and identifying targets or environmental

hazards. The most commonly applied methods are described and evaluated for forensic searches. Seismic

reflectionor refractionand CHIRPSare usefulfor deep,openwaterbodies andidentifyinglargetargets,yet limited

in streamsand ponds.The useof ground penetrating radar (GPR) on water (WPR) is of limited usein deep waters

(over 20 m) butis advantageousin thesearchfor non-metallic targetsin small ditchesand ponds.Large metal or

metal-bearing targets can be successfully imaged in deep waters by using towfish magnetometers: in shallow

waters such a towfish cannot be used, so a non-metalliferous boat can carry a terrestrial magnetometer. Each

device has its uses, depending on the target and location: unknown target make-up (e.g. a homicide victim with

or withouta metal object) maybe best locatedusinga range of methods (the multi-proxy approach), depending

on water depth. Geophysics may not definitively find the target, but can provide areas for elimination and

detailed search by dogs and divers, saving time and effort.

2009 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

2. The problem The need to search water bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

3. Geophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

4. Hydrogeophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

4.1. Seismic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

4.1.1. Seismic reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4.1.2. Seismic refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4.2. CHIRP sub-bottom profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

4.3. Side scan sonar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

4.4. Ground penetrating radar (GPR, here used as WPR or water penetrating radar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

4.5. Magnetometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

4.6. Other techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5. Published case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5.1. Search for a victim of drowning: Gr egory Reedy, Oregon USA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5.2. Search for sunken snowmobile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

5.3. Evaluation of polluted pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.4. Search for sunken jetski . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.5. Search for diseased animals in a ditch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.6. Search for a homicide victim in a reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.7. Search of a ditch for the body of a badger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.8. Search for explosives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Science and Justice 50 (2010) 141149

Corresponding author.

E-mail address: [email protected] (A. Ruffell).

1355-0306/$ see front matter 2009 Forensic Science Society. Published by Elsevier Ireland Ltd. All rights reserved.

doi:10.1016/j.scijus.2009.09.001

Contents lists available at ScienceDirect

Science and Justice

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i j u s
mailto:[email protected]://dx.doi.org/10.1016/j.scijus.2009.09.001http://www.sciencedirect.com/science/journal/13550306http://www.sciencedirect.com/science/journal/13550306http://dx.doi.org/10.1016/j.scijus.2009.09.001mailto:[email protected]

7/30/2019 1-s2.0-S1355030609001348-main(1)

2/9

1. Introduction

Geophysicists are sometimes asked by law enforcement officials

(police, environment agency, customs, and the military) and search and

rescue personnel, to provide advice on assisting searches of water, which,

by its very nature, is a challenging environment. Likewise, geophysicists

and other Earth scientists, used to using geophysics on land or from an

airborne platform, may not be aware of how different shallow (typically,

0

20 m water depth) survey methods perform on and in water. Thus wefelt the need for a review paper that examines a recurrent request made

to us: what geophysical devices can be deployed on and in freshwater?

This question is asked because of a lack of knowledge amongst those

conducting the search regarding the capabilities of geophysics, especially

in water and often in freshwater. This knowledge gap may be that the

investigator does not know of the existence of water-borne geophysics,

or may be that they over-estimate the capability of the method. On the

latter point, it is often the case that the technique will not necessarily

find the sunken object, but may be used to excludeareas of the lake, river

or pond, such that other methods (e.g. divers, search dogs, draining) may

be targeted, saving time and effort.

2. The problem The need to search water bodies

Common methods of disposing of homicide victims [1], still-born

neonates, diseased animals and incriminating materials (contraband,

weapons, drugs, explosives [2]) may include: burialin soil or sediment;

cremation; dissolution; encasement in concrete; crushing (e.g. in a

scrapped vehicle) and deposition in a deserted or covert location [1]

(caves, abandoned mines or bodyof water). The latter location presents

particular opportunities and challenges for the investigator because

preservation of the material andassociated evidence maybe better than

some of the other environments mentioned, yet the search can prove

difficult. Unlike land, freezers, scrapped cars or buildings, water bodies

deeper than wading depth (about 1 m) cannot easily be searched by

teams of personnel. In shallow waters, wading may obscure visibility

by sediment stirring, can be hazardous (tripping on objects, slipping

on mud, becoming stuck) and disturb the scene/deposition site [1]; indeeper waters the search is difficult unless the water is clear, or cadaver

(victim recovery) dogs and boats are available [3]. The deposition of

homicide victims in marine waters has been considered previously [4],

yet without access to a pier/jetty or boat large enough to carry the

suspect material, the perpetrator is dependant on disposal from the

shoreline, which may be exposed to view and may not provide much of

a challenge for the search team. Consequently, access to freshwater

bodies (lakes, rivers, ponds, ditches, reservoirs) is easier for the per-

petrator, making these, sometimes enclosed from view locations com-

mon disposal sites [5]. Whilst water-borne physical evidence (micro-

organisms, sediment) can be used in thesameway as soil to potentially

exclude a suspect from all but one location [5], as mentioned above, the

subsequent search of this water body may prove challenging if possibly

very rewardingin terms of evidencerecovery.The latteris critical,as it iscommon for the perpetrator to dispose of weapons, coverings (bags,

carpet, tarpaulins) at the same location as the body [1]/contraband, due

to the principle of minimum effort expenditure [5]. Thus it is essential

that the water body be searched in an appropriate (i.e., using a method

that is fit for purpose) and efficient manner. Draining the water body,

trawling with grappling hooks, deployment of scent dogs and divers

may all be considered. Draining may be impractical for environmental

and cost reasons; trawling may not be efficient and may damage and

compromise evidence; scent dogs may not react to well-wrapped ma-

terials and divers may not have good visibility. Furthermore the nature

of moving waters makes accuracy of search by any means (scent dogs,

divers or trawling) the most difficult. Most water bodies have flow

movements and therefore positioning, marking and equal coverage of

any water search is challenging. Nonetheless, the latter two methods

(dogs and divers) are ideal when used conjunctively with geophysics as

a means of target definition.

3. Geophysics

Geophysics encompasses a range of non-destructive and non invasive

means of remotely investigating subsurface of the Earth, commonly

deployed in forensic searches [5]. Geophysical methods measure the

Earth's physical properties providing specifi

c information governed bytheir own make-up, best known to investigators when searching soil and

sediment. The use of geophysical methods on (or in) water is slightly

different to land in that freshwater is chemically less variable (vertically

and horizontally) than soil yet is mobile over shorter periods of time.

Nonetheless, each geophysical method not only characterises the sub-

surface but can identify inhomogeneous features or objects that are not

characteristic of the surrounding host material in water, water-covered

sediment or soil. Identification of these anomalies is oftenthe objective of

a geophysical survey e.g. utility surveying or target identification [6,7].

Objects are readily identified when a contrast is sufficiently large to alter

the geophysical signal depicting the anomaly as an alien feature of the

subsurface, i.e., different physical and/or chemical properties than the

surroundings inwhichit is located. Thesuccessof geophysicsis dependent

upon the presence of a contrast between features, layers, objects and the

ability of a geophysicist to interpret the data. Geophysics fills the void of

mesoscale data via its depth of exploration and spatially dense sampling.

[6]. Geophysical surveying will not always remove the need for invasive

studiesbut by maximising therate of ground cover [7]surveying withthe

use of geophysics means any need for invasive methods will be minimal.

