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Ground penetrating radar and geoforensics for criminal investigations: The value of collaboration between law enforcement and academia

June 12, 2024  By Grant Wach, Trevor Kelly, Lauren Morris, Kristie McVicar and Don Stienburg



Ground-penetrating radar (GPR) (the towable system shown in Figure 1) has become well-known to the public because of its usage on the popular TV show CSI, and the very sobering former residential school sites where unmarked graves have been detected.

Figure 1. An alternative towable GPR setup for rougher terrain (Morris, 2021).

GPR is a valuable tool for law enforcement officers (LEOs) to collect evidence in criminal investigations, yet they may not be trained in the use of the technology. In academia, GPR is often used for archeological and geological research. While the objectives may differ between law enforcement and academia, the science behind the method remains the same and opportunities exist for collaboration. At the Dalhousie University Basin and Reservoir Laboratory (DU-BRL), GPR systems (e.g., Figure 1) and software are used for geological, archaeological and forensic/geoforensic investigations, including the identification of marked and unmarked burial sites, and aiding with criminal investigations.

GPR methodology

GPR is a non-destructive and non-penetrative geophysical tool for imaging the shallow subsurface with a variety of practical applications. These include forensic and law enforcement, military, mining and quarrying, infrastructure, utility locating, archaeology and paleontology, geotechnical and environmental, and agricultural and forestry. GPR uses pulsed radio waves to detect changes in the velocity of materials, effectively imaging the contrast between objects and layers in the subsurface. GPR can provide detailed imaging of the shallow subsurface (< 10m) through the transmission and reflection of electromagnetic pulses (Figure 2).

Four basic principles determine the capabilities and limitations (Table 1) of GPR systems:

  • Velocity
  • Dielectric permittivity
  • Reflectivity
  • Attenuation

GPR’s capability to detect both metallic and non-metallic buried objects has led to an ever-widening use in forensics.

Capabilities

Limitations
  1. Detect large targets at depth
  1. Difficulty detecting small targets at depth
  1. Identify contrasts in subsurface materials
  1. Materials with similar velocities cannot be distinguished apart
  1. Achieve great depth of imagery through ice
  1. Salt water prohibits imaging altogether
  1. Reach deeper depths with lower frequency antennae
  1. Tradeoff between resolution and antennae frequency
  1. Gather 3D imagery through a grid system
  1. A single line cannot produce 3D imagery
  1. Identify presence of a target with a single line
  1. A single line cannot define the shape of a target

Table 1. The capabilities and limitations of GPR.

Uses of GPR in law enforcement investigations

Clandestine graves

The search for buried victims and remains is perhaps the most common use of GPR in law enforcement investigations. It can perform remarkably well because the remains, particularly if not severely composed or desiccated, will have a much different conductivity than the soil surrounding the object. GPR can be particularly effective in identifying casualties that have been wrapped in plastics or other types of fabric layers because the surface of these wraps will tend to improve the reflection of the GPR waves and help to reduce the processes of decomposition.

Hidden stockpiles

GPR is also extremely effective in detecting buried caches, including weapons or other munition stockpiles (metallic and non-metallic). It’s not uncommon for perpetrators to promptly bury this type of evidence to conceal proof of their activities. While metal detectors are only useful for detecting metallic objects, GPR can reveal non-metallic shapes and items, such as stashes of drugs and money, as well as soil disturbances caused by the burial of these items.

Underground cavities and structures

Discovering underground structures, such as a criminal refuge or underground passageway, is also a task that complements itself well to GPR. Using a GPR system, LEOs can promptly scan an area of interest to reveal any subsurface structures in the soil created by voids without the need for any type of excavation. This greatly reduces the time required to identify these targets and lessens costs.

Items concealed in walls

Another ideal area for concealing criminal evidence is in the walls of structures. They usually offer longstanding storage for items of various sizes and shapes; and it is challenging for LEOs to access those items once the wall is resealed. Specific configurations of GPR can be employed for identifying objects hidden in wall cavities and other structural features of buildings. This is especially true in criminal investigations where there is the possibility of damaging these concealed objects while attempting to retrieve them.

Figure 2. a) How ground-penetrating radar works (Morris, 2022).

Figure 2. b) Typical configuration of GPR System used by the Basin & Reservoir Lab (Morris, 2022).

Excavations are costly, damaging and are too slow

Excavating clandestine graves and areas comprising possible buried evidence, whether through heavy machinery or by hand, increases the likelihood of site contamination and disturbance. To mitigate damage and decrease the likelihood of compromising the evidence, the process of excavating is generally undertaken at a very slow pace, however it does not eliminate the risk entirely. The slow speeds significantly increase excavation times, which usually means more money spent on equipment rentals and operator wages. GPR is a typical go-to solution because it can be pushed or towed along the ground surface without producing any subsurface disturbance and can normally be accomplished quickly and efficiently.

