The use of geoscience methods for terrestrial forensic searches
Pringle, J.K.1*, Ruffell, A.2, Jervis, J.R.1, Donnelly, L.3, McKinley, J.2, Hansen, J.1,
Morgan, R.4, Pirrie, D.5 & Harrison, M.6,7
1
School of Physical Sciences & Geography, Keele University, Keele, Staffs., ST5
5BG, UK.
2
School of Geography, Archaeology and Palaeoecology, Queen’s University, Belfast,
BT7 1NN, UK.
3
Wardell Armstrong LLP, 2, The Avenue, Leigh, Greater Manchester, WN7 1ES, UK.
4
Department of Security and Crime Science and UCL JDI Centre for the Forensic
Sciences, University College London, 35, Tavistock Square, London, WC1H 9EZ,
UK.
5
Helford Geoscience LLP, Menallack Farm, Treverva, Penryn, Cornwall, TR10 9BP,
UK.
6
Commander Forensic Operations, Australian Federal Police, GPO Box 401,
Canberra, ACT 2601, Australia.
7
Faculty of Applied Science, Building 3, University of Canberrra, ACT 2601,
Australia.
Corresponding author: Jamie Pringle.
Email: j.k.pringle@esci.keele.ac.uk Fax: +44 (0)1782 733737
1
Abstract
Geoscience methods are increasingly being utilised in criminal, environmental and
humanitarian forensic investigations, and the use of such methods is supported by a
growing body of experimental and theoretical research. Geoscience search techniques
can complement traditional methodologies in the search for buried objects, including
clandestine graves, weapons, explosives, drugs, illegal weapons, hazardous waste and
vehicles. This paper details recent advances in search and detection methods
conducted at a macro-scale (kilometres to metres), with case studies and reviews.
Relevant examples are given, together with a generalised workflow for search and
suggested detection technique(s) table. Forensic geoscience techniques are continuing
to rapidly evolve to assist search investigators to detect hitherto difficult to locate
forensic targets.
Keywords: geoscience; forensic; burials; remote sensing; geochemistry; geophysics
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1. Introduction
Pye and Croft (2004) define forensic geoscience as “the application of geoscience and
wider environmental science techniques to investigations that could potentially be
brought before a court of law”. As such, it encompasses a number of sub-disciplines,
such as forensic geology, forensic geophysics, forensic soil science, environmental
forensics, forensic mapping, geomatics and remote sensing. There is also an overlap
with related disciplines, such as forensic archaeology, forensic engineering and
forensic botany (Ruffell and McKinley, 2008), which has driven recent discussions on
defining these varied scientific terms for clarification purposes (Ruffell, 2010).
Various geoscience investigative methods are increasingly being used as a search tool
by law and environmental enforcement agencies, as evidenced by the recent
publication of a number of case studies (Fiedler et al. 2009a; Ruffell et al. 2009a;
Missiaen et al. 2010; Pringle and Jervis, 2010; Novo et al. 2011), research articles
(Dekeirsschieter et al., 2009; Jervis et al. 2009; Arosio, 2010; Hädrich et al., 2010;
Dionne et al. 2011; Schultz and Martin, 2011) and textbooks (Killam, 2004; Pye and
Croft 2004; Pye, 2007; Ruffell and McKinley 2008; Chainey and Ratcliffe, 2008), and
the organisation of several international ‘forensic geoscience’ meetings (e.g. Ritz et
al. 2009). Forensic geoscience is currently considered not only to be an emerging
discipline that can bring significant benefits to policing, but an application of
geoscience methods that can provide important results in environmental,
humanitarian, military and engineering investigations.
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Ruffell and McKinley (2005) reviewed the early history of forensic geoscience and
the main sub-disciplines with relevant case studies. Traditionally, forensic geoscience
methods involved the analysis of soil and materials as trace evidence to determine if
there was an association between a suspect or other object or item and a scene of
crime. The quest for associative evidence was replaced by the legally-robust
exclusionary principle (Chisum and Turvey, 2007; Morgan and Bull, 2007a), an ethos
that has its place in the context of this article and forensic searches, whereby all areas
without a buried target are excluded, leaving only those with possible targets. The
traditional use of soil or sediment analysis was expanded in the 1990s when
geoscience methods started to become applied for forensic searches for buried or
concealed objects, largely because of the widespread use of remotely-sensed data and
increasing ease of use and good quality outputs from shallow geophysical methods.
Since the turn of the millennium, there has been an increased use of forensic
geoscience methods in law enforcement, environmental and humanitarian search
investigations which have correspondingly led to an increased number of published
articles, along with more academic papers on a range of experimental and theoretical
research projects as are detailed in this paper.
Geoscientific methods are being increasingly utilised and reported upon by forensic
search teams for the detection and location of clandestinely buried material. In these
situations, burials are usually shallow (less than 3 m below ground level or bgl). The
forensic objects being searched for vary from illegally buried weapons and
explosives, landmines and improvised explosive devices (IEDs), drugs and weapons
caches to clandestine graves of murder victims and mass genocide graves. In
addition, the disposal of toxic waste in illegal dumps is a significant and growing
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issue (Ruffell and Dawson, 2009). Water-based forensic geoscience surveys have
been undertaken to assist police and environmental divers, especially in water with
poor visibility or large search areas, but these will not be covered in this review.
Parker et al. (2010) provide a comprehensive review of forensic geophysical searches
within freshwater bodies, which includes seismic, side-scan sonar, magnetics and
water penetrating radar (WPR). For marine forensic geophysics searches we would
refer the reader to Missiaen and Feller (2008), Leighton and Evans (2008), Missiaen
et al. (2010) and Reynolds (2011) for case studies.
This review focuses on the use of geoscientific methods for the search for objects or
substances that are concealed in the ground and whose presence there constitutes
some form of civil, criminal or humanitarian crime. For example, objects may be
buried in the ground as part of an attempt by a criminal to evade justice (e.g. the burial
of evidence, such as a the corpse of a murder victim); other objects may be buried as a
form of storage in order to facilitate later crimes (e.g. stashes of weapons or illegal
drugs); or hazardous materials may be disposed of illegally in the subsurface in order
for criminals to avoid the costs of legally disposing of such materials. As a result of
this definition, this review does not cover the substantial aspect of forensic geoscience
that relates to the study of trace evidence that may link offenders to crime scenes; we
would refer the reader to other published material (e.g. Pye and Croft, 2004; Ruffell
and McKinley, 2008; Morgan et al. 2008; Morgan et al. 2010) or reviews (e.g. Ruffell
and McKinley, 2005; Morgan and Bull, 2007b). This review focuses on articles
predominantly published within the last ten years due to the rapid expansion of this
area. The review paper structure progresses through an idealised search workflow,
from remote sensing and desk-based studies to site reconnaissance, and ends at
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intrusive investigations.
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2. Overview of current police and environmental search practices
2.1 Criminal law enforcement context
There are four core functions that can describe all law enforcement activity, namely
investigation, interview, search and recovery (Harrison and Donnelly, 2009; Larson et
al. 2011). Without locating the evidence (e.g. a victim’s body), obtaining a successful
conviction can be very difficult. A UK law enforcement definition of search is “the
application and management of systematic procedures and appropriate detection
equipment to locate specific targets” (Harrison and Donnelly, 2009). Traditional law
enforcement search methods have involved large-scale searches of areas with
personnel ‘finger-tip/line searches’ and often conducting trial-and error excavations of
suspect areas. These methods are still used and can prove very effective; however,
law enforcement planning searches now have many more methods to assist their
work.
Currently in the UK, a search strategist is involved at an early stage during a case
investigation for target detection. They will decide upon ‘the most cost effective way
to achieve the minimum standard (resolution) required for a high probability of search
success’ (p.201, Harrison and Donnelly, 2009). In other countries a search may not
be as methodical, investigations may not be standardised and a variety of techniques,
experts and scientific rigour are undertaken, depending upon local experience (see
Larson et al. 2011). Usually forensic search investigators will use a host of proven
methods for detecting targets, which can include scenario-based, feature focused,
intelligence-led and lastly systematic Standard Operating Procedures (SOPs).
