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Journal ArticleDOI

A controlled monitoring study of simulated clandestine graves using 3D ground penetrating radar

06 Sep 2015-Near Surface Geophysics (EAGE Publications)-Vol. 15, Iss: 3, pp 274-284

AbstractA controlled three-dimensional ground penetrating radar monitoring study over simulated clandestine graves was conducted near Pretoria, South Africa, in which the detectability of graves as a function of post-burial interval was assessed, as this is of particular interest to local forensic investigators. It was demonstrated that the site-specific environmental parameter (a clay-rich loamy soil with poor drainage) and heavy seasonal rainfall (as confirmed by ground-penetrating-radar-derived soil moisture estimates) drastically compromised the long-term grave detectability, especially when adopting a three-dimensional depth slice analysis approach. It is also seen that the disturbed burial zone is the major contributor to the total grave anomaly rather than the buried body due to the combination of environmental parameters and the absence of buried artefacts. This paper also advocates the combined use of different data representations (two-dimensional and three-dimensional) to increase the likelihood of detecting subtle grave anomalies.

Summary (3 min read)

INTRODUCTION

  • Ground penetrating radar (GPR) is a widely used and versatile near-surface geophysical method that has been applied to a diverse range of earth science problems; for example, utility detection, concrete and infrastructure studies, mining and hydrogeophysics.
  • 3D GPR surveys typically involve a grid of equidistant parallel profiles acquired in two perpendicular directions to optimise spatial resolution.
  • It is clearly evident that the detection of clandestine burials using GPR is a challenging problem that is highly dependent on a range of site-specific, cultural and environmental variables and since forensic geophysics is also a relatively new application field, there is a definite need for a variety of benchmarking studies to advance the knowledge in this field.
  • The survey parameters include shallow clandestine burials with no body coverings or other artefacts, a clay-rich loamy soil characterised by poor drainage and a dominant environmental variable in the form of a seasonal rainfall spike.
  • This controlled monitoring study also represents the first of its kind in South Africa and possibly on the African continent.

TEST SITE AND SURVEY APPROACH

  • For the purpose of the taphonomic study, provision was made for a total of 40 simulated clandestine graves; the layout was a regular grid of ten rows of four graves each.
  • This red loamy soil, which has relatively high clay content, is characterised by poor drainage.
  • The taphonomic study required the excavation (removal of carcass and backfilling of grave) at prescribed intervals (7 days, 2 weeks, 1 month, 3 months and 6 months).
  • This approach ensured that GPR data were acquired for a variety of scenarios, ranging from relatively fresh burials (3 days) to older than 180 days, as listed in Table 1.

DATA PROCESSING AND VISUALISATION

  • For each of the six GPR surveys the same approach to processing and analysis was followed: Individual 2D profiles were first processed using the REFLEXW software (by Sandmeier Scientific Software).
  • (1) A time-zero correction; (2) mean subtraction to eliminate unwanted low-frequency components from the data; (3) automatic gain control (AGC) to amplify low-amplitude ranges; and (4) background removal to suppress horizontally coherent energy and to emphasise anomalies that vary laterally such as diffractions, also known as Standard processing steps included.
  • In addition to the 2D analyses described above the respective 3D data sets were processed for the purpose of generating depth slices.
  • The key included with Figure 3 details the notation used in these plots.

DISCUSSION OF RESULTS

  • In previous similar studies it was pointed out that both the buried body as well as the disturbed burial zone contributed to an observed clandestine grave anomaly (Schultz and Martin, 2012; Salsarola et al., 2015).
  • The impact of these variables at the FABF site becomes evident when monitoring and comparing the changes in GPR responses for the respective surveys: 03 November 2014.
  • It is also not possible to discriminate between the responses of undisturbed grave that are ~2 months old (e.g., graves 9-12) and those of graves that were excavated and backfilled more recently (e.g., graves 17-20); this adds further weight to the inference that in this particular environment the disturbed burial zone is the major contributor to the total grave response.
  • The depth slice in Figure 6 reveals that the contrast between grave anomalies and background and hence the detectability of graves has deteriorated significantly between early December 2014 and mid-January 2015.
  • A method that uses the average radar velocity in the near surface to derive the apparent soil permittivity, and subsequently a soil water content estimate, was used.