4. Hydrogeophysics

Geophysics plays a vital role inexploring the aquaticenvironment. The

unknown nature of the subsurface is increased due to the water body

covering the land; as a result geophysics has been adapted for exploration

of this medium (see references below). The principal benefit can be

identified as geophysics may provide spatially distributed data models

of physical properties in regions that are difficult to sample... [8]. Hy-

drogeophysical methods that may be adapted for the forensic searchinclude study of sedimentation archives [9], groundwaterflow modelling

[10], engineering faults and failures [11], sediment classification [12],

benthic mapping [13] and aquatic engineering [14]. However it is still a

research technique that can be referred to as state-of-the-practice [6]

rather than state-of-the-art meaning it is developing and geophysicists

are still learning the organisation of the multitude of parameters to en-

hance data output. The shallow water environment poses several opera-

tional problems that have a detrimental effect on the results of a

geophysical survey [15]. The hydrogeophysical methods most applicable

to surveying freshwaterbodies will be considered: seismic: reflection and

refraction; side scan sonar; CHIRPs; ground penetrating radar (GPR);

magnetometry and multi-sensor platforms. For each method, as fresh-

water has minimal influence on acquisition, it is the suitability of the

method for locating different materials that will be considered in detail.

4.1. Seismic methods

Two seismic methods are considered: reflection and refraction; both

of which are typically applied to exploration seismology. Seismology

with relation to this work is the scientific study of seismic waves prop-

agating through the Earth. Seismic methodologies (whether reflection or

refraction) are used to measure the travel time of propagating acoustic

waves created by a sound source resulting in ground movement which is

then recorded by an array of ground sensors, usually hydrophones. Boyd

[16] details how seismic experiments were first conducted by Robert

Mallet, 1845. Mallet used containers of mercury at various spacing dis-

tances and recorded the time it took each container to ripple after an

explosion. From this date it has been accepted that the acoustic energy

142 R. Parker et al. / Science and Justice 50 (2010) 141149

7/30/2019 1-s2.0-S1355030609001348-main(1)

3/9

wave's travel throughthe subsurface canbe likened to thatfirst noticed in

the mercury containers by Mallet, i.e., hemispherical propagating waves

[16]. Seismic reflection and refraction use acoustic wave (short duration

and constant frequency) propagation to measure changes in acoustic

impedance [17]. Seismic waves travel through the subsurface as body

waves, whilst they travel along the Earth's surface as surface waves. It is

the body waves which are of relevance for reflection and refraction

success. Body waves can then be further divided to p waves which

propagate through the subsurface medium at a faster rate and s waveswhich are slower in propagationability. Pwaves are generally used as the

recording sourcefor seismic surveys [16]. Seismic reflectionand refraction

measure different elastic properties: both methods can be adapted for

water surveying by means offloatation devices or bank side layout of

equipment using waterproofed cables [18].

4.1.1. Seismic reflection

Anselmetti [12] describe seismic reflection as a method which

provides an image of the subsurface and is particularly useful in delin-

eating sedimentary layers, bedrock outcrops, depth to acoustic basement

andother geological features beneaththe watersediment interface. Itisa

measurement of the travel time of an acoustic wave which reflects from

an interface between differing densities [15]. The transmitted acoustic

waves and in particular the p wave are described as wave fronts and

raypaths (wave fronts are perpendicularto raypaths) at a given frequency

selected by the researcher. Seismic energy travels through the material

until a point at which reflections are generated, causing the p wave to

be reflected from the boundary. Changes are detected by piezoelectric

transducers or hydrophones on the surface where they are then con-

verted to frequency, filtered and displayed. Wave reflections are ulti-

mately caused by sudden change in elastic properties or density of a

medium, i.e., change in acoustic impedance [17]. The success of seismic

reflection profiling is dependenton a frequencyselection thatbest fits the

research objective [20]. Frequency determines the depth of penetration

but also the given resolution. Frequency resolution is inversely propor-

tional to depth penetration [20]. An accepted rule of frequency is high

frequency waves constitute short wave lengths which result in low

penetration depths but increased resolution output. Alternatively low

frequency waves constitute long wave lengths that result in highpenetration depths but low resolution outputs. As a result the researcher

must select a frequency best suited to their research objective. Generally

within seismic reflection high frequencies are 3.5 to 14KHz whilst low

frequencies are 300 to 2000 Hz. Interpretation of seismic data yields

subsurface velocity information attributable to each subsurface material

with differing acoustic impedance values. One advantage of seismic

reflection profiling is the continuity of a cross section profile [15] giving a

continuous visualisation of the water base and sediments below, such

as that recorded by Anselmetti [12] when using the method to research

Western Swiss Lakes (to refine the Holocene lake level curves). Fur-

thermore continuous surveying gives an effective horizontal resolution

based on theselectionof suitable pre-determined surveyline spacing; this

can become a limitation when knowledge and experience lacks resulting

in poor horizontal resolution [12]. The output of a seismic reflectionsurvey is greatly superior to that of refraction however it does come at an

economic cost. A 3-D seismic reflection survey produces an image of the

subsurface geometry for imaging small objects and complex subsurface

geometries [21]. Seismic reflection can be applied to hydrogeophysics in

that the inability of water to transmit shear waves makes collection of

high quality reflection data possible even at very shallow depths that

would be impractical to impossible on land [18]. A general limitation of

seismic reflection profiling is the inadequate ability for a precise depth

measurement to be made. Velocities obtained from the survey may be

10% and can be up to 20% inaccurate of their true velocities [19], result-

ing in inaccurate depth measurements and exact feature locations. This

limitation is not a major problem when the survey is used for target

definition, to be followed by extraction of the feature/object being re-

searched. Seismic reflection data interpretation is a skill which requires a

qualitative approach and is labour intensive resulting in the need for an

experienced researcher who has interpreted previous (similar) data. Lack

of user knowledge and interpretation ability is a large limitation to any

research objective. Seismic reflection can be expensive and computer

intensive [15]. Data collection size can at times be overwhelming causing

an over complication of a survey site and ultimately leading to survey

failure and wrong interpretations [15]. In relation to small (less than a

few tens of metres) and shallow (less than the draft of the survey boat,

generally a few metres) water bodies, seismic refl

ection may proveproblematic as it requires a surveyspacing of several metres. Furthermore

it is imperativethat thereceiver spacing is adaptedto thefrequencyrange

to avoid spatialaliasing of thedata [21]. Ina small shallowwaterbodythis

may not always be possible and seismic reflection output will be of poor

quality as a result. It is also necessary in reflection surveying to suppress

the surface waves, particularly in shallow subsurface surveys as they can

record high amplitudes which impede the recording of the true body

waves (p waves)withinthe Earth.A further limitationof seismic reflection

for use in shallow water bodies is the hindrance caused to acousticwaves

by gas or air bubbles. The influence of gas or air bubbles causes an

increased rate of scattering of the propagated wave resulting in a poor

output. Signal processing is required to remove ringing which is a com-

mon feature in shallow water bodies. Anselmetti [12] recognise that

seismic surveying is difficult in gas-filled sediments, shallow water and

some on shore settings. Alternatively Anselmetti [12] also acknowledge

the benefit of acquiring seismic reflection in conjunction with ground

penetrating radar (or rather water penetrating radar, or WPR, see below)

to allow comparison of the data outputs. Seismic reflection as a solo

technique is most successful in open, deep water bodies when sediment

penetration (e.g. submerged and sediment-covered targets) is required.

Materials of similar density to the host water or sediment will not be

imaged: small targets (e.g. cadavers) require high frequency seismic in-

puts that will not achieve good sediment penetration. For this reason, in

concurrence withcost, size of surveylines/surveyboat andthe problemof

gas-prone sediment, seismic reflection may only be required for specialist

forensic searches of large, non-metallic (see the use of magnetometers,

below) objects in large, deep water bodies. These environments share

many common characteristics to marine locations, with attendant res-

trictions on perpetrator activity (see above) and thus, as yet, there areno publications on the forensic use of seismic reflection in freshwater.