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Scenario-based will use available case intelligence and psychological profiling.
Feature focused will identify physical landmarks that may have been used by the
offender to relocate a burial site. Intelligence-led will be based on available case
information (including covert surveillance) and lastly SOPs will provide a proven
search strategy. These aspects are discussed in more detail by Harrison and Donnelly
(2009) and Larson et al. (2011) with law enforcement investigation processes
comprehensively described by Walton (2006) and Geberth (2006).
2.2 Environmental law enforcement context
The illegal burial of waste is becoming an increasing problem globally as costs of
legitimate disposal and recycling have dramatically increased. For this reason,
various geoscience methods have been used in the detection of buried waste and later
prosecution of suspects. Ruffell and Dawson (2009) document various environmental
forensic investigations of illegal waste in Northern Ireland. Illegal disposal of waste
has to ‘be detected and characterized, before a criminal charge can be brought against
the perpetrators’ as Ruffell and Kulessa (2009) state. Ruffell and Kulessa (2009)
detail the successful searches and characterization of illegally buried foot-and-mouth
animal mass graves and illegal conductive and non-conductive waste deposits. Suggs
et al. (2002) provide comprehensive guidelines and resources for conducting
environmental crime investigations in the US. The discovery and determination of the
type and properties of illegal waste can be undertaken by search practices covered
later in this review. Determining exactly when an illegal burial occurred, which has
implications for both the present and past landowners on whose land materials have
been buried, is still currently very difficult (Ruffell and Dawson, 2009). Morrison
8
(2000) provides a useful overview of environmental forensic techniques and their
possible applications for age dating and source identification for environmental
litigation purposes.
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3. Identification of the search area(s)
Before detailed physical investigations commence, it is first necessary to identify the
limits of the search area or areas. This may be undertaken by conducting a ‘desktop
study’, combined with a reconnaissance site visit to the search area(s). Usually a site
visit should involve the forensic geoscientist, the civil, criminal or environmental
investigator depending upon the search type and potentially a behavioural profiler if
applicable. A common misconception by non-geoscientists is that there exists one
piece of geoscientific equipment that is successful in all search environments for all
target types. In our view this mistake can be costly – beginning a search with no prior
knowledge of the target size, shape or makeup, or of the local geology, soils and
history of the site will inevitably mean that the wrong equipment and approach is
adopted and the search will be compromised. There are a variety of geoscientific
methods that can be utilised to provide critical background information to implement
a satisfactory search strategy. These are now briefly reviewed.
3.1 Remote sensing
Brilis et al. (2000a,b) and Grip et al. (2000) provided a comprehensive overview of
remote sensing methods in forensic investigations, which includes aerial photography,
topographic mapping, satellite imagery and global positioning systems (GPS)
applications. Brilis (2000a) details the usefulness of early forensic balloon unbiased
photographs, taken just after the 1906 San Francisco earthquake, to detail the location
and extent of damage caused both by the earthquake itself and by non-earthquake
insurance homeowners setting fire to their own homes.
10
Aerial photographs can be used to identify unusual features and/or patches of
vegetation (France et al. 1992; 1997; Hunter and Cox, 2005), especially when taken
when the sun is at a low angle, which can accentuate more subtle features (France et
al. 1992; 1997; Hunter and Cox, 2005). Historical photographs are also a useful
resource when they can be compared to recently acquired imagery to determine both
potential locations of burials and the time when the burial had taken place (Ruffell
and McKinley, 2008). Other forensic-relevant features may also be identified, e.g.
sediment plumes caused by the dumping of a body and associated objects in a lake
(Ruffell and McKinley, 2008) and to identify access points from roads to a suspect
location (e.g. Ruffell and McKinley, 2008). Access points are prime search locations
as heavy objects are rarely carried more than 150 metres before being buried (Killam,
2004). The time of day and year when the photograph was taken is obviously key for
some searches and in certain environments (for instance in woodlands), the technique
may not be that helpful due to vegetation cover. Brilis et al. (2000b) detail an
environmental forensics case study, using photographs to detail the damage and extent
of environmental damage caused by lead-zinc mining in Missouri, USA, which, when
combined with historical lease and ownership information, determined the cleanup
liability by respective site-owning companies. Abbott (2005) used historical
photographs to show that a mining fraud case defendant could not have improved a
road as it was eroded before it was alleged to have been used. Ruffell and McKinley
(2008) detail the use of photographs in three case studies of discovering and
monitoring environmental pollution in Northern Ireland.
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Ultraviolet (UV) photography is increasingly being used in forensic investigations to
map the maturity of vegetation - on the premise that any vegetation growing over
recently buried material will be younger than the surrounding plants (Ruffell and
McKinley, 2008). Localised vegetation growing over a burial may also have different
characteristics to background areas; containing different species and with more or
stunted growth for example (Dupras, 2006; Killam, 2004) that Larson et al. (2011)
attributes to both localised pH changes. Ruffell and McKinley (2008) document a
case study in Northern Ireland where illegal toxic waste positions were identified
using infra-red imagery (IR), even when the perpetrators were aware of this method
and had tried to hide the heat emission from these deposits by covering them with peat
and soil. IR and UV photography are also often undertaken to non-intrusively
examine scenes of crime, e.g. to detect ground disturbance and to determine tyre
travel positions before physical examination of the scene commences (Ruffell and
McKinley, 2005). Satellite imagery datasets are many and varied; hyperspectral
imagery, for example, has proven useful to determine locations of individual
clandestine graves and mass graves in areas of rapid vegetation growth, changing
land-use and humid climates in Colombia (Ruffell and McKinley, 2008; Equitas,
2010). Silvestri et al. (2008) document how remote sensing identified illegal buried
waste on an Italian flood plain using remote sensing methods. Light Detection And
Ranging (LiDAR) airborne data capture systems can be used to very rapidly collect
laser scan three-dimensional points in most weather conditions. This has a significant
advantage over aerial or ground photographs (Ruffell and McKinley 2008). Suspect
burial areas can also be rapidly scanned to create a high resolution digital elevation
model and later interrogated, with no need for photographic rectification. This
technique is routinely used by footwear tread analysts to collect scene data (Geradts
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and Kelizer, 1996; Bennett et al. 2009). Thermal imaging equipment may be aircraft
mounted (e.g. Dickinson, 1976), vehicle mounted or hand-held (Davenport, 2001;
Statheropoulos et al. 2011). Hand-held devices have been used for landmine
detection, and objects can be detected even if temperature differences between the
target and background levels are less than 1° C (Deans et al. 2006). Rotary and fixed
wing unmanned air vehicles (UAV’s) are now becoming mainstream with UK and
international Police Service personnel enabling “real time” remote sensing to be
conducted over a search area with the added ability of being flown in a covert manner.
For both clandestine burials (e.g. Dickinson, 1976) and illegal waste deposits (e.g.
Ruffell and McKinley, 2008), the optimum time to conduct a thermal imaging survey
is just before sunrise or just after sunset, and this is thought to be due to localised
increases in temperatures and different rates of heat gains/losses of disturbed soil
compared to the surrounding environment (Davenport, 2001; Larson et al. 2011).
Clandestine graves are thought to be most easily detected with thermal techniques
during the first few weeks of burial (Dickinson, 1976, found buried animal was
detectable for up to 17 days post-burial), although this time frame will be influenced
by the local geology, ground water level and soil type(s); after this time, it gets
progressively more difficult to pinpoint burial locations using this technique (Larson
et al. 2011). Silvestri et al. (2008) document how a combination of thermal imaging
and recognition of stressed plant communities identified illegal waste deposit
locations in a floodplain environment. Although a survey area can be covered rapidly,
thermal imaging can be adversely affected by environmental factors such as poor
weather conditions, for example, high rainfall and elevated humidity (Dickinson,
1976).
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3.2 Geomorphology
Geomorphological mapping has been developed in a forensic context through the use
of a combination of topographic and hydrological maps to sub-divide and prioritise
potential search sub-area(s). This technique has proven useful for locating clandestine
graves of murder victims. Ruffell and McKinley (2008) detail a missing person
search in Northern Ireland in which the search area was divided into ten different subareas, based on a landform mapping approach, visibility from roads and paths, etc.