31 March 2015

  • Between January and March 2015 the observed anomaly contrast and grave detectability continued to decrease rapidly and this is evident in both 2D and 3D data .
  • Even the anomalies associated with the relatively fresh graves 27-32 (post-mortem interval of 11-12 days) are not as prominent and easy to detect as in the first two 3D surveys.
  • An interesting consequence is that some of the grave anomalies on the 2D section appear easier to identify than the corresponding anomalies on the selected depth slice .
  • This case study highlights an important pitfall of 3D GPR forensic surveys: one cannot always rely solely on depth slice information when searching for subtle anomalies such as those anticipated for clandestine graves.

16 April 2015

  • In the two weeks subsequent to the 30 March 2015 survey, the upper portion of the soil profile dried out appreciably and the target-host contrast increased again as seen in the depth slice for the 16 April 2015 survey shown in Figure 9.
  • The southwestern corner of the survey area however is still characterised by a relatively high degree of clutter and unwanted noise; this anomalous zone correlated with an area where drainage / drying out appeared to occur much slower than over the rest of the grid.
  • Note that the graves associated with the most recent soil disturbances manifest most prominently on the displayed depth slice; for example, the three-day-old graves 33-36, 40 and ~ one-month old graves 27-32; also, the recently excavated graves 1, 3,4,6 and 7.
  • This is further confirmation that for this specific environment the clandestine grave anomaly is primarily the response of the disturbed soil zone.

31 August 2015

  • A final 3D survey was completed on 31 August 2015, towards the end of the dry winter months.
  • The results are similar to that of the earlier April survey; however, an overall deterioration in image quality is apparent .
  • This gradual deterioration could be attributed to the effect of the wet-dry cycle and continued settling of the disturbed soil zones.
  • There are nevertheless indications that some of the observed anomaly contrasts have increased slightly from April to August 2015 , which can be attributed to the decrease in soil moisture content between April and August 2015 .
  • It is however clear that only those graves that were last disturbed after or towards the end of the rainy season are still reasonably detectable.

CONCLUSIONS

  • This controlled study confirmed that 3D GPR is a valuable tool when conducting forensic grave searches.
  • The site-specific combination of soil type (A Hutton form clay-rich loam, characterised by poor drainage) and clandestine burials (absence of coffin or artefacts that could enhance the target responses) resulted in the detectability of clandestine graves to be highly dependent on the season and to a lesser extent on post-mortem interval.
  • Soaking seasonal rains contributed to an increase in soil moisture content and ground conductivity and an associated increase in radar signal attenuation, noisier data, more rapid settling of the disturbed burial zone and consequently lower observed anomaly contrasts.
  • The deterioration of grave responses over time due to the previously described variables may result in very subtle anomalies on individual depth slices; it is good practice to consider a series of successive slices in conjunction with corresponding 2D radargrams when trying to identify possible grave anomalies.
  • It would also be useful to know whether the addition of clothes, plastic bags or other body coverings would perhaps increase the detectability window of opportunity.