However, as stated above, these methods are state-of-the-practice and in

analogous searchenvironments [12] mayprove usefulfor locating specific

objects.

4.1.2. Seismic refraction

Refraction surveying is a popular method for land surveying [2].

Refraction surveying is similar to that of the reflection method with

relation to the p wave principle however it differs in measurement: a

similar boat, seismic source and array of geophone detectors is required

(Fig. 1). The seismic refraction method measures travel time of waves

refracted along an acoustic interface [19]. Thetransmitted acoustic waves

(and in particular the p wave) travel through the surface until a point at

which the wave reaches an acoustic boundary. The penetration of thewave into the boundary and subsequent strata is similar to that of a light

beam penetrating a glass prism andtherefore it is governed by Snell's Law

[16]. As the ray path of the p wave intersects the boundary it causes a

change in direction ultimately due to a change in velocity, i.e., the path

recorded is the quickest ray path which the wave would travel between

interchanging boundaries of differing velocities [16]. The success of

seismic refraction profiling is similar to that of reflection, i.e., it is highly

dependentonthe frequencyselectedas a bestfit forthe researchobjective

[20]. As previously described, frequency determines the depth of pen-

etration but also the given resolution. Frequency resolution is inversely

proportional to depth penetration. One great advantage of seismic re-

fraction profiling is that it is generally a cheaper alternative to that of its

sister method however it is still relatively expensive in relation to other

geophysical surveys [18]. Data interpretation is much simpler and data

143R. Parker et al. / Science and Justice 50 (2010) 141149

7/30/2019 1-s2.0-S1355030609001348-main(1)

4/9

processing is less than that of reflection. A further advantage of seismic

refraction profiling is the continuity of a cross section profile giving a

continuous visualisation of the water body bottom [12,21]. Furthermore,

continuoussurveyinggives an effectivehorizontal resolutionbasedon the

selection of suitable pre-determined survey line spacing; this canbecome

a limitation when a lack of knowledge and experience results in poor data

output. Seismic refractiondoes produce adequate depth resolution for thedepiction of subsurface features. An important limitation is thatrefraction

is only successful if the speed of propagation through the Earth increases

with depth [19]. Seismic refraction is limited by depth penetration, i.e.,

depthsless than 100ft (appx.70 m).Greater lengths of array displacement

across the surveysite (at least threetimes the length of the desired depth

[19]) would aid a greater depth of penetration however in small survey

areas this is not possible. In relation to array displacement, the larger the

spreadof geophones,the more degraded therecorder resolutionbecomes.

Survey size is a trade-off between penetration and resolution similar to

frequency selection (as in reflection, above and WPR, below). Seismic

refraction is limited by the presence of air and gas-prone water and

sediment: the influence of gas or air bubbles causes an increased rate of

scattering of the propagated wave resulting in a poor output. Additionally

refractionoftenrequires an acoustic source which is much largerthan thatfor seismic reflection. This larger source is required to be able to be

detected between the source and receiver. The size and in particular type

of acoustic sourcecan becomeproblematic especiallyin shallow sheltered

areas: the shot energy required to produce the acoustic wave for depths

greater than 100ft could ultimately lead to too large an explosive charge

causing concernover safety [16]. Ithasbeen noted byChaet al. [22] whilst

researching for engineering applications, that a possible advance in seis-

mic refraction is to deploy the hydrophones directly on the sub-bottom

thus removing the ray path within the water body however this requires

a more complicated surveying technique as well as an increase in costs

and risks (possible loss of equipment). Overall, the refraction method's

application to aquatic environments is limited, notably so in shallow

waters. As a method it yields reliable results when applied to a lake or

lagoon, where the surface is flat and velocities increase with depth [22].

4.2. CHIRP sub-bottom profiler

A compressed high intensity radar pulse or CHIRP is a sub-bottom

profiler which transmits a linear, frequency modulated (FM) acoustic

pulse with a desired frequency bandwidth that is inverselyproportional

to the resolution which will be output [21]. A frequency modulated

pulse effectively means that the frequency of the transmitted pulse

adjusts linearly with time. The use of an FM pulse results in an im-

provement in signal to noise ratio which is problematic in other seismicmethods, i.e., seismic reflection. The combination of FM pulse, correct

filtering (compression of the swept FM signal into short duration wave)

and efficient surveying methods ultimately leads to increased resolu-

tion in conjunction with penetration which results in successful data

interpretation by the researcher [23]. The CHIRP source (towfish) is

positioned directly in the water and towed beneath the water surface

at a given speed, thesystem requires the same sort of boat and towfish,

but without the geophone array, as the seismic method ( Fig. 1). It is

a method by which a continuous vertical survey of the sedimentary

make-upof a water body can be recorded [24]. Like seismic reflection it

propagates through the water surface and then the subsurface using

acoustic waves. The acoustic waves measure acoustic impedance

vertically through the subsurface via the reflective nature of each dif-

fering material type it comes into contact with. The first boundary

reflection recorded by CHIRP is between the water and sediment

followed by the sequential layers beneath; each individual interface

reflects the acoustic wave and is recorded. CHIRP is versatile in that it

canbe readily applied to both shallow watersand marineenvironments.

However as with many other methods, boat draft and depth of towfish

limit the survey to the navigablechannel [20]. The mainadvantage of

the CHIRP sub-bottom profiler is the dependence on the bandwidth of

the transmitted pulse and not the wave length [23]. The CHIRP can

project long pulses into the water and thus increase the range and

effectiveness of the method but retain resolution quality. It essentially

reduces the trade-off between signal range and resolution quality and

can provide imaging of up to 20m of the subsurface, described by

Gutowski [21]. Limitations of CHIRP in relation to use in shallow water

bodies is the depth required to tow the towfish. Shallow water bodies

may not be sufficiently navigable to float the towfish [24]. CHIRP, likeseismic reflection is also affected by gas bubbles in the sediment or

water body. Laverty and Quinn [24] describe acoustic blanking where

reflections are faint or absent due to gas absorption of acoustic wave

energy. They also describe the affect of sediment types on the acoustic

wave of CHIRP. Forexamplethey show evidencethat a mixedgraveland

fine sediment lake bottom results in chaotic internal reflections whilst

gravels provide irregular strong signals. As a result the sedimentary

type may require to be known before the CHIRP is implemented and

poor results retrieved as a result.

4.3. Side scan sonar

Side scan sonar is similar to CHIRP however it sends a focussed

acoustic beam at right angles to the vessel's track [15]. Most side scansonar systems measure approximately 100200 m either side of the

vessel. It is typically a narrow beam with only 2 width and a vertical

angle of approximately 50 [15]. The beam design enables the sea

floor to be imaged in similar detail to a photographic image. Side scan

sonar surveys the lithology and terrain of the water body floor, as a

resulta fairlyflat andsmooth sea floordoes notproduce a good output

on the data display. Side scan sonar requires a raised surface in order

for a return signal; it provides a graphic representation of how ma-

terials on the lake floor interact with acoustic energy [24]. It is the

irregularities of the lake floor which will cause increased back scatter

of the signal to be recorded and depicted more readily on a side

scan sonar image e.g. rocks and boulders as well as covertly-sunken

objects. Furthermore the return signal is affected by the material in

which it has come into contact with. Some material will reflect better

Fig. 1. The seismic reflection survey. Similar larger-boat and towfish arrays are used

for the CHIRP and towfish magnetometer systems discussed in text. The survey was

modified after.