Chainey and Ratcliffe (2008) provide a useful review of GIS and crime mapping, with
case studies using map information to aid forensic investigations, often coupled with
mobile telephone record locations. Historical maps may provide useful information
for assessing what could be present on a site prior to a search. For example, areas that
now have buildings may have been agricultural land and visa versa. Brownfield
(disused urban land) sites can prove particularly challenging with numerous buried
objects. Similarly, records of land use, held in Public Records offices or with local
history societies can be used. Ruffell et al. (2009) document how historical maps and
photographs were used to identify the search area of 19th Century pauper graves in
Northern Ireland.
3.3 Geology and soil mapping
Knowledge of the local geology and soil types can be critical in forensic
investigations. Williams and Aitkenhead (1991) discuss how the lack of knowledge
of the local geology and the poor understanding of the overlying landfill geochemistry
in Loscoe, Derbyshire (UK) culminated in a methane gas explosion and subsequent
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destruction of a domestic house in 1986 seriously injuring three occupants. Lee
(2004) also described a house explosion in 2004, where on investigation it was found
that rather than a local gas leak or malicious damage, methane from underlying coal
seams were the cause of the explosion. Geology can determine where objects can and
cannot be buried and should, therefore, be considered prior to forensic searches (see
Section 4.4). In some cases geological and soil trace evidence may be available and
can help, to identify potential search areas and also, potentially as significantly, to
exclude areas (Fitzpatrick et al. 2009). As such available geological trace evidence
can aid an intelligence led search. Over the last decade national and regional ground
and remotely sensed surveys have generated large-scale spatial digital databases and
maps of solid and superficial geology, soil properties and soil geochemistry. These
have been used for a range of applications including geological and soil resource and
baseline quality mapping, economic prospecting, environmental studies as well as
prediction of the soil provenance for forensic purposes. As well as global soil
information databases (e.g. Harmonized World Soil Database v.1.0, European Digital
Archive of Soil Maps (EuDASM) which covers the different continents, the UN Food
and Agriculture Organizations (FAO) and the European Soil Database (ESDB)), many
countries also have their own higher resolution databases for geology and soils (e.g.
British Geological Survey (BGS) geochemical survey (G-BASE), The Geological
Survey of Northern Ireland (GSNI) Tellus Survey, the United States Department of
Agriculture SSURGO database, the Australian Soil Resources Information System
(ASRIS) and New Zealand National Soils Database (NSD)). These databases are
used most effectively when combined with geological or soil expertise in assessing
aspects of the landscape such as whether soft ground occurs such that burial is
possible (Ruffell and McKinley, 2008).
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Geological mapping combined with expert knowledge was used by Ruffell and
Dawson (2009) in an environmental forensic case of illegal waste movement and
burial in N. Ireland. Trace evidence of bauxites and laterites associated with a small
geological outcrop indicated the location of an illegal dump. The use of soil databases
for forensic applications has been questioned because of the lack of comparability of
forensic specimens (in terms of sample support and quantity of material) with those
collected for generating databases (e.g. Rawlins et al. 2006; Lark and Rawlins 2008;
Pye and Blott 2009), but discussion of this as trace evidence is beyond the scope of
this paper.
3.4 Metal detectors
Metal detector search teams are used during either initial search investigations or as
part of a phased programme, for example, when looking for shallow (i.e. <0.5 m bgl)
buried metallic material in appropriate environments, confirmed by control studies
(Davenport, 2001; Rezos et al. 2010). Example clandestine burial search case studies
using metal detectors are also detailed in Nobes (2000) and Pringle and Jervis (2010).
There are also magnetic object detection devices that can be used in water as well as
diver-operated submersible metal detectors.
3.5 Search dogs
Specially trained search (sometimes referred to as detector) dogs are also used during
either initial search investigations or as part of a phased programme and can be used
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to identify a variety of different buried objects, commonly IEDs (Curran et al. 2010),
drugs, contraband and human remains. Specialist search dogs looking for human
remains are sometimes referred to as cadaver dogs (see Rebmann et al. 2000). Field
trials conducted by Lasseter et al. (2003) showed relatively low success rates for the
cadaver dogs searching for buried human remains (20-40%) compared to 55-95%
success rates for surface human remains (Komar, 1999). Dupras (2006) provides a
good summary of the principles and limitations of using cadaver dogs to locate
clandestine burials of human remains. Other authors have shown that probing the
surface (sometimes referred to as venting) can release gases which dogs can then
scent and thus increase their success rate (Hunter and Cox 2005; Dupras, 2006),
although this may be slow and could potentially destroy forensic evidence. Optimal
environmental conditions for specialist detector dogs to operate in a suspect area have
been found to be air temperatures of 4º C – 16º C, at least 20% humidity, a wind
speed of at least 8 km h-1 and moist ground (France et al. 1992; 1997; Larson et al.
2011). There is also emerging case work experience that barometric pressure can
have a significant effect on the ability of detector dogs to locate sub surface human
remains.
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4. Reconnaissance
4.1 Assessing the search area(s)
A reconnaissance visit to the search area(s) should include an assessment of any
features and obstacles that may influence the choice of areas to be searched, the
potential search methods that could be utilised and their importance (Killam, 2004;
Harrison and Donnelly, 2009). For example, a relatively small search area surrounded
by metal fences may mask any targets resolved by a ground conductivity survey.
Experienced field geomorphologists and intrusive probers, forensic botanists, search
investigators, profilers and soil analysis experts can be involved at this stage (see
Sections 3 and 5 for details). Analysis of the main soil types within the search area is
important as some types may preclude certain search methods to be utilised, for
example (see Discussion), and the importance of characterising the search area soil for
archaeology is shown by Walkington (2010).
4.2 Conceptual geological model, Diggability and RAG maps
Search models for mineral exploration were adapted for forensic purposes during the
1990s for clandestine grave searches of the Pennines in the UK (see Harrison and
Donnelly, 2009). These conceptual geological models (CGMs) provide information
of the expected target(s) size, age, geometry, depth and surrounding ground
conditions. Time since burial will be important in the search for human remains as
optimum detection technique(s) may vary (see Pringle et al. 2012a). Ground
conditions include the local geology, geomorphological, geophysical, geotechnical
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and hydro-geological regimes (see Harrison and Donnelly, 2009). CGMs should
facilitate and partly pre-determine the optimum choice(s) of the investigative search
technique(s). Obviously as the investigation progresses and new information is
gathered, the initial model will need continual refining. Figure 1 is shown as a
generalised example, although see Harrison and Donnelly (2009) and Killam (2004)
for detail. Related to the CGM is a diggability survey, a qualitative assessment of the
ease by which soils and superficial deposits may be excavated and reinstated. This
can provide information on the difficulty of excavation (which may rule out an area),
the achievable burial depth and time taken to undertake such an exercise. It can also
provide information on the local subsurface, aid the identification of any resulting
features remaining after burial and refine the initial conceptual burial model. This
approach has been used in other forensic areas, for example, in environmental
forensic investigations after a mass disaster such as the Hurricane Katrina devastation
in New Orleans, US, where geotechnical assessment of the catastrophic canal bank
collapse was undertaken to determine the likely causes (Seed et al. 2008).
Once the preliminary CGM and diggability surveys are completed, a simple RAG
(Red-Amber-Green) prioritisation map of the survey area(s) should then be developed
to help focus and prioritise search resources.
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5. Phased site investigations
Once the identified search area and site reconnaissance stages have been completed,
the forensic search investigator(s) will normally then review available case and site
data and decide upon the optimum search technique(s). These technique(s) will be
case and survey site specific (see Discussion) and may well be a combination of
techniques – with perhaps an early survey phase rapidly covering the survey area and
identifying anomalous areas before a further phase of surveys are undertaken focused
on the identified anomalous areas. For example, Nobes (2000) undertook a bulk
ground conductivity survey within woodland before later Ground Penetrating Radar
(GPR) surveys over the conductivity identified anomalous areas led to success at
identifying a clandestine grave position. There are an ever-expanding number of
search techniques that could be utilised as Ruffell and McKinley (2008) document
and some that have already been discussed in the site reconnaissance section. Other
forensic search techniques will now be reviewed; near-surface geophysics in a law
enforcement, environmental and humanitarian context is referred to here as the top 10
m bgl as the maximum depth of search although more usually it is the top 1 m bgl
(Fenning and Donnelly, 2004).