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A controlled monitoring study of simulated clandestine graves using 3D
ground penetrating radar
M. van Schoor
1
, W.C. Nienaber
2
and A. Marais-Werner
2
1
Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa
2
University of Pretoria, South Africa
E-mail: mvschoor@csir.co.za
ABSTRACT
A controlled 3D GPR monitoring study over simulated clandestine graves was conducted near Pretoria, South
Africa, in which the detectability of graves as a function of post-burial interval was assessed, as this is of
particular interest to local forensic investigators. It was demonstrated that the site-specific environmental
parameter (a clay-rich loamy soil with poor drainage) and heavy seasonal rainfall (as confirmed by GPR-derived
soil moisture estimates) drastically compromised the long-term grave detectability, especially when adopting a
3D depth slice analyses approach. It is also seen that the disturbed burial zone is the major contributor to the
total grave anomaly rather than the buried body due to the combination of environmental parameters and the
absence of buried artefacts. This paper also advocates the combined use of different data representations (2D
and 3D) to increase the likelihood of detecting subtle grave anomalies.
INTRODUCTION
Ground penetrating radar (GPR) is a widely used and versatile near-surface geophysical method that has been
applied to a diverse range of earth science problems; for example, utility detection, concrete and infrastructure
studies, mining and hydrogeophysics. Although the most common fields of application are still utility detection
and concrete and infrastructure studies (Pers. Comm., Greg Johnston, Sensors & Software Inc., 22 September
2016), the use of GPR is on the rise in the field of forensic geoscience (Schultz, 2012; Pringle et al., 2012).
The popularity of GPR is due to its capability to survey large areas in relatively short times and at a relatively
high (cm-scale) resolution. Barone et al. (2016) also highlighted the onsite real-time processing capability of
modern GPRs as a further key benefit. Most commercially available GPR systems employ a constant-offset
transmitter and receiver antennae configuration that can be mounted on a push-cart. A 3D or grid survey
1

approach is commonly used to cover large areas for the purpose of depth slice extraction and 3D data
visualisation. 3D GPR surveys typically involve a grid of equidistant parallel profiles acquired in two
perpendicular directions to optimise spatial resolution. Data acquisition is usually conducted at a single
operating frequency in the range 100-1000 MHz and the resulting depth of investigation is typically limited to
the first few meters of the near-surface. In GPR there is a trade-off between range and resolution: increasing the
operating frequency implies a higher resolution, but at the cost of a decreasing range associated with the
corresponding increase in attenuation; similarly, a lower operating frequency will enable a greater depth of
investigation, but at a reduced mapping accuracy.
GPR studies that relate to grave detection can be classified as either of a heritage or a forensic nature. In
heritage-related studies the typical survey aim is to identify unmarked cemetery graves; for example, Fiedler et
al. (2009), Hansen et al. (2014) and Barone et al. (2016). Forensic studies typically involve the somewhat more
challenging task of searching for clandestine burials; for example: Pringle et al. (2008) advocated the use of a
multi-technique approach to identify possible clandestine burials in an urban environment and using GPR as a
follow-up tool for obtaining better resolution on selected anomalies; however, the challenges associated with
detecting subtle clandestine burial anomalies using GPR in the noisy and heterogeneous ground conditions of
urban environments were highlighted. Schultz (2008) studied the monitoring of pig cadaver burials in sandy
soils, and emphasised the difficulty of detecting anomalies after a few months because the disturbed zone does
not present as strong a GPR contrast as soils with distinct horizons or higher clay content might. Doolittle and
Bellantoni (2010) focused on the site-specific applicability of GPR in a soil type, which is generally considered
as favourable for GPR due to its low clay content; however, the lack of well-developed soil horizons and the
associated relatively low contrast of the disturbed (burial) zone, as well as the presence of undesirable scattering
bodies such as tree roots, rock fragments, animal burrows and modern cultural debris are described as
challenges. Schultz and Martin (2012) also reflected on the previously mentioned problem of detecting
clandestine burial in sandy environments, particularly when the body is buried without any artefacts or
coverings that may enhance the grave anomaly to some extent. In fact, the difficulty in detecting clandestine
burials is often attributed to the absence of a coffin, container or other artefacts (Pringle et al., 2008; Doolittle
and Bellantoni, 2010; Novo et al., 2011). The covert nature of such burials and the fact that no record exists
imply a greater degree of uncertainty in terms of anticipated location, orientation, age and depth of burial.
Another variable that may adversely affect the GPR detectability of clandestine burials is the soil moisture
2