University of Rhode Island, 2008 (www.dosits.org).

http://www.dosits.org/http://www.dosits.org/

7/30/2019 1-s2.0-S1355030609001348-main(1)

5/9

than others whilst some may absorb a significant part of the energy

resulting in poor backscatter for example fine sediments. In relation

to shallow water surveying side scan sonar is possible but again is

dependent on the water body size. Dobinson and McCann [15] des-

cribe how shallow water systems are available but are limited to a

frequency range of 100 to 500KHz resulting in a range resolution

of 2050 cm of the subsurface lake floor. High frequencies, such as

500KHz to 1 MHz give excellent resolution but the acoustic energy

travels a shorter distance

[20] and vice versa for low frequencies. Themajor limitation of side scan sonar in shallow water is the depth of

the water body. An extremely shallow water body will not provide

sufficient depth to attach and float the sonar beneath the boat. Side

scan sonar is ideal for rapid searching for surface objects in mod-

erately-deep (more than a few metres, and less that hundreds of

metres) waters. Sediment-covered objects may not be resolved unless

still upstanding.

4.4. Groundpenetrating radar (GPR, hereusedas WPRor water penetrating

radar)

The first use of radar as a general application for surface variance

was by Christian Hulsmeyer, an engineer from Dusseldorf, Germany.

Hulsmeyer patented thefirst worldwide use of radar technology in 1904.

Thepatent canbe summarised: if a beam of radio waves were transmitted

which impinged on an object, such as a ship, then some of those waves

would be reflected back to a receiver adjacent to the transmitter, thus

revealing the presence of the object [25]. In 1910 Gotthelf Leimback and

Heinrick Lowy attained a patent for use of radar for detection of buried

objects. This is first known use of surface penetrating radar or GPR as we

now know it. Lowy and Leimback used a continuous radar wave via

surface antennas before Hulsenbeck in 1926 patented a pulse radar. GPRis

a high resolution electromagnetic technique that is designed primarily to

investigate the shallowsubsurface of the Earth [26]. It is a methodology

which uses electromagnetic radar waves that propagate into the Earth's

surface. The GPR technique is similar in principle to seismic and sonar

methods [27]. GPRconsists of onetransmitting antenna andone receiving

antenna, usually placed on the ground, but these devices can also be

lowered on long cables down boreholes and wells. It is the reflection ofa pulse from electromagnetic radiation by which homogeneous and

inhomogeneous material can be recorded. The transmitter antenna sends

out a single pulse (wavelet) which will contain a number of frequencies

based on the antenna being used but will be referred to by the central

frequency. The wave will travel through the host material at a velocity

determined by the dielectric permittivity of the material. The wave will

continue at the same velocity untilsuchtime that it meetsa material with

a different permittivity to the host. At the point of interaction with an

inhomogeneous material the wavelet is scattered and detected by the

receiving antenna. A GPR wave records the interaction between the ini-

tially transmitted electromagnetic wave and several factors [28] namely:

the spatial variation within the complex/material; the EM properties of

the Earth material (dielectric permittivity, electrical conductivity and

magnetic permittivity). The velocity of the travelling wave is determinedby the dielectric permittivity of the material: put simply a wavelet of the

same central frequency governed by the same antenna being used on the

same GPR machine when passedthrough two differingmaterials over the

same distance will inevitably arrive at the receive antenna at different

times.Permittivity therefore determines travel time. Energyis returned to

the surface to be received at the receiving antenna anytime there is a

contrast in dielectric properties; the amplitude of the return energy is

illustrative of the degree of contrast at the interface of the differing

materials. Dielectric permittivity and electrical conductivity are complex

frequency dependent parameters that describe the microscopic electro-

magnetic properties of materials. Whilst dielectric permittivity controls

velocity it is also true that electrical conductivity controls/affects atten-

uation. Asa result GPR is not a viablechoicefor surveying inclayrichareas

where 510% clay content can reduce penetration depth to less than 1m

[28]. As previously mentioned upon impingement with an object/feature

thewave scatters andthis canoccur in fourways.First,SpecularReflection

Scattering (based on the laws of reflection, where the angle of reflectionis

equal to theangleof incidence)where thewave impinges on aninterface

it scatters according to the shape and roughness. Second, Refraction

Scatteringsome of the wave is refracted back into the material based on

Snell's Law. Third, Diffraction Scatteringthe bending of EM waves when

the waveis partially blockedby a sharp boundary, creating semi-coherent

energy patterns which disperse in several directions. Fourth, ResonantScattering (Ringing)occurs when the wave bounces back and forth

between boundaries of the object. The length of time taken for the signal

to return depends upon the permittivity contrasts. Antenna selection

plays a large rolein determining penetration depth andresolutionquality.

Transmitting antennae are transducers, i.e., they convert electric currents

on the metallic antenna elements to transmitted electromagnetic waves

that propagate into the subsurface (vice versa of this description for

receiving antennae that capturing electromagnetic radiation). Antenna

frequencies generally range from 101000 MHz and selection of the

correct operating frequency is necessary for a successful visual output. In

lowconductivity settings such as drysand andgravels, lowfrequency GPR

systems of 50100 MHz can achieve penetration depth of up to several

tens of metres, whilst high frequency antenna of 450900 MHz achieve

penetration of one to several metres [27]. Resolution is therefore

determined by the period of the emitted pulse which is controlled by

the frequency. General purpose GPR systems use dipole antennas that

typically have a two octave band width [29] meaning that frequencies

vary between one half of thecentrefrequency and double the samevalue,

i.e., an antenna with a central frequency of 300 MHz generates an overall

frequency of 150 MHz to 600Mhz.However as previously stated they are

referred to as the centre frequency by users [28]. The pulse length is

inversely proportional to the centre frequency [30]. Low frequency

antennas have a long wave length and thus greater penetration yet poor

resolution compared to higher frequency antennas. As expected higher

frequencies consist of shorter wave lengths and thus shorter penetration

depths areonly possible.Standard dipole antennas radiate energyinto the

ground in an elliptical wave taking on theappearance of a cone like shape

or conical beam [31], with the apex of the cone at the centre of the

transmitting antenna resulting in the ability to trace a hyperbola [31]. Thekeyfactors to consider when selectingantenna frequencies for surveys are

as follows: presumed electrical properties of the ground at the site; depth

necessary for survey; size and dimensions of features (the stratigraphic

resolutionrequired);site access; presence of possible outsideinterference.

Antenna orientation will also play a role in the quality of the data col-

lected;the two orientations are parallel and perpendicular to the traverse,

however there arevariationsin which the antennas canbe orientated [32]

althoughthe effectof these variants on freshwater surveyinghas notbeen

documented. When electromagnetic wave energy makes contact withan

object within the subsurface energy is reflected and traces a feature on

the GPR trace known as a hyperbola. Depending upon the location of

the object and the location of the antenna above it on the surface the

hyperbola develops a cone like appearance. The convexity of the cone will

be dependent uponthe travel distance,i.e., if positioneddirectly above theobject the convexitywill be acuteand become more obtuse with distance

from it. It is the reflected events in a radar section that trace out a

hyperbola (Huisman et al., 2003 [27]). The main advantages of WPR are

that it is a sub-bottom profiler meaning it can be used for sedimentation

studies, civil engineering, hydrological studies and many more. Its ability

to be used in a variety of locations (for example small dams, loughs,

reservoirs) is its strongest advantage over many of the other geophysical

methods. Additionally as a sub-bottom profiling method it provides a

continuous reading of the subsurface giving clarity of the ground surface

or lake bed as an entiretyratherthanselected locations. WPR may be used

to measure large areas quickly resulting in good coverage of the survey

location. From this subsequent decisions can then be made without any

previous means of invasive research. Further advantages of WPR include

the high resolution pseudo image of the subsurface which is similar in


7/30/2019 1-s2.0-S1355030609001348-main(1)

6/9

quality to that ofseismic reflection.Data collection requires minimal effort

depending on survey size however interpretation does require geophys-

ical knowledge. WPR is relatively cost effective, when compared to other

geophysical methods described in this work, and it is generally accepted

that the cost of surveying is justified by the effective 3D outputs gained.