5.1 Seismology
On a large scale, seismology has been used in the investigation of international
incidents involving explosions and impacts, as the energy released can be detected by
seismic networks. Examples of forensic seismology include the investigations of the
Kursk submarine disaster (Koper et al., 2001), the Lockerbie (Scotland) aeroplane
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crash (Redmayne and Turbitt, 1990), the Oklahoma City (US) bombing (Holzer et al.,
1996) and the Nairobi US Embassy bombing (Koper et al. 2001; Koper, 2003).
Seismology could also potentially be useful for investigating covert underground tests
of nuclear weapons (Douglas et al., 1999). Seismo-acoustic methods are showing
promise to detect Improvised Explosive Devices (IEDs), mine and Un-eXploded
Ordnance (UXO) locations (see Xiang and Sabatier, 2003; Donskoy et al. 2005; Sutin
et al. 2009). These methods utilise differences between the elastic, acoustic and
reflective properties of buried objects and the surrounding soil (see Reynolds, 2011
for background). A major advantage of this technique is that it works well for nonmagnetic material, e.g. for detection of plastic mines, and can discriminate mines
from smaller, metallic non-target material. However, the data can take a relatively
long time to analyse compared to other techniques, and it is currently difficult to
detect deeply buried material (see Sutin et al. 2009). Hildebrand et al. (2002)
documented that high resolution seismic reflection surveys were effective in locating
a dead pig in a wooden coffin at 2 m bgl in an unmarked grave, if closely-spaced
geophones were utilised. However, they also note that GPR surveys were as effective
in detecting the graves and could be completed much faster. Multi-channel analysis
of surface waves (MASW) is showing forensic potential; for example, Tallavó et al.
(2009) detail how this method could detect 2 m bgl buried wooden trestles that
supported Canadian railway embankments. A micro-seismic approach has been
shown by Arosio (2010) to have the potential to be used to locate earthquake
survivors trapped under rubble if sensors are placed around the site and crosscorrelating seismic sources are used. Reynolds (2011) gives a good overview of both
seismic reflection and refraction surveys, and provides some examples of their use in
a forensic context, such as in environmental forensic searches to characterise landfill
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sites or locate buried toxic waste material. Case studies of seismic methods used to
characterise contaminated land sites and illegally buried solid waste are given by
Cardarelli and Di Filippo (2006) and Grandjean (2006) respectively. Reynolds (2002)
detailed an environmental forensic survey of a polluted pond in North Wales, UK,
where a seismic reflection survey was utilised to characterise the thickness of toxic tar
waste at the bottom of the pond.
5.2 Bulk ground conductivity
There has been limited use of Electromagnetic (EM) surveys for law enforcement
investigations, which is surprising considering its relatively rapid survey rate and
ability to be carried through woodland environments, where other techniques may not
be useful. Frohlich and Lancaster (1986) deployed a Geonics™ EM-31 instrument in
Jordan to locate and characterise unmarked burials and tombs, with resulting target(s)
contrasts with background levels dependent on the proportion of silt present within
the graves. Nobes (2000) documented the successful search for buried human
remains in a wood, initially by utilising an EM survey to identify anomalous areas,
with follow-up GPR investigations over these suspected burial areas. Nobes (2000)
also suggested that the victim’s clothing may have trapped decompositional fluids
during dry conditions (see section 5.3) which were detected by EM methods. France
et al. (1992) also found EM surveys could successfully locate simulated clandestine
burials of pig cadavers in the Western US. Larson et al. (2011) suggested low-energy
EM fields could be detected from burials as bone is piezoelectric. Witten et al. (2001)
also conducted an initial EM survey to look for mass graves in Tulsa, USA, before
follow-up magnetic and GPR investigations were undertaken. Pringle et al. (2008)
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conducted a controlled experiment in a UK urban garden environment and found that
a Geonics™ EM-38 instrument, which is designed to be placed on the ground and
therefore should be less susceptible to above-ground interference, did not resolve the
target ‘grave’. This was attributed to the local urban environment and ‘made ground’
nature of the burial site. Nobes (1999) also found difficulty using EM methods to
locate unmarked graves in a cemetery in New Zealand, due to differentiating
anomalies from significant background effects caused by fence boundaries and local
topography. Interestingly Nobes (1999) found that the ‘head’ ends of unmarked
graves were easier to identify than the ‘foot’ ends.
Saey et al. (2011) have shown a combined EM induction sensor approach can be used
to detect UXOs in former WWI battlefields in Belgium. High resolution time-domain
EM surveys have also shown promise for UXO detection (Pasion et al. 2007).
Researchers have also used EM methods to detect landmines (Combrinck, 2001) and
buried weapons in a controlled environment (Dionne et al. 2011), although equipment
resolution and background variations in soil type can make the detection of small
targets problematic. EM survey equipment needs to be carefully calibrated to the
local site conditions and can also be significantly affected by above-ground
conductive objects, e.g. metal fences, electricity pylons, cars, etc., which may
preclude their use in certain search areas and environments, such as urban areas (see
Reynolds, 2011; Milsom and Eriksen, 2011). Electromagnetic (EM) surveys can be
used for environmental forensic geophysical surveys (see Reynolds, 2011), as the
target is usually more conductive than background site materials. Bavusi et al. (2006)
detail a case study in which an EM survey was used to characterise a waste dump in
Southern Italy. Vaudelet et al. (2011) shows an urban contaminant case study
23
characterising different source sites. As conductivity surveys using conventional
instruments (such as the Geonics™ EM31 or EM38 [Geonics Ltd., Mississauga,
Canada]) are directional, they can focus on either the top 5-8 m bgl (using the
horizontal model component or HMD) or up to 15 m bgl (using the vertical mode
component data or VMD), depending upon the suggested depth of burial bgl and the
local site ground conditions.
5.3 Electrical methods (resistivity)
Resistivity is the reciprocal of conductivity, with resistivity surveys actively
measuring the bulk ground resistivity of a volume of material below the sample
position (see Reynolds, 2011 and Milsom and Eriksen, 2011 for background and
operation). Typically, fixed-offset resistivity surveys are utilised for searches, due to
their relatively rapid coverage of the ground, although Electrical Resistivity
Tomography (ERT) 2D profiles have also been collected over simulated clandestine
burials (Pringle et al. 2008) and illegal waste dump sites (Ruffell and Kulessa, 2009)
which also gives depth information. Basic 2D resistivity modelling can also be
undertaken to gain a better understanding of resistivity survey results (see Scott and
Hunter, 2004). Resistivity surveys have the advantage of being less affected by
above-ground material (i.e. background ‘noise’) when compared to EM surveys as
probes are shallowly placed into the ground surface. They also work well in clay-rich
soil types (see Pringle and Jervis, 2010). However, resistivity survey results can be
problematic if undertaken after metal detector search teams have searched for buried
items due to disturbances and small voids as detailed by Pringle and Jervis (2010).
24
Resistivity has been successfully used to locate unmarked burials in cemeteries (e.g.
