Geoforensic search to crime scene: Remote sensing, geophysics, and dogs

In the absence of surface indications of burial sites, law enforcement or humanitarian organizations are faced with the difficult task of focusing large‐scale ground searches to a manageable excavation area. A geoforensic‐based survey may exclude parts of the landscape for reasons such as diggability or viewshed analysis but leave areas still too large for invasive exploration. This work examines how drone‐based remote sensing, geophysics, and search dogs may be combined to narrow such searches. Here, we ask the reader to consider two examples where forensic geomorphology and land use provided a range of possible burial locations. Following this is a multi‐proxy approach to similar dilemma, with a search‐to‐scene case study using remote sensing (drone photography), geophysics, ground probes, and search dogs. This approach is not presented as a definitive guide, but serves as an example of the conjunctive use of well‐studied methods to approach a common problem in geoforensics.


Abstract
In the absence of surface indications of burial sites, law enforcement or humanitarian organizations are faced with the difficult task of focusing large-scale ground searches to a manageable excavation area. A geoforensic-based survey may exclude parts of the landscape for reasons such as diggability or viewshed analysis but leave areas still too large for invasive exploration. This work examines how drone-based remote sensing, geophysics, and search dogs may be combined to narrow such searches. Here, we ask the reader to consider two examples where forensic geomorphology and land use provided a range of possible burial locations. Following this is a multi-proxy approach to similar dilemma, with a search-to-scene case study using remote sensing (drone photography), geophysics, ground probes, and search dogs. This approach is not presented as a definitive guide, but serves as an example of the conjunctive use of well-studied methods to approach a common problem in geoforensics.

K E Y W O R D S
geoforensic search, geomorphology, geophysics, orthophotography, remote sensing (drones)

Highlights
• The problematic issue of where to focus a forensic search is addressed.
• The link between Search and Scene is made using geophysics as a proxy method.
• Remote sensing can prioritize locations for ground search teams.
• GPR is demonstrated as one possible geophysical means of focusing a search. may be hard to visualize, so we provide two examples where the information leading to the general location is considered reliable but leaves too much uncertainty for excavation and thus become a crime scene. We then take a third case as a more fully worked up study to explore this, using drone remote sensing and geophysics (GPR) to focus ground probing for VR (victim recovery) or cadaver dog deployment. We have faced this gray area between the Search and establishment of an excavation Scene many times, including these examples.

| T WO FORENS IC G EOMORPHOLOGY E X AMPLE S
After we present these, we take one case and consider how different proxy indications may be combined to focus investigations, potentially saving time and money.

| Rural valley
A missing persons inquiry led police to suspects last seen with the potential victim. A combination of CCTV, mobile telephone data, and vehicle telematics indicated suspect vehicles were at a rural location for more than an hour at night. The area of interest comprises a postglacial terrain of drumlins (low, often oval mounds/small hills), formed on hard rock (Lower Paleozoic greywackes). Peat bogs and mires occur in inter-drumlin hollows. The location has a minor road, where two or three vehicles could park unobserved, as shown in the drone (UAV, or unmanned aerial vehicle) data in Figure 1A. dog indication would normally be considered, but these were not undertaken here, as the investigation moved elsewhere. However, F I G U R E 1 (A) Photomosaic of 60 drone images, acquired at Case Study 2.1 (Rural Valley), using a DJI MavicPro™ in autonomous flight, stitched using DroneDeploy™ (under license) to provide one orthoimage for context; (B) topography, modeled from the data in A, to demonstrate covert, low ground with presumed diggable soils and elevated ground, with thin soils and lack of cover. this case study serves as an example of how to manage investigation of a search area and provide focus for geophysics, ground probing, canine deployment, and possible excavation. These questions are explored in Case Study 3 below.

| Upland forest plantation
As in Case 2.1 (above), this area was considered following a criminal confession. The perpetrator, who worked in tree planting, stated they knew of their partner's affair with a third party and enticed them to their picturesque, if remote workplace one Sunday evening. The supposed offender had access to a locked forest gate, from where they drove toward a hilltop wildfire viewpoint and telecommunications mast near an area for vehicle parking ( Figure 2). The witness stated that they stopped at a Y junction in the tracks, where the victim was assaulted, murdered, dragged downhill, and placed in a ditch that was to be regraded by the perpetrator the next day.
Thus, the search focus was on three Y-junctions in the forest track

| Background
The search location considered was repeatedly indicated to be of interest as a homicide victim burial location from background information, yet was not pursued as the location presented some dif-

| Methods and results
The site was surveyed using orthogonal drone photography [5,6] with a DJI Mavic Pro 2. This geolocated imagery was used as a showed an anomaly at the western side of this grid. Such a mixed approach, comprising grids for 3D analysis and 2D profiling is entirely standard and widely published in geophysics and forensic science journals [7,8]. The work published here is not a discussion of geophysics or GPR acquisition, so the reader is referred to publications cited above ( [7,8], and references therein). Seven profiles in this grid and in the 4 m × 4 m grid to its east ( Figure 3B) showed insubstantial, if persistent anomalies that were consistent with drainage utilities, taking water from the raised roadway to the south, northward.
We processed data from the 20 m × 11 m grid ( Figure 3C,D) in MalaVision™ online software; this grid having the only substantial GPR anomaly ( Figure 3C). A horizontal time-slice with medium RGB threshold at 17 ns depth (c. 75 cm below ground surface, Figure 3C,D) shows numerous anomalies, some coincident with that seen on the 2D data (insets on Figure 4B) that were unexplained or presumably caused by a buried utility. By increasing the red band of the RGB threshold, we effectively accentuate the radar wave reflection amplitude [8], suppressing noise and leaving the most significant GPR return ( Figure 4A), which was consistent with the bright spot on 2D data ( Figure 4B). 2D and 3D data may be positively compared in a 3D GPR cube ( Figure 4B).

F I G U R E 4
Outputs for Per-urban Case Study (Peri-Urban Search). (A) 3D cube of GPR amplitude (from Figure 4D); (B) 3D cube of combined 2D GPR data, replacing the anomaly first detected ( Figure 4B) back into context; (C) example of police dog handlers probing the ground for scent release over the identified anomaly.

| CON CLUS IONS AND RECOMMENDATIONS
The question of how to refine a Search to possible excavation Scene was demonstrated for the problematic forensic geomorphology examples in Section 2. These are intentionally incomplete to prompt the reader into considering what should be done next. This is answered with our Case Study (Section 3) using one geophysical technique (GPR). The process of homing in on an area suitable for excavation may be achieved using other appropriate proxies such as drone or aerial remote sensing [5,6], geophysics such as resistivity or electromagnetics [9,10], vegetation mapping [11], or less-targeted probing over the whole area for victim recovery dog deployment [12,13]. Should results from any of these proxies be inconclusive, then a conjunctive, or orthogonal approach should be adopted [14] to build up a refined conceptual model of the search area and individual anomalies.

ACK N OWLED G M ENTS
Equipment loan was kindly facilitated by Mike Langton (GuideLineGeo) and Rob Storrar (Sheffield Hallam University). Site access was provided by Trevor Cooke (Police Service of Northern Ireland) and Caroline Trimble (Indiwoods Ltd.). The work was considerably improved by two anonymous reviewers, who we thank.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors have no known conflicts of interest.