With relation to shallow water surveys WPR is at an advantage due to

the equipment being of a compact size in comparison to the other

geophysical techniques described. WPR equipment can be loaded onto a

small infl

atable boat [33] and towed through shallow waters which arenot able to host surveys such as seismic or CHIRP (Fig. 2). Increasingly

smaller GPR devices are becoming commercially available making them a

viable option for lock surveying and dam surveying.

The main limitation of WPR is its inability to be used in saline envi-

ronments. This is due to conductivity resulting high attenuation and little

penetration producing excessive ringing. This makes the application of

WPRlimited to freshwater lakes,ponds, dams andupper sections of rivers.

Tidal estuarine environments are presumed to be not possible locations

for GPR application, although this requires testing. Conductivity of the

water should be considered before application of GPR. A further problem

with GPR in shallow water is the coupling of the antenna to the water

surface. Waterproofing of the antenna removes the presence of air

providing better coupling resulting in a peak frequency change. However

manywatersurveysfloattheantenna in a dinghyor similarobject causing

a gap between the water surface and the antenna. Several water surveys

have been conducted; documentedsurveys range from identifying infilled

scour holes by Forde et al. [34] to identifying unexploded objects by Pope

et al. [35]. Unlike the search for buried objects, the above works did not

need to consider antenna efficiency, i.e., type, frequency, and orientation

in shallow waters. This is perhaps due to the need for perfect coupling

of water and antenna interfaces to remove an impedances and thus

attenuation of the radar signal. The examples mentioned previously use

low frequency antenna and gained mixed results: there appears to be no

consistency of data. Forde et al. [33] suggest a range of 100500 MHz

based ondepth with depthsas little as 12m being successfully surveyed

with 500 MHz. Pope et al. [33] successfully surveyed objects using

100 MHz antennae but found direct floatation problematic as wave

movement causes alteration of hyperbola. In addition to direct water

surveying by means offloatation on a boat or water proofing, antennashave also been suspended by cable over river courses [36]. However this

results in a large air wave and is perhaps only a solution when safety is

an issue with regards to water currents and water velocity. Borehole

antennae have theadvantageof being waterproof and able to be lowered

into water, avoiding the energy loss caused by deep water: the dis-

advantage is that the antenna and cable can get caught on objects and

possibly lost.

4.5. Magnetometers

The search for ferrous objects, or of subtle differences in magnetic

potential, is best conducted using a boat-borne magnetometer. Two

methods of deployment are possible. In open water of substantial

depths (over 5 m) the magnetometer may be deployed as a towfi

sh:the device looks similar to the CHIRP largely and is deployed away

from metallic objects in the boat. In shallow water, or where the

towfish may become snagged, terrestrial magnetometers may be

placed on a flotation device (Fig. 3), or be used by the operator in a

rubber boat.

The prevalence of metal in submerged objects of forensic, disaster-

related (e.g. shipwrecks) and archaeological interest makes the device

very useful indeed [37], with many commercial companies advertis-

ing their use in cable and pipe detection, shipwreck bounty reclaims

and the search for lost or disposed of valuables. Magnetometers may

use a total field sensor and or electromagnetic induction: Nelson and

McDonald [38] describe a dual-sensor device (deploying both types)

for the discovery of unexploded ordnance and mines in water. The

common devices, having a towfish, are limited in their use to open

waters with reasonable (a few metres) depth to allow manoeuvring

and avoid snagging. A terrestrial magnetometer system has been

deployed (Fig. 3) on a platform that can be rowed (with plastic oars)

or towed by a metal-free operator. The latter has proven to be useful

in enclosed ponds,ditches and streams: see Case study 5.3 (below) for

a description of the use of boat-borne magnetometer surveys.

4.6. Other techniques

In addition to those geophysical methods already mentioned

several other techniques may be considered.

LiDAR is a method which utilises laser projection to determine dis-

tance, or more readily in this case water depth(infra-red for position of

water surface and blue-green for depth of water column). The mea-

surement is based on the round trip travel time for thelaser pulse [39].The pulseused is typically 4 to 10 kHz. It is a sub-bottom profiler which

under homogeneous water conditions canpenetrate water depthsof up

to 60m [40]. At locations of excellent water clarity the main advantage

of bathymetric LiDAR over passive imaging systems is its capacity to

measure at two to three times the Secchi Depth[40]. However the

accuracy of this technique is hindered when the water becomes op-

tically dirty [41]. LiDAR can be affected further by aquatic vegetation

and air entrainment.

Radiometrics and gravimetry are two further techniques which may

be considered. Both are complex methods for calculation of physical

parameters of the Earth material/objects under survey. Gravimetrics

is the study of the gravitational field or in particular the changes in

gravitational pull or acceleration. Density variations of the Earth's ma-

terials cause changes in the force of gravity meaning they can bemeasured in terms of gravitational changes. Radiometric surveying

measures Earth materials which contain radio-emitting isotopes to

obtain gamma-ray data. Such data may provide improved resolution of

data in areas which are problematic to survey. Both techniques can be

complex and difficult to obtain but in a survey situation where they are

believed fit for purpose they can be sources of invaluable data.

5. Published case studies

5.1. Search for a victim of drowning: Gregory Reedy, Oregon USA

Alaimo (2003: [42]) reports on this tragic case, wherein 38-year-old

Gregory Reedy waded into the weed-infested Herbert's Pond (Douglas

County, Oregon) in order to retrieve a broken remotely-controlled

Fig. 2. A regular ground penetrating radar system, here placed in a small boat for water

penetrating radar surveys.


7/30/2019 1-s2.0-S1355030609001348-main(1)

7/9

model motor boat in the evening of 25th May, 2003. By nightfall, Mr.

Reedy had not returned to shore, sparking a local search, followed by a

police operation. Alaimo (2003) provides the background to the story:

Diversdid a painstaking grid searchof thenorthwest corner of thepond

usingsonar equipment and cadaver-sniffing dogs. A specialsearch crew

visiting from the United Kingdom tooka half-day off from theStephanie

Condon case to help out in the recovery effort. Condon is a Riddle girl

who disappeared in 1998. In fact Alaimo [42] is incorrect in quoting

sonar equipment, as the photograph in her article shows the twin-

hulled, metal-free boat being rowed acrossthe pond, running GPRat the

same time. Alaimo continues in the photo caption: Search and rescue

personnel Scott Robbins of Eugene, standing, and Mark Harrison of

England talk with Douglas County Sheriff's Office divers Scott Batsch,

left, and Brandon Sardi as they search for the body of 38-year-old

Gregory C. Reedy of Myrtle Creek in Herbert's Pond Monday afternoon.