Buck, 2003) and ancient tombs (e.g. Persson and Olofsson, 2004; Powell, 2004;
Matias et al. 2006) although local variations in soil moisture content, particularly in
heterogeneous ground in relatively dry conditions, can affect survey success by
masking target location(s) (Ellwood et al. 1994) and/or result in numerous non-target
anomalies being imaged (see Pringle et al. 2012a). Resistivity datasets can also be
highly affected by coarse-grained and larger (e.g. cobble) soil types, as well as saline
conditions (e.g. Pringle et al. 2012b). Milsom and Eriksen (2011), however, point out
that obtaining data from the area surrounding graves can be problematic due to the
inability for probes to penetrate concrete, tarmac or other hard surface. Resistivity
surveys have been successful in locating clandestine graves; for example Ellwood et
al. (1994) imaged three anomalies when searching for the hanged Texan gunfighter
William Preston Longley. Scott and Hunter (2004) and Pringle and Jervis (2010) both
detail the use of resistivity surveys in rural Welsh fields in their respective
unsuccessful missing person searches. Pringle and Jervis (2010) used semiquantitative analysis of resistivity data to identify anomaly locations, and compared
the results to simulated study results. Published control studies over simulated burials
(e.g. Cheetham, 2005; Jervis et al. 2009) have therefore proved useful for forensic
investigators for comparison with active search data; these studies typically used a
small grid survey pattern (0.25- to 0.5-m-spaced data point samples) so that targets
could be resolved. Low resistivity anomalies with respect to background values
would be expected to occur over clandestine burials of murder victims, due to
increased soil porosity (Scott and Hunter, 2004) and the presence of highly conductive
decomposition fluids (see Jervis et al. 2009; Pringle et al. 2010). However, Jervis et
al. (2009) found that high resistivity anomalies, with respect to background values,
25
were observed over clandestine burials with a wrapped pig cadaver, and concluded
that the wrapping prevented decomposition fluids being released into the ground and
the wrapped cadaver presented a barrier to electrical currents. This temporal study
has been continued and found resistivity anomalies to be present at least three years
post-burial (see Figure 2 and Pringle et al. 2012a). Decompositional fluid
conductivity has been documented to change markedly over time since burial, rapidly
increasing in year one and slowly increasing in year two, before decreasing by the end
of year three post-burial (see Pringle et al. 2010). These variations will undoubtedly
be different in different soil types and in different regions and environments in the
world. Juerges et al. (2010) documented that decompositional fluid may be detectable
below surface pig remains, even when no physical evidence of the cadaver remained.
Resistivity methods (particularly ERT) are a standard investigatory tool for
environmental forensic investigations. Environmental applications of the technique
include looking for leachate leaking from landfills (see Reynolds, 2011) and
contaminant plumes from urban sites (see Vaudelet et al. 2011), identifying where
illegally buried solid waste may be located (Cardarelli and Di Filippo, 2004; Ruffell
and Kulessa, 2009) and studying potential aquifer contamination by graveyards
(Matias et al. 2006). Ruffell and Kulessa (2009) also document a Northern Ireland
case study where a combination of ERT and GPR surveys were used to locate animal
mass graves from the 2001 foot-and-mouth cattle epidemic.
5.4 Ground penetrating radar (GPR)
26
Ground penetrating radar (or GPR) is a commonly used near-surface geophysical
technique for the detection of unmarked grave locations, clandestine graves and
various other buried materials, with radar reflections caused by contrasts in dielectric
permittivity (Figure 4). Successive 1D radar traces are collected to build up vertical
2D profiles of the sub-surface, with 3D datasets being collected if time permits (see
Jol, 2009 for background theory and operational detail). Commonly bi-static, fixedoffset transmitter-receiver antennae with fixed frequencies are used to acquire a series
of 2D profiles. GPR has arguably one of the highest resolutions of near-surface
geophysical survey techniques and data can be obtained relatively quickly, depending
upon the equipment, antennae frequency and trace sampling spacing utilised. It is one
of the most commonly used techniques for near-surface target detection and widely
used in the geotechnical industry for buried utilities (see, for example, Metje et al.
2007). GPR has also been shown to work successfully to locate snow avalanche
survivors (see Instanes et al. 2004). However the method does not work well in saline
environments due to poor penetration (see Pringle et al. 2012b). There have also
been studies showing reduced penetration in damp clay-rich soils although if this is
homogenous it is not a big problem (see Nobes, 1999); more of an issue is
heterogeneous ground as is discussed later in this paper.
There are a number of published papers that describe the use of GPR in cemeteries to
locate unmarked graves (e.g. Kenyon, 1977; Ellwood, 1990; Bevan, 1991; King et al.
1993; Nobes, 1999; Davis et al. 2000; Watters and Hunter, 2004; Powell, 2004;
Fiedler et al. 2009a) and searching for a clandestine grave in a cemetery (Ruffell,
2005b). Conyers (2006) suggests GPR can detect the disturbed grave back-fill, the
coffin itself, its contents and any grave ‘fluid’, although Nobes (1999) highlights that
27
age of burial is important with associated variability in burial styles and
decomposition causing target detection and signature to very considerably, even
within the same burial site. Historical graves can often be difficult (though not
impossible) to detect due to the limited skeletal remains and the process of soil
compaction (e.g. Vaughan, 1986). Published clandestine grave searches using GPR
include Mellet (1992), Calkin et al. (1995), Davenport (2001) and Billinger (2009).
Nobes (2000) detailed a GPR survey which followed up on anomalous areas
identified in an EM survey looking for a clandestine grave in woodland. Novo et al.
(2011) also detail a difficult GPR search for a grave in a mountainous area. The use
of legal cemetery graves as a proxy for covert burials is dubious due to the different
burial depths (typically 2 m bgl for graves and 0.5 m bgl for clandestine graves).
However, some rapid legal burials may provide suitable approximations for
clandestine graves. For example, Davis et al. (2000) document a GPR survey for
unmarked and shallowly buried, Spanish Influenza victims from 1918 in Svalbard,
Norway. Ruffell et al. (2009a) showed a GPR case study of mass graves from the
Irish ‘Potato Famine’ in the 19th Century. Ruffell and McKinley (2008) also point out
that the additional material in legal graves (coffin, clothes, lime, disinfectant,
embalming fluid, etc.) that may prevent results from being comparable to clandestine
burials.
Controlled grave studies, whereby animal remains (typically pig cadavers) are used as
a proxy for human remains, have been used to determine whether GPR could be
successful to locating a target. An early example of this was that of France et al.
(1992) a multi-disciplinary study in the USA. Strongman (1992) also published a
series of case studies, using 5 year old bear carcasses with comparison to case results.
28
Miller (1996) also documents control studies and shows how historical graves can be
utilised as comparisons. More recent control study examples include sequential GPR
monitoring over large (Schultz et al. 2006) and small (Schultz 2008) pig cadavers and
a simulated clandestine grave with accompanying GPR time-slices (Pringle et al.
2008). There are currently differing views amongst researchers on which set
frequency GPR antennae should be utilised for forensic searches. GPR antennae
commonly range from 50 MHz to 2000 MHz; Ruffell et al. (2009a) suggest 200 MHz
was optimal to detect shallow buried unmarked historical graves, and used 400MHz to
image an individual burial, and Schultz and Martin (2011) showed both 250 and 500
MHz antennae could be utilised to detect simulated clandestine graves, whilst others
suggest higher frequencies (e.g. Buck, 2003 used 800 MHz frequency GPR antennae
to locate unmarked graves). The general consensus seems to be that optimum
detection frequency will depend on target size, depth bgl, geology and soil type.
Ruffell (2005b) gives a good review of some of these parameters, showing sandy soils
are optimum for GPR surveys; this was also supported by France et al. (1992);
although note surveys will always be site specific and depend upon depositional
processes. For example, Nobes (2007) detailed five burial sites in New Zealand with
different soil types and found sandy soil did not show targets, as depositional
processes mimicked grave responses, whereas the homogenous clay-rich soils could
be processed to resolve graves. High clay content soils and saline conditions (Pringle
et al. 2012b) also significantly reduce radar penetration depths. A current major
limitation is the speed of data collection that precludes whole fields to be surveyed,
and the significant post-field data processing time to optimise datasets. There has also
been research to numerically model the expected GPR responses from buried human
remains in different soil types (Hammon et al. 2000). Cassidy (2007a) reviews GPR
29
numerical modelling methods for a variety of buried targets. Millington et al. (2011)
go further and invert synthetic GPR data from a simulated clandestine grave for
automatic target location purposes although this is currently still being developed.
Solla et al. (in press) used a different approach using finite-difference time-domain
(FDTD) modelling of GPR data acquired over simulated buried forensic objects and
used ground-based photogrammetric methods to calibrate the modelling.