content, due to the associated increase in bulk soil conductivity (implying a decrease in GPR range). However,
in some cases (depending on the combination of soil type and state of decomposition) an increase in soil
moisture may actually contribute to an enhancement of grave anomalies (Schultz and Martin, 2012; Molina et
al., 2015). It is clearly evident that the detection of clandestine burials using GPR is a challenging problem that
is highly dependent on a range of site-specific, cultural and environmental variables and since forensic
geophysics is also a relatively new application field, there is a definite need for a variety of benchmarking
studies to advance the knowledge in this field.
In South Africa, the applicability of GPR to forensic cases has in recent years attracted the attention of criminal
investigators, with the South African Police Services (SAPS) acquiring several GPR units. However, the
frequency of use for clandestine grave searches is low due to insufficient training and poor technical support of
the investigating officers; for example, during the period January 2015 to July 2016, GPR was only used in three
out of 32 clandestine grave search cases (Pers. Comm. Colonel L Rossouw, SAPS VIC, 25 July 2016; Major W
Ngoma, SAPS Gauteng Crime Scene Management Unit, 26 July 2016). There clearly exists a need to empower
local crime scene investigators in the use of geophysical tools such as GPR and case studies such the one
described in this paper can make a positive contribution in this regard.
Between September 2014 and May 2015, researchers from the Forensic Anthropology Research Centre of the
University of Pretoria (UP) conducted a taphonomic study at a controlled test site. The aim of this study was to
investigate the rate and pattern of decomposition associated with shallow, clandestine burials in the local soil
and environmental conditions. Pig cadavers were used as proxy for human bodies as is commonly done in such
studies (e.g., Schultz, 2008; Molina et al., 2015). Although geophysical monitoring did not originally form part
of the taphonomic research plan, it was decided to run a time-lapse GPR monitoring experiment in parallel with
the taphonomic study and as part of a research collaboration between UP and the Council for Scientific and
Industrial Research (CSIR).
Four 3D GPR surveys were conducted between November 2014 and March 2015 and preliminary results and
findings were reported on in Van Schoor et al. (2015). Two additional 3D GPR scans were conducted between
April 2015 and August 2015 and are reported on for the first time in this paper. A more quantitative assessment
3

of local soil moisture conditions was also added and these additional data sets helped to support and expand on
the previously reported findings.
In the context of earlier forensic geophysics studies relating to the application of GPR for clandestine grave
detection, this study involves a unique combination of survey parameters and methodologies, not previously
described. The survey parameters include shallow clandestine burials with no body coverings or other artefacts,
a clay-rich loamy soil characterised by poor drainage and a dominant environmental variable in the form of a
seasonal rainfall spike. In terms of methodologies, the relationship between soil moisture and 3D GPR
performance is investigated by considering both rainfall data and GPR-derived soil moisture content estimates.
This controlled monitoring study also represents the first of its kind in South Africa and possibly on the African
continent.
TEST SITE AND SURVEY APPROACH
The Forensic Anthropology Body Farm (FABF) of the University of Pretoria is located on an experimental farm
(Miertjie le Roux), 30 km east of Pretoria. For the purpose of the taphonomic study, provision was made for a
total of 40 simulated clandestine graves; the layout was a regular grid of ten rows of four graves each. The
dimensions of individual graves were approximately 2.2 m x 1.2 m x 0.75 m, with a spacing of 2.5 m between
graves; the total effective area of the final grid was approximately 42 m x 17 m (Figure 1)
The FABF site is located in a warm and temperate climatic region of South Africa in which the summer (Dec-
Feb) receives a much higher rainfall than the winter (Jun-Aug). The GPR study described here therefore
commenced in late spring, just after the onset of the rainy season. The soil cover at the FABF site can be
classified as a Hutton form soil (Pers. Comm., Garry Paterson, Agricultural Research Council, Institute for Soil,
Climate and Water, 20 December 2016). This red loamy soil, which has relatively high clay content, is
characterised by poor drainage. The poor drainage, coupled with the elevated electrical conductivity often
associated with this type of clay-rich soil, is expected to impact negatively on the performance of the GPR
method.
4