Data from the survey were used in real-time to direct two police diversto anomalies that could have corresponded to thebody of Mr. Reedy, as

mentioned by Hardisty [5]. The description of conditions in Alaimo's

article [42] demonstrates the usefulness of GPR in providing targets in

these types of environments: The weeds grow nearly to the surface of

the pondand were so thick that divers described the environment as

oneof the mostdifficult they have ever searched through. Thedetails of

the GPR survey are confidential, but it is known that Mr. Reedy's body

was retrieved not long after the police search concluded (M. Harrison,

pers comm.). He was buried some three weeks later.

5.2. Search for sunken snowmobile

Sjstrm (2002: [43]) provides an account of the use of GPR in the

search for a snowmobile in an Arctic lake, wherein such vehicles safelynavigate frozen lakes throughout the winter, but come the spring, the

ice thins but the rider cannot see this and may sink. Sjstrm states that

such accidents are common, although rarely fatal. He [43] continues

Onesnowmobile accident happened in thespringof 2002 in a lake near

Ammarns, Sweden. Right after the accident some GPS coordinates

where taken on the edge of the ice to keep a reference The problem

here was that the snowmobile [had] sunk. In addition to this the bottom

of the lake has a slope of about 45 down to about 35m depth.

Sjstrm [43] then makes a very valid point for this review: The first

attempts to search for the snowmobile with the help of scuba divers

failed dueto very dark water and the steep slope that made bottom grid

searching more difficult. The team decided to wait until the ice had

melted completely, and deployed a GPR system with 100 MHz un-

shielded antennas linked to a GPS. The antennas were placed at thebottom of a small plastic boat and a survey grid of 150 m length lines,

3 m apart established. The tenth profile to be gathered displayed a clear

hyperbolae, which was confirmed by collecting an orthogonal line. The

location was noted and the snowmobile recovered by divers.

5.3. Evaluation of polluted pond

Reynolds (2002: [44]) provides a case study that is equally-well

suited to the search for hidden waste, toxic waste or environmental

forensics. In this case, a lagoon in North Wales (United Kingdom) had

been used forover 180 years as a dumping groundfor coalwaste, metals

drums (numbering at least 1000) of unknown chemicals, sulphuric acid,

tar and predominantly heavy oil (mixed with bentonite), hence the site

was referred to as an acid tar lagoon. In order to begin remediation,environment agencies needed to know the location of the metal barrels,

areas of elevated pollution and the geometry (for volumetrics) of the

lagoon. Reynolds [44] considers these targets and demonstrates the

deployment of a boat-borne magnetometer (for location of metal

targets) and a seismic survey (for thegeometry of the lagoon). Electrical

methods (resistivity, electromagnetic surveying, GPR) would not have

achieved theaccuracyor penetration of such material as theacid tar that

most likely occurred in the lagoon. Both magnetometer and seismic

equipment were deployed on a twin-hull boat (the MagCat), moved by

acid-resistant ropes and positioned using a prism on the boat and

robotic geodimeter on the land. The results, especially the magnetic

anomaly map are very convincing, demonstrating the fit for purpose

nature of understanding what is being searched for, and selecting

appropriate geophysical tools in this unusual environment.

Fig. 3. Three views of the MagCat system a regular terrestrial magnetometer

deployed from a floating platform (see Published Case Studies: Evaluation of Polluted

Pond, by Reynolds, 2002: [44]). Photo courtesy of Chris Leech, Geomatrix Systems Ltd.


7/30/2019 1-s2.0-S1355030609001348-main(1)

8/9

5.4. Search for sunken jetski

Ruffell (2006: [33], also summarised in [5]) recounts the use of GPR in

resolvinga disputeover events that occurred duringthe collision between

a jetski and speedboat in a lake used for recreation in Ireland. In the case,

the speedboat collided with the jetski, causing damage to the former and

sinking the latter. The speedboat driver claimed the jetski moved into his

path: the jetski operator claimed to be stationary. Thus the position and

nature of the damage to the jetski was critical to evaluate, a situation nothelpedby thedispute onlycoming to light some monthsafter theincident,

when visibility-obscuring silt had been deposited in the lake. The damage

couldeasily be assessed by simply bringing thevehicle to surface, but this

would destroythe jetski's position. A number of tests were undertakenby

Ruffell [45] using different antenna frequencies in order to establish the

optimum frequency for depth to target imaging, as well as to determine

what other objects may be present that may confuse the results. Local

intelligence suggested numerous metal targets would be present from

boatingand fishing nearby. It was thus thesize of thismostly plastic target

that would provide a location. 200 MHz antennae were deployed, as this

frequency overcame the problem of silt-covering but could resolve the

sizeof thejetski. A grid pattern of lines was gathered by [45] and onelarge

target was confirmed as the jetski by divers.

5.5. Search for diseased animals in a ditch

Ruffell & McKinley (2008: [5]) describe how a report from a farm

worker regarding his former employer led environmental agencies to

search a flooded quarry and nearby ditch, using GPR. The quarry was

too polluted to provide good surveying conditions but the survey of

the ditch produced excellent results. These were obtained by the

investigators placing 200 MHz antennae in a rubber boat that was

dragged along the ditch. A diseased and possibly abused calf cadaver

was recovered and used in prosecution of the perpetrator.

5.6. Search for a homicide victim in a reservoir

Ruffell & McKinley (2008: [5]) describe a search for a missing personin Northern Ireland that included areas of farmland, quarries, ditches,

building sites and the sea. A public appeal generated some specific

information: that the keeper of fishing boats on a large freshwater

reservoir was had noticed one boat had been used at about the time of

thesupposed abduction.Another witness reported activity at one endof

this lake at the same time. Scent dogs were deployed on boats that

showed interest in certain areas of the lake. As Snovak [4] go to state:

The problem remained of how to focus any diver-based search in

waters of 4 m depth with unknown thickness of silt below. Boat-borne

GPR was considered and the following positive and negative attributes

determined. The survey deployed low frequency (50 and 100 MHz

antennae). These have deep penetration and large footprint, effectively

seeing more of the water and sediment. Anomalies were marked with

weighted buoys and then re-surveyed using lower-frequency, 100 and

200 MHz antennae, in order to locate any anomalies. The main target

identified was recovered as a tree stump.

5.7. Search of a ditch for the body of a badger

In this case, Parker et al. [45] used WPR to locate a badger that hadbeen placed in a sack with rocks, and thrown into a ditch (similar to

Case 5.5, above). A 200 MHz antenna was placed in a small rubber boat

(rib) withthefloor slats removed to facilitate good connection with the

water. Twenty meter-long fibre optic cables were then purchased to

allow remote control of the survey while the boat was towed along.

Problems encountered included unstable sections of the ditch bank and

snagging of cables on vegetation. Where targets were observed on the

radar monitor, these were marked on the adjacent bank. The two

anomalies were recorded, the first of which was the badger.

5.8. Search for explosives

In this case, one of the authors [46] described how a raid on a terrorist

house in one location caused associates in a nearby farmhouse to dumpsealed and weighted barrels of explosives in a pond. The investigators

noted evidence of activity near the pond (footprints in mud) and ordered

a search. Theexplosives were knownto be Semtexand ammonium nitrate

withfuel oilmixesin plasticbarrels weighed down with eitherleadfishing

weightsor rocks. Boat-bornemagnetometers andmetal detectorsfailedto

producea response, as thesuspectedmetal wasnot present. A dog trained

in detecting explosives was not available. Oily patches were observed in

some locations: WPR located over 30 anomalies, some of which were the

explosive-filled barrels, often where surface oil coincided with the WPR

anomaly.