GPR can be used as a first pass to help pinpoint areas that will necessitate further investigation, thereby eliminating the speculation from excavating locations based on little to no evidence. It can also differentiate regions that have seen recent disturbance or have non-original soils present. A GPR technician would ordinarily require a day or less to survey a 1-acre location while being able to provide specific locations of importance in real-time to focus resources on. This tightens the search area and substantially reduces the time spent searching for evidence, thereby conserving money in the process.

Collaboration with law enforcement

Researchers at the DU-BRL have assisted with criminal investigations on several occasions employing geoforensic techniques. The details of these investigations are strictly confidential; however, the type of work can be described as 1) site analysis for homicide and missing persons cases and 2) site investigations and searches for buried objects.

Aiding in criminal investigations is mutually beneficial for LEOs and researchers. The LEOs gain insight into their crime scene that may not have been possible without the option of visualizing the subsurface through geoforensics including GPR, and researchers are able to practically apply research, deploy their equipment on a range of terrains and conditions, and identify areas where their workflows can improve. Publication of results of non-criminal investigations provide peer-reviewed corroboration of methodology, analysis and results to establish the scientific validity of geoforensic techniques such as GPR, to be entered as sound and acceptable evidence in a court of law or inquiry.

GPR can be used as a first pass to help pinpoint areas that will necessitate further investigation.

In May 2022, a workshop was held at the DU-BRL for 25 LEO’s to learn about the use of geoforensics and GPR in their investigations. Attendees were provided the opportunity to learn about geoforensic techniques and concluded with hands-on GPR experience as they searched for buried items at a mock crime scene.

Other geophysical/geoforensic techniques

While GPR is the most popular surficial geophysical instrument for collecting subsurface data during criminal investigations, additional equipment may also yield useful information, particularly if paired with other data collection instruments. These include seismic reflection and refraction; multichannel analysis of surface waves (MASW); electrical resistivity tomography (ERT); magnetometer; electromagnetics (EM); and a gravimeter.

Concluding thoughts

The use of GPR and interpretation techniques can be taught relatively easily. A recommendation would be for police forces to have individuals trained in this technology, much like there are police divers, dog handlers and forensic specialists. This would provide rapid access to the technology for investigations and an in-house technical specialist that could decide what equipment is best suited and warranted for investigations.

GPR does not produce the fantasy results depicted on some TV shows, but it can offer valuable insight to LEOs needing to perform thorough subsurface site investigations. The academic and law enforcement communities both benefit from the integration of GPR and other geoscience methods for criminal investigations.

Acknowledgements

We would like to acknowledge our collaboration with the law enforcement agencies in Nova Scotia and our colleagues at the International Union of Geological Sciences (IUGS) Initiative on Forensic Geology (IFG), particularly Drs. Laurance Donnelly and Alastair Ruffell, and Mike Sanders principal archaeologist with Cultural Resource Management Group.

References

  • Kelly, T.B., Wach, G.D., O’Connor, D.E. (2021) The technical challenges and outcomes of ground-penetrating radar: a site-specific example from Joggins, Nova Scotia. AIMS Geosci., 7 (1) pp. 22-55. DOI: 10.3934/geosci.2021002
  • Kelly, T.B. and Wach, G.D. 2021. Geoforensics – Innovative 3D modelling GPR Data for Graveyards and Criminal Investigations. Subsurface Views 32(66), 4-6.
  • Kelly, T.B., Angel, M.N., O’Connor, D.E., Huff, C.C., Morris, L.E., and Wach, G.D. (2021b) A novel approach to 3D modelling ground-penetrating radar (GPR) data – A case study of a cemetery and applications for criminal investigation. Forensic Science International, 325. DOI: https://doi.org/10.1016/j.forsciint.2021.110882
  • Morris, L.E. 2021. Digital photographs of GPR surveying from the Debert area, Nova Scotia.

Grant Wach, Trevor Kelly, Lauren Morris and Kristie McVicar are all members of the Dalhousie University Basin and Reservoir Lab in Halifax, N.S. Led by Professor Grant Wach, the Basin and Reservoir Lab serves as a research facility for undergraduate and graduate students, post-docs and visiting scientists from the energy industry, government and other academic institutions. Don Stienburg (Staff Sgt. Retired) is formally with the Halifax Regional Police – Investigations.


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