FIGURE 1: 3D GPR surveys conducted at the FABF site
5

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Abstract: The dependence of the dielectric constant, at frequencies between 1 MHz and 1 GHz, on the volumetric water content is determined empirically in the laboratory. The effect of varying the texture, bulk density, temperature, and soluble salt content on this relationship was also determined. Time-domain reflectometry (TDR) was used to measure the dielectric constant of a wide range of granular specimens placed in a coaxial transmission line. The water or salt solution was cycled continuously to or from the specimen, with minimal disturbance, through porous disks placed along the sides of the coaxial tube. Four mineral soils with a range of texture from sandy loam to clay were tested. An empirical relationship between the apparent dielectric constant Ka and the volumetric water content θv, which is independent of soil type, soil density, soil temperature, and soluble salt content, can be used to determine θv, from air dry to water saturated, with an error of estimate of 0.013. Precision of θv to within ±0.01 from Ka can be obtained with a calibration for the particular granular material of interest. An organic soil, vermiculite, and two sizes of glass beads were also tested successfully. The empirical relationship determined here agrees very well with other experimenters' results, which use a wide range of electrical techniques over the frequency range of 20 MHz and 1 GHz and widely varying soil types. The results of applying the TDR technique on parallel transmission lines in the field to measure θv versus depth are encouraging.

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Additional excerpts

  • ...The relationship used to derive the SWC (𝜃) from the apparent soil permittivity is based on an empirically determined equation (Topp et al., 1980): 𝜃 = −5.3 × 10−2 + 2.92 × 10−2𝜀 − 5.5 × 10−4𝜀2 + 4.3 × 10−6𝜀3 (2) The transient behaviour of the estimated SWC for the FABF site over the GPR study…...

    [...]


Journal ArticleDOI
Abstract: We present a comprehensive review of methods to measure soil water content with ground penetrating radar (GPR) We distinguish four methodologies: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient For each of these four methodologies, we discuss the basic principles, illustrate the quality of the data with field examples, discuss the possibilities and limitations, and identify areas where future research is required We hope that this review will further stimulate the community to consider ground penetrating radar as one of the possible tools to measure soil water content

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Abstract: We present a comprehensive review of methods to measure soil water content with ground penetrating radar (GPR). We distinguish four methodologies: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient. For each of these four methodologies, we discuss the basic principles, illustrate the quality of the data with field examples, discuss the possibilities and limitations, and identify areas where future research is required. We hope that this review will further stimulate the community to consider ground penetrating radar as one of the possible tools to measure soil water content.

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"A controlled monitoring study of si..." refers methods in this paper

  • ...The method, which is described in Huisman et al. (2003), involves deriving average soil velocities from diffraction hyperbolae....

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TL;DR: Overall, cadavers in sand were easily detected for the duration of this study at 21.5 months, and in clay it became increasingly difficult to image the pig cadaver over the first year of burial, even when they still retained extensive soft tissue structures.
Abstract: Ground-penetrating radar (GPR) was used to monitor 12 pig burials in Florida, each of which contained a small pig cadaver. Six of the cadavers were buried in sand at a depth of 0.50-0.60 m, and the other six were buried in sand at a depth of 1.00-1.10 m to represent deep and shallow burials that are generally encountered in forensic scenarios. Four control excavations with no pig interment were also constructed as blank graves and monitored with GPR. The burials were monitored for durations of either 13 or 21 months, and were then excavated to correlate the decomposition state of the cadaver with the GPR imagery. Overall, this study demonstrated that it may be difficult to detect small cadavers buried in sand soon after they are skeletonized because the area surrounding the body, or the grave, may not provide a strong enough contrasting area to be detected by GPR when compared to that of the surrounding undisturbed soil. Also, depth of burial appears to influence grave detection because bodies that are buried at deeper depths may be detected for a longer period of time due to reduced decomposition rates.

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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, 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.

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"A controlled monitoring study of si..." refers background in this paper

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Frequently Asked Questions (2)
Q1. What are the contributions in "A controlled monitoring study of simulated clandestine graves using 3d ground penetrating radar" ?

A controlled 3D GPR monitoring study over simulated clandestine graves was conducted near Pretoria, South Africa, in which the detectability of graves as a function of post-burial interval was assessed, as this is of particular interest to local forensic investigators. This paper also advocates the combined use of different data representations ( 2D and 3D ) to increase the likelihood of detecting subtle grave anomalies. 

Finally, several follow-up forensic research possibilities can be suggested: for example, a study to quantify the detectability of graves after a second summer rainfall season ; that is, up to two years after burial.