6. Conclusions

Having examined the most common geophysical techniques applica-ble to the freshwater environment it is evident that researchers have a

good selection of geophysical technology (Table 1). However it is lack of

understanding of the basic advantages and disadvantages of each tech-

nique that may hinder surveys. Eachgeophysical method is vulnerable to

several limitations, some more so than others. Seismic reflection and

refraction are most readily applied to large open water bodies and suffer

from several problems once the water body is reduced to a small, shallow

feature. CHIRP, sonar and towfish magnetometers also succumb to this

limitation also whilst the MagCat [44] and WPR does not: the latter is

limited by saline concentration. Where intelligence suggests that metal

may be present in the object to be searched for, in open, deep waters, the

Table 1

Summary of the methods described for the geophysical search of freshwater bodies, with advantages and limitations indicated.

Method Detection Water depth of operation Width/length of water body

Seismic

(reflection and

refraction)

Changes in compressive strength of subsurface,

from 0 m to over 10 km.

Boat-borne requires draft of boat, usually more

than 2 m upwards.

Requires a streamer with geophones ~10 m or

more.

CHIRPS Changes in compressive strength of subsurface.

From 0 m to ~100 m.

Shallower thanseismic(~ 1.5 m), butproblemof

snagging towfish.

Shorter thanseismic ~5 m, again manoeuvringof

towfish limits.

Sonar/side

scan sonar

Topography, lake/river bed, no subsurface

penetration.

Boat draft, can be ~ 1 m. No long streamer required boat length required

(~3 m). Positioning of towfish may limit.

Water penetrating

radar

Dielectric contrast (chemistry) of surface and

subsurface, from 0 to ~20 m.

From 0 m. From 1 m (for shallow targets needing a small

antenna).

Magnetometer Changes in metal content, from surface to tens of

metres, depending on size of metal object.

From 0 m (boat-or Magcat-borne) to length of

towfish wire (~50 m, could be deeper).

Boat-borne (small, or Magcat), 34 m, with

towfish, at least 50 m.

LiDAR Surface texture, no sediment penetration. 05 m, depending on light penetration. 10 m up to tens of km.

Radiometrics Radioactive emission, low radiation at surface to

high radiation when buried.

From 0 m upwards, depending on radioactive

emission.

23 m (hand-held device at surface to tens of km

(airborne platform).


7/30/2019 1-s2.0-S1355030609001348-main(1)

9/9

towfish magnetometer has a distinct advantage over all other methods, as

the towfishcan be lowered to be close to the sediment surface (if used in

conjunction witha depth sounder to avoid snagging and equipment loss).

If metal is thought to be present in shallow, restricted waters, then a

regular, terrestrial magnetometer may be deployed from a rubber or

plastic boat, with no metal on board at all: this can be quite problematic

but if overcome, could likewise prove a very useful method, as shown by

[44] for the MagCat. If non-metallic objects are being searched for, then if

large enough,in deep water, seismic is appropriate, especiallyif the objectis possibly covered by sediment: if not, then sonar is also ideal ( Table 1).

For small objects in deep water, a GPR (WPR) borehole antenna can be

lowered to the sediment surface (again, a depth sounder will be required,

possibly in an advance boat to warn of upstanding snags to the antenna

and cable). If non-metallic objects are thought to occur in small water

bodies, then seismic, CHIRP, sonar and magnetometers will not be

suitable, where WPR is. If there is no intelligence concerning the metal

content of the target in shallow water, GPR can be used, as this reacts to

metal as well, but would be best being deployed with a terrestrial mag-

netometer in a metal-free environment. Two messages from this review

are apparent. First, a knowledge of the target to be identified is essential,

as is the size of the water body, its chemistry, depth and sub-bottom

sediment and geology. Second, the methods described are rarely capable

of directlyfinding theobjectbeingsearched for: they arebetter thoughtof

as a range of assets available to the search coordinator to limit the time

dogs and divers spend on and in thewater. Geophysical devices are target

identifiers and alloweliminationof large areas of open water, leading to a

quicker and less hazardous search.

References

[1] W.D. Haglund, M.H. Sorg, Advances in forensic taphonomy, Human Remains inWater Environments, 2001, pp. 202216, Chapter 10.

[2] H.H. Nelson, J.R. McDonald, Multisensor towed array detection system for UXOdetection, Geoscience & Remote Sensing 39 (2001) 11391145.

[3] A. Snovak, Barron's Guide to Search and Rescue Dogs, Baron, New York, 2004.[4] J. Hardisty, Hydrodynamic modelling as investigative and evidential tools in

murder enquiries: examples from the Humber and Thames, in: K. Pye, D. Croft(Eds.), Forensic Geoscience: Principles, Techniques and Applications, GeologicalSociety of London, 2003, 34 March 2003.

[5] A. Ruffell, J. McKinley, Geoforensics, Wiley & Son, Chichester, 2008 288 pp.[6] D.A. Robinson, A. Binley, N. Crook, D.F. Day-Lewis, P.A. Ferre, V.J.S. Grauch, R. Knight,

M. Knoll, V. Lakshmi, R. Miller, J. Nyquist, L. Pellerin, K. Singha, L. Slater, Advancingprocess-based watershed hydrological research using near-surface geophysics: avision for, and reviewof, electrical and magnetic geophysical methods, HydrologicalProcesses 22 (2008) 36043635.

[7] P. Kearey, M. Brooks, I. Hill, Chapter 1: The Principles and Limitations of ExplorationMethods' in An Introduction to Geophysical Exploration, WileyBlackwell, 2002.

[8] H. Vereecken, A. Binley, G. Cassiani, A. Revil, K. Titov, Applied Hydrogeophsics,NATO Science Series, IV Earth and Environmental Sciences, vol. 71, Springer,Amsterdam, 2004.

[9] F.P. Haeni, Application of continuous seismic-reflection methods to hydrologicstudies, Ground Water 24 (1) (1986) 2331.

[10] B.P. Hansen, W.W. Lapham, Geohydrology and simulated groundwater flow. Ply-mouthCarver Aquifer, southeastern Massachusetts, US Geological Survey, 1992,WRIR 90-4204.

[11] S.R. Gorin, F.P.Haeni, Use of surface-geological methods to assess riverbedscour atbridge piers. U.S. Geological Survey Water Resources Investigations Report, 1989.

[12] F. Anselmetti, M. Fuchs, M. Beres, SedimentologicalStudiesof WesternSwiss Lakes

with High Resolution Reflection Seismic and Amphibious GPR Profiling, 10thinternational conference on ground penetrating radar, 21st24th June, Delft, TheNetherlands, 2004.

[13] B.D. Wilson, J.A. Madsen, Acoustical methods for bottom and sub-bottom imagingin estuaries: benthic mapping project to identify and map the bottom habitat andsub-bottom sediments of Delaware Bay, Sea Technologies 47 (6) (2006) 4346.

[14] X. Xingxin, et al., Case study: application of GPR to detection of hidden dangers tounderwater hydraulic structures, Journal of Hydraulic Engineering 32 (2006)1220.

[15] A. Dobinson, D. McCann, Application of marine seismic surveying methods toengineeringgeological studies in thenear shore environment,Quarterly Journal ofEngineering Geology and Hydrogeology 23 (1990) 109123.

[16] T.M. Boyd, Seismic Methods:Reflection and Refraction, Introduction to GeophysicalExploration, Colorado School of Mines, 2003, http://galitzin.mines.edu/INTROGP/

notes_template.jsp?url=SEIS%2FNOTES%2Fsintro.html&page=Refraction%3A%20Notes%3A%20Method%20Overview, [last accessed 10/12/2008].

[17] F. Hassani, P. Guevremont, M. Momayez, A. Sdari, K. Saleh, Application of non-destructive evaluation techniques on concrete dams, International Journal of RockMechanics and Mineral Science 34 (1997) 34.