Acheroy (2007) reviewed both remote and field detection of anti-personnel mines
using GPR methods, EM sensors and remote sensing. He also documented the main
current police and military search forensic geophysical detection equipment which are
a combination of GPR and metal detectors, with Bruschini et al. (1998) showing a
case study of these combined methods. Lopera et al. (2007) showed how numerical
filtering of GPR signals can reduce non-target cluttering of GPR datasets when
looking for shallow buried landmines, using an example from Colombia. Lopera and
Milisavljević (2007) documented how important the knowledge of soil type is to
predict the GPR responses from metallic and non-metallic landmines. Furthermore
Sato et al. (2004) provided information on the potential for using GPR antennae
arrays to obtain common mid-point (CMP) multi-fold datasets for landmine detection,
a significant improvement than the typical fixed-offset 2D profiles commonly
acquired. For environmental forensic target detection, there are case studies given in
Reynolds (2011), ranging from mapping of contaminated land, pinpointing of illegally
buried, toxic waste (see Orlando and Marchesi, 2001; Ruffell and Kulessa, 2009) to
mapping groundwater contamination from landfills (Davis and Annan, 1989),
chemical (Brewster and Annan, 1994) and hydrocarbon spills (Cassidy, 2007b;
Bermejo et al. 2007). These can often be observed by radar signal attenuation and
30
dielectric permittivity contrasts, although note geophysical detection can be
temporally variable depending upon contaminant size, concentration and dispersal
rates (see Greenhouse et al. 1993; Sauck et al. 1998).
5.5 Magnetometry
Highly sensitive magnetometers have had varied success in forensic applications.
Ancient archaeological graves have been shown to have high magnetic susceptibility
(Linford, 2004), proposed as due to being long term mineral changes caused by
bacterial action. However magnetic results over simulated recent clandestine burials
in a variety of depositional environments have proved to be not that useful (see
Juerges et al. 2010). Ellwood (1994) and Witten et al. (2000) encountered difficulties
in locating 19th Century graves in cemeteries and a mass grave from 1921,
respectively, using magnetic methods. Magnetic susceptibility analysis was
undertaken on illegally dumped soil on a motorway in China that caused multiple
fatalities and successfully identified its origin (Manrong et al. 2009). Pringle et al.
(2008) pointed out that magnetic susceptibility datasets can also be used for quality
control checking of magnetic gradiometry datasets: e.g. for assisting with the removal
of magnetic data spikes. Recent field trials by the authors have shown magnetic
susceptibility methods are optimum to detect buried metallic targets beneath a
domestic patio (Figure 4) versus total field and gradient methods (see Reynolds, 2011
for background). Magnetic surveys collected by helicopters flying at a low altitude
have also proved useful in identifying UXOs; Billings and Wright (2010) provide a
good example from a former army range in Canada. For land-based surveys for UXO
detection, case studies using specialised magnetometers have been published on
31
multi-sensor 3-axis magnetometers (Munschy et al. 2007), quad-sensor arrays
(Billings and Youmans, 2007) and borehole magnetometry (Zhang et al. 2007).
However, Butler (2003) details the importance of understanding the background
magnetic susceptibility for identifying and locating UXOs and uses case examples
from Indiana and Hawaii, USA. In environmental forensic applications, Marchetti et
al. (2002) detail how magnetic methods identified where over 160 illegally buried
solid metal drums were located, with a recent paper showing how test sites can aid
magnetic data interpretation (Marchetti and Settimi, 2011).
5.6 Forensic geomorphology
A forensic geomorphologist can conduct visual inspections of the ground surface to
assess likely burial positions (see Owsley, 1995). Generally, the lower density of
backfilled soil and the buried target itself may initially result in a raised mound on the
ground surface (Killam, 2004). However, over time some disturbed soil may become
compacted as material density increases, resulting in localised ground depressions.
Ground depressions can become even more pronounced in searches for human
remains when cadaver chest cavities collapse (Rodriguez and Bass, 1985). Other
scientific observations that may indicate the presence of clandestine graves include
unusual localised vegetation changes, including lower species diversity/maturity,
improved/decreased vegetation growth (see France et al. 1992; Killam, 2004; Hunter
and Cox, 2005; Caccianiga et al. 2012), evidence of animal scavenger activity (France
et al. 1992; Turner and Wiltshire, 1999; Hunter and Cox, 2005; Dupras et al. 2006;
Wilson et al. 2007), entomology (see Amendt et al. 2007 for a detailed review) and
human activity, including discarded burial tools (Killam, 2004).
32
5.7 Intrusive probing
Owsley (1995) suggests that suspected burial areas be intrusively probed, allowing the
operator to `feel’ whether disturbed soil or even a buried cadaver is present, although
this may compromise forensic evidence. To quantify this process, suggestions have
used a spring gauge mounted on a probe to obtain quantitative measurements of soil
strength (Ruffell, 2005a) or a penetrometer (Killam, 2004); the resulting data points
can then be gridded, contoured and analysed to pinpoint anomalous areas with low
soil strength with respect to background values. Both methods are essentially using
the concept of ‘diggability’ applied to ground searches (see Section 4.2), in that a
perpetrator cannot bury an object where ground is hard or the soil is too thin. Fiedler
et al. (2008) detailed a successful intrusive probing case study pinpointing World War
Two mass graves in dense woodland in Germany that precluded the use of other
search methods. Note however the potential for damage to buried material and hence
potentially crucial forensic evidence.
5.8 Soil analysis
For the location of human remains, the chemical alteration of surrounding soil can be
detected, with authors suggesting a programme of soil samples from search area(s)
should be analysed to locate anomalous high element concentration regions with
respect to background values (Tibbett and Carter, 2008; Benninger et al. 2008;
Dekeirsschieter et al. 2009), especially for Nitrogen (Carter et al. 2008; Van Belle et
al. 2009) and human-specific carbon (Bull et al. 2009). Decomposition fluids
33
associated with buried animal remains can be detected using resistivity (Jervis et al.
2009), with fluid analysis showing them to be highly conductive when compared to
background soil water values (Pringle et al. 2010). McKinley et al. (2008) detail a ‘no
body’ forensic search case in Western Ireland using chemical analysis as one potential
search tool. Decomposition fluids and Volatile Organic Compounds (VOCs) have
also been shown to vary in elemental concentrations over time (see Juerges et al.,
2010 and Vass et al. 2004; 2008 respectively).
Electronic gas detectors (referred to as methane probes), which are able to analyse gas
drawn through metal probes inserted into the ground, have also shown promise for
grave detection (Killam, 2004; Hädrich et. al, 2010) although results should be used
with caution in case values are due to other emitting sources (e.g. from peat). Buried
sampler modules collecting gases over time have also been suggested as an alternative
detection method (Ruffell, 2002). It has also been suggested that decomposition gases
can be detected by laser spectroscopy (Ruffell, 2002), and GC-MS analysis of human
specific VOCs by headspace sampling methods (see e.g. Hoffman et al. 2009;
Lovestead and Bruno, 2011).
5.9 X-ray techniques
A continuous operating scanning X-ray source for landmine detection using
backscatter X-rays has been developed and it has been proved possible to distinguish
between plastic and metal mines due to relative mass density differences between
targets and background materials (Niemann et al. 2002; Yuk et al. 2006). However,
the equipment is not easily portable, and safety concerns relating to X-ray levels limit
34
the depth to which signals can penetrate in soil. A field portable X-ray fluorescence
spectrometer has been developed; Bergslien et al. (2008) discuss forensic geoscience
search applications, including location of cremated human remains, and identifying
both the environmental polluting source of illegal waste and tracking its movement
through the local environment.
5.10 Nuclear Quadrupole Resonance
Nuclear Quadrupole Resonance (NQR) is a solid state radio frequency technique
which has proven highly effective for the detection of explosives. It works by
inducing a radio frequency pulse into the ground through a coil, which causes the
explosive nuclei to resonate and an electrical potential to be recorded (see Anferova
and Grechishkin, 2005). NQR can distinguish between different forms of the same
compounds, e.g. TNT polymorphs, and has been shown by Somasundaram et al.
(2008) to be relatively unaffected by different soil conditions. However, the
technique has low signal to noise ratios indicating further data processing research is
required to make this technique usable in the field and the technique cannot penetrate
metallic material.
5.11 Radiometrics
Commonly used to locate radioactive rocks for exploration, few environmental
forensic searches using these techniques have been published. Ketterer and
Czechenyi (2008) reviewed atomic analytical analysis techniques, detailing plutonium
surveys at the Nevada atomic test site, USA, the Chernobyl plume in the Ukraine and
35
atomic accidents in Palomares, Spain and Thule in Greenland. Discussion was also
given to using plutonium as a marine tracer, showing it can be traced from the Pacific
Proving Ground to the North Pacific. There are issues with collecting enough data to
be statistically valid and geometric correction factors due to topography variations
(see Milsom and Eriksen, 2011).
36
6. Post-search
Once the fieldwork has been concluded and the data has been processed and
interpreted, a report should be written, detailing the survey’s findings and providing a
prioritised list of area(s) for intrusive investigation, if any (see Harrison and Donnelly,
2009 for more detail). These will obviously be target specific which may vary
considerably, depending on whether the target is a clandestine grave, explosive
material or contaminated land. All data may be questioned in a court of law, so
original and processed information, interpretations, samples, etc must be time and
date stamped and held in a secure, tamper-proof manner. All such items must adhere
to the state rules of chain of custody (see Chisem and Turvey, 2007). The civil,
criminal or environmental investigating officer will then decide on the next
investigative steps. This may or may not involve the geoscientist, depending on
whether further geoscientific investigations are required or not. Follow-up law
enforcement or environment agency options will correspondingly vary, from
excavation and identification of illegally buried material, mine/object clearance,
recovery of human remains or land remediation respectively. For the readers’
information, when a physical excavation is unsuccessful, there is generally an
effective exit strategy in place so that excessive use of manpower and resources is not
used (Harrison and Donnelly, 2009). This does not usually have any input from the
geoscientist so will only be briefly discussed.
6.1 Topsoil removal
37
Topsoil removal is a relatively straightforward method that is appropriate for buried
body searches once the search area has been narrowed down to a relatively small area
or when other search methods have failed (Killam, 2004). This involves the repeated
removal of thin soil surface layers, either until evidence of disturbance is encountered
or a depth is reached such that the area can be ruled out (Hunter and Cox, 2005). This
can be performed using machinery (e.g. Killam, 2004) or by hand-held digging
implements (e.g. Dupras et al., 2006). After enough soil has been removed, disturbed
areas can usually be identified by different coloured soil (see Dupras et al., 2006).
Note however the potential for damage to buried material and hence potentially
crucial forensic evidence. In potential crime scenes it is at this stage of the operation
that the forensic geoscientist will have handed responsibility on to an appropriately
qualified forensic archaeologist (Hunter and Cox, 2005).
6.2 Exploratory trenches and excavations
The final stage of any forensic search is often undertaken by careful trenching and
excavation of prioritised anomalous areas, usually by trenching suspected anomalous
areas pinpointed by the investigations suggested (Hunter et al. 2001; Cheetham, 2005;
Hunter and Cox, 2005). This phase is overseen by the crime scene manager or
environmental investigative officer with trained archaeologists or other relevant
personnel present. A systematic and controlled approach is utilised, for example
when excavating a clandestine grave, this includes detailing the stratigraphy and
relative dating, the grave contents, identifying the burial cut and tool marks whilst
retaining the material relative context. Hunter and Cox (2005) and Dupras (2006)
detail forensic archaeology excavations and best practice methodologies with Skinner
38
(1987) detailing best practice specifically for mass grave recoveries. Hunter and Cox
(2005) also show 29 clandestine burial case studies and mass grave investigations in
former war zones. Walkington (2010) also describes the importance of analysing
suspected areas for forensic archaeology. However, it is important to note that
excavations may not always be successful, with suspect burial areas in fact being due
to something more benign, for example, a test trench for a potential site development
as Ruffell et al. (2009b) document.
39
7. Discussion
Every forensic search of the subsurface will be unique, with different target(s) sizes
(e.g. child to adult clandestine burials), burial depth(s) below ground level (typically
the deeper buried the forensic target, the harder it may be to locate), style(s) of burial
(e.g. for discovered clandestine graves they are commonly wrapped, clothed or
naked), time since burial with its accompanying target variables (see Figure 1),
background soil type(s) (see below), local bedrock (which often affects overlying soil
type(s)), depositional environment (e.g. urban, rural, woodland, moorland, etc.),
vegetation type(s) (e.g. grasses, woods, urban gardens, etc.), local climate (e.g. arid to
humid), water table depth bgl, and many other specific variables. Each of these
target(s) and background site variables will have a limited to very important effect on
optimum search technique(s) and their subsequent target detection success or failure,
so it is very difficult to produce specific guidelines for forensic searches for all
eventualities. However, a generalised table to indicate the potential for search
technique(s) success for relatively common buried forensic target(s) has been
generated in this review (see Table 1). It is important to note that (1) thermal imaging
techniques will be dependent on time since burial (see Section 3.1) and (2) Water
Penetrating Radar survey detection success rates will depend upon target(s) size,
bottom sediment type(s), water depth, salinity and temperature (Parker, pers. comm.).
Side-scan sonar surveys are also dependent on relatively fine-grained sediments
surrounding the target to reduce non-target reflectors (see Dupras, 2006) and waterbased magnetometers would normally only be highly effective in relatively quiet nonmagnetic background sites.
40
Forensic target(s) in Table 1 are deliberately generalised; it would obviously be more
difficult to successfully locate (using magnetic methods) a buried single weapon than
a collection of them. Similarly illegally buried waste would obviously be easier to
detect if it was of significant size and had a high contrast from background materials,
for example buried metallic waste in a rural environment, than relatively small nonmagnetic material in heterogeneous ground. A mass grave may be easier to locate
using remote sensing methods whereas single clandestine graves are much harder to
detect remotely, and very often difficult to detect within a suspect site (see Hunter et
al. 2001).
The generalised search table differentiates between sand and clay site soil type endmembers, although, of course, this is also simplistic as most soils will lie between
these two, normally be heterogeneous and the end members ignore coarser grain-sized
soil types (e.g. river gravels, etc.). Soil type is, however, important to consider for
forensic searches for a variety of reasons. Firstly certain soil types (generally clayrich types) result in poor GPR penetration depths, the success rate of element analysis
detection (generally better in sandy soils) and decomposition rates with accompanying
fluid plumes (generally faster and larger respectively in sandy and coarser grain-sized
soils compared to clay-rich types). The search table also tries to judge the influence
of the local search generalised environment, covering the main depositional settings
that include woodland, rural, urban, coastal and underwater environments
(mountainous not included as not a common depositional scenario, although see Novo
et al. 2011). Local burial environment can be important to consider when undertaking
a survey, for example, urban areas will have potential difficulties for search teams to
use some specialist equipment (e.g. conductivity surveys if adjacent to metal
41
fencing/objects) and potentially have many non-target objects buried within the
survey site (see Drahor, 2011). However, environments are also deliberately general
in the table, for example, a GPR survey in woodland with widely-spaced deciduous
trees may be successful (see Nobes, 2000), however, it may be difficult to
differentiate buried forensic targets from tree roots in coniferous woodland with
closely-spaced trees. Exactly where the search site is in the search environment may
also have potentially large variations in potential target detection success, for
example, in coastal environments, technique(s) target detection success may vary
widely depending on whether it is in sand dunes versus the actual foreshore
environment (see Pringle et al. 2012b). Similarly a concrete surface may preclude
certain search technologies (e.g. resistivity) from being utilised due to not being able
to insert probes into the ground; however, GPR methods may be optimal in this urban
environment (see Billinger, 2009).