[18] Enviroscan, Seismic Refraction Versus Seismic Reflection, 2003 http://www.enviroscan.com/html/seismic_refraction_versus_refl.html, last accessed 10/12/2008.

[19] New Jersey Department of Environmental Protection, Chapter 8: GeophysicalTechniques' in Field Sampling Procedures Manual, 2005 http://www.state.nj.us/dep/srp/guidance/fspm/pdf/fsmp2005.pdf, last accessed 29/01/09.

[20] D. Fitzgerald, G. Zarillo, S. Johnston, Recent developments in the geomorphicinvestigations of engineeredtidal inlets, Coastal EngineeringJournal45 (4) (2003)565600.

[21] M. Gutowski, J. Bull, J. Dix, T. Henstock, P. Hogarth, T. Hiller, T. Leighton, P. White,Three-dimensional high resolution acoustic imaging of the sub-seabed, Journal ofApplied Acoustics 68 (2008) 412421.

[22] Y.H. Cha, C. Jo, H. Suh, Water bottom seismic refraction survey for engineeringapplications, Geosystems Engineering 6 (2003) 4045.

[23] Teledyne Benthos, 2007, Teledyne Benthos Undersea Systems and Equipment,http://www.benthos.com/ [last accessed 29/01/09].

[24] B. Lafferty,R. Quinn, C. Breen,A side-scans onarand highresolution Chirpsub-bottomprofile study of the natural and anthropogenic sedimentary record of Lower LoughErne, northwestern Ireland, Journal of Archaeological Science 33 (2005) 756766.

[25] B. Kendal, An overview of the development and introduction of ground radar to1945, Journal of Navigation 56 (3) (2003) 343352.

[26] J. Daniels, Ground Penetrating Radar Fundamentals. U.S.EPA Report, Appendix 121,2000.

[27] J.A. Huisman, S.S. Hubbard, J.D. Redman, A.P. Annan, Measuring soil and watercontent with ground penetrating radar: a review, Vadose Zone Journal 2 (2003)

476491.[28] R. Knight, Ground penetrating radar for environmental applications, Annual

Review of Earth and Planetary Science 29 (2001) 229255.[29] L. Conyers, D. Goodman, Ground Penetrating Radar: An Introduction for Archae-

ologists, Altamira Press, USA, 1997.[30] J.L. Davis, A.P. Annan, Ground-penetrating radar for high-resolution mapping of

soil and rock stratigraphy, Geophysical Prospecting 37 (1989) 531551.[31] Gruber, S., and Ludwig, F., 1996. Application of Ground Penetrating Radar in

Glaciology and Ground Frost Prospecting [unpublished].[32] H.M. Jol, C.S. Bristow, GPR in sediments: advice on data collection, basic processing

and interpretation, a good practice guide, in: C.S. Bristow, H.M. Jol (Eds.), GroundPenetratingRadarin Sediments, vol. 211, Geological Society, London, 2003, pp.927,Special Publications.

[33] A. Ruffell, Underwater scene investigation using GPR in the search for a sunkenjet ski, Northern Ireland, Journal of Science and Justice (2006).

[34] M.C. Forde, D.M. McCann, M.R. Clark, K.J. Broughton, P.J. Fenning, A. Brown, Radarmeasurement of bridge scour, NDT&E International 32 (1999) 481492.

[35] J. Pope, R.D. Lewis, T. Welp, Beach and Underwater Occurrences of Ordance at aFormer Defence Site/; Erie Army Depot, Ohio, Army Corps of Engineers CERCReport 96-1, 1996.

[36] J.E.Costa, K.R. Spicer,R.T. Cheng, F.P.Haeni, N.B.Melcher, E.M.Thurman, W.J.Plant,W.C. Keller, Measuring stream discharge by non-contact methodsa proof-of-concept experiment, Geophysical Research Letters 27 (2000) 553556.

[37] E.T. Hall, The use of the proton magnetometer in underwater archaeology,Archaeometry 9 (1966) 3244.

[38] H. Nelson, J. McDonald,Multisensor towedarray detectionsystem for UXOdetection,IEEE Transactions Geoscience and Remote Sensing 39 (June 2001) 11391145.

[39] Z. Bowen, R. Waltermire, Evaluation of Light Detection and Ranging (LIDAR) formeasuring river corridor topography, Journal of American Water ResourcesAssociation 38 (2002) 3341.

[40] C. Wang, W. Philpot, Using SHOALS LIDAR system to detect bottom materialchange, Geoscience and Remote Sensing Symposium, vol. 5, 2002, pp. 26902692.

[41] S. Pe'eri, W. Philpot, Increasing the existence of very shallow-water LIDARmeasurements usingthe red-channelwaveforms, Geoscience and Remote SensingSymposium, vol. 45, 2007, pp. 12171223.

[42] M. Alaimo, Family finds Myrtle Creek drowning victim. News Review Today,Tuesday, May 27th, 2003, 2003, http://www.nrtoday.com/article/20030527/

NEWS/105270001&parentpro file=search.[43] T. Sjstrm, Searchingfor a sunken snowmobile in an arctic lake! MalaGeoscience

Newsletter, 2002, 2002 www.malags.se/snowmobile.[44] J.M. Reynolds, The role of environmental geophysics in the investigation of anacid

tar lagoon, Llwyneinion, North Wales, UK, First Break 20 (2002) 630636.[45] R. Parker, J.M. McKinley, A. Ruffell, Application of ground-penetrating radar to shallow

freshwater bodies for forensic search, 15th European Meeting of Environmental andEngineering Geophysics, 2009, Conference Abstract. http://www.eage.org/events/index.php?eventid=110&Opendivs=s3.

[46] A. Ruffell, L.J. Donnelly, L. Dawson, The future of geoforensics. Geoscientsists atcrime scenes, Geological Society of London, 17th December, 2008 ConferenceAbstract, 2008, http://www.stratascan.co.uk/pdf/EIGG/Forensic%20Geoscience%20Group%20Programme.pdf.

http://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.state.nj.us/dep/srp/guidance/fspm/pdf/fsmp2005.pdfhttp://www.state.nj.us/dep/srp/guidance/fspm/pdf/fsmp2005.pdfhttp://www.benthos.com/http://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.malags.se/snowmobilehttp://www.eage.org/events/index.php?eventid=110&Opendivs=s3http://www.eage.org/events/index.php?eventid=110&Opendivs=s3http://www.stratascan.co.uk/pdf/EIGG/Forensic%20Geoscience%20Group%20Programme.pdfhttp://www.stratascan.co.uk/pdf/EIGG/Forensic%20Geoscience%20Group%20Programme.pdfhttp://www.stratascan.co.uk/pdf/EIGG/Forensic%20Geoscience%20Group%20Programme.pdfhttp://www.stratascan.co.uk/pdf/EIGG/Forensic%20Geoscience%20Group%20Programme.pdfhttp://www.eage.org/events/index.php?eventid=110&Opendivs=s3http://www.eage.org/events/index.php?eventid=110&Opendivs=s3http://www.malags.se/snowmobilehttp://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.nrtoday.com/article/20030527/NEWS/105270001&parentprofile=searchhttp://www.benthos.com/http://www.state.nj.us/dep/srp/guidance/fspm/pdf/fsmp2005.pdfhttp://www.state.nj.us/dep/srp/guidance/fspm/pdf/fsmp2005.pdfhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.htmlhttp://www.enviroscan.com/html/seismic_refraction_versus_refl.html

Documents

1-s2.0-S1355030609001348-main(1)