The generalised table also assumes optimum search technologies are available and
optimum data sampling strategies are chosen for the respective search target(s) within
their specified local environment(s). Small forensic target(s) would normally require
closely-spaced sampling points but this may depend upon target contrast with
background materials and time available onsite. For example, a forensic search to
detect a metallic knife using metal detection equipment in a metal scrap yard would
have a relatively low detection success rate due to their being little contrast between
the target and the background environment. The table also assumes experienced
search operators are undertaking the surveys which also increase the chances of
success as Billings and Youmans (2007) document (86% target location success for
experienced versus 54% for inexperienced magnetic data interpreters); not only would
42
they choose appropriate search detection equipment and sample data spacing, an
experienced forensic GPR team, for example, would undertake soil sampling and trial
2D profiles using all set antennae frequencies to determine the optimum frequency for
a site before a full forensic search survey is initiated. GPR data processors may then
use relatively advanced processing techniques, for example, to remove any aboveground interference effects (see Pringle et al. 2009) or remove average background
radar responses to reveal target anomalies (see Nobes, 1999).
Combinations of search techniques are considered best practice which would
normally increase the chances of detection success; this is relatively common in
geotechnical (see Sloan et al. 2007) and archaeological investigations (see ArciniegaCeballos et al. 2009) for examples. Published forensic examples include multidisciplinary searches for the hanged Texan Gunfighter William Preston Longley
(Ellwood et al. 1994), the Nobes (1999) EM and GPR study to locate unmarked
graves and a three year monitoring study of buried pig cadavers showing both GPR
and bulk ground resistivity surveys should be collected if the style of burial (e.g.
wrapped or naked) is unknown (Pringle et al. 2012a). Multi-technique environmental
forensic case studies are provided by Cardarelli and Di Filippo (2004), who used
electrical and seismic methods to characterise illegal landfills, and Boudreault et al.
(2010), who used electrical, magnetic and GPR surveys to characterise a contaminated
urban site.
The timing of the forensic search survey may be important in seasonal climates;
Pringle et al. (2012a) monitoring study showed that winter surveys were statistically
more likely to be successful at target detection rather than surveys undertaken in
43
summer months, concluded due to soil moisture content variations. Combining
techniques also allow the efficient use of time and resources for the best chances of
detection success. A workflow of a forensic search investigation has been generated
that again has been deliberately generalised (Figure 5). Of course the workflow will
need to be altered for each search and refined when further case information becomes
available. A major part of the search effort is undertaken prior to any field
investigations; this has benefits in time, manpower and obviously cost. This review
also strongly recommends that site reconnaissance, including visual inspection, a
diggability survey and perhaps limited trial surveys should be undertaken prior to
more comprehensive site survey work (see Harrison and Donnelly, 2009 and Larson
et al. 2011 for more information). Forensic searches normally do, of course,
encounter problems which Larson et al. (2011) document; whether a case could have
inaccurate or compromised evidence of suspect’s movements, false (positive or
negative) search dog alerts, misreading of soil profiles, misinterpretation of forensic
search datasets, confused evidential records and even investigator bias (see US
National Research Council Report 2009 for more information). It is therefore crucial
to improve search protocols and standardise search methods to ensure scientific
rigour, consistence and highest standards of investigation as Harrison and Donnelly
(2009) and Larson et al. (2011) document.
44
8. Conclusions
From this review it is clear that forensic geoscience methods aid terrestrial searches in
many and varied ways; their applications and recently published case studies are ever
expanding, even since the publication of the Ruffell and McKinley (2005) review;
search methods thus require standardisation and protocols to maximise the chances of
target detection success. Whilst certain countries, such as the UK, are attempting to
standardise methods, other countries globally are just starting to see the potential (see,
for example, Mazari 2010) and applying geoscientific search methods differently and
with subsequent varied success rates as other authors point out (see Larson et al.
2011). Many of the case studies, site investigations and research papers reviewed in
this paper indicate the benefits of multi-disciplinary investigations with integration of
historical and modern information, remote sensing and multiple phases of site work,
to make efficient time savings and to use appropriate equipment and technologies to
focus on search targets. Creating a conceptual target model and continually refining
this when new information becomes available will assist forensic search teams in
developing a detailed and focussed search plan. Key considerations when deciding on
optimum geoscience search techniques and methodologies can be exhaustive and case
specific but should be as a minimum: target type (composition), expected target size,
time since burial, likely depth below present ground level, the local burial
environment, site geology and soil type. Table 1 summarises preferred methodologies
for detection of specific target(s) but, of course, searches will be case and site specific
so this table should not be used as justification for preferred search methodologies to
be utilised on a search case. Clearly there is much empirical and applied research still
to do in order that current search models can be further refined and the capabilities of
45
geoscience methods for ever more effective search operations can be refined. It is
hoped that both researchers and practitioners will continue to evolve in synergy and to
further develop knowledge and research, be quantitative in approach and publish more
research and case study articles to further disseminate current developments.
Acknowledgements
The authors (other than John Jervis, Jamie Hansen and Mark Harrison) are committee
members of the Forensic Geoscience Group, a specialist sub-group of the Geological
Society of London, whose stated mission is to promote the study and understanding of
forensic geoscience nationally and internationally, promoting good practise for
academics and practitioners alike. The International Union of Geological Sciences
(IUGS), Initiative on Forensic Geology (IFG) is also acknowledged for its
contributions to this paper. Mike Ferguson and Sarah Corrigan of the Home Office
Centre for Applied Science and Technology (CAST) are acknowledged for magnetic
and metal detection equipment advice. Emily Giddings is thanked for providing some
burial statistics and Leigh Noble is thanked for UXO/IED detection information.
Three anonymous reviewers have significantly improved the manuscript.
Role of the funding source
The respective author research funders had no involvement in any studies shown,
writing or decisions regarding submitting this paper for publication.
Disclosure Statement
46
There are no actual or potential conflicts of interest by the co-authors.
47
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Figure Captions:
Figure 1. Simplified conceptual geological model (CGM) of a murder victim’s
clandestine grave in a rural, sandy soil environment for (A) recent burial and (B) old
burial. Potential location indicators are shown. Modified from Harrison and
Donnelly (2009) and Pringle et al. (2012a).
76
Figure 2. Map view examples of fixed-offset (0.25 m spaced) resistivity data over a
naked pig cadaver (left), empty grave (middle) and wrapped pig cadaver (right)
respectively that were buried in semi-urban sandy soil. Respective burial weeks are
shown. Note the low (blue) resistivity ‘plume’ was caused by conductive
decomposition fluids from the naked cadaver.
77
Figure 3. Mapview example of contoured magnetic susceptibility data (SI units) over
a simulated forensic patio test site (see key for target).
78
Figure 4. Ground Penetrating Radar (GPR) 450 MHz dominant frequency 2D profile
over a domestic patio (marked) identifying the locations of two forensic targets
(marked and shown).
79
Figure 5. Generalised search programme workflow (inc. = including). Note this
figure does not differentiate between target size, burial depth, soil type, burial
environment and other important case specific factors. Modified from Harrison and
Donnelly (2009).
80
81
Target(s)
Soil type:
sand
clay
Remote
sensing
Photo- Infra
graphs -Red
Geomorph
-ology /
probing
Thermal
imaging
1
Specialist
search
dogs
Seismology /
Sidescan
sonar
Site work
Near-Surface Geophysics
Cond- Resist- GPR
Maguctivity
ivity
netics
Metal
detector
Element
analysis
Unmarked
grave(s)
Clandestine
grave(s)
UXOs/
IEDs
Weapons
Drug / cash
dumps
Illegal
waste
Influence of search environment on chosen method(s) (above) effectiveness
Woods
Rural
Urban
Coastal
Underwater
²
82
Table 1. Generalised table to indicate potential of search techniques(s) success for buried target(s) assuming optimum equipment configurations.
Note this table does not differentiate between target size, burial depth and other important specific factors (see text). Key: Good; Medium;
Poor chances of success. The dominant sand | clay soil end-types are detailed where appropriate for simplicity, therefore not including peat,
cobbles etc types (a more wide ranging summary of geophysical techniques versus soil types can be found in Reynolds, 2011). 1Time post-burial
dependent. ²Water Penetrating Radar (WPR).
83