Location Techniques

Detection and recovery of buried bodies have for long concerned anthropological and archaeological studies. It became more recently a focus of forensic investigations. Whatever the context (clandestine graves, mass graves), locations are very complex because of the search area: surface and time elapsed since the burial. Indeed, modification of soil growth of vegetation can mash a burial site several months after. Such a task is generally time-consuming and requires personnel (safeguarding of the scene, recovery with replacement of staff) and material that is more or less sophisticated (detection material, engine and light by night). They include non-intrusive foot search methods such as observation (vegetation, soil) and use of air scent dogs (Killam 1990) or aerial observation (photography and video, infrared). Intrusive ground search methods include techniques that could cause damage of the site by the use of probes, combustible gas vapour detectors and soil analyses: pH, chemical composition, organic content, etc.

Other search methods are based on geophysical prospecting (magnetic surveying, metal detectors, ground-penetrating radar, resistivity or electromagnetic surveying), or remote-sensing. Parapsychological methods have also been used sometimes (Killam 1990).

13.4.1.1 Non-intrusive Ground Search Methods

Existing techniques performed at ground level are more or less sophisticated and dedicated to a relatively reduced surface area. They imply that the site is accessible to personnel.

13.4.1.1.1 Testimonies

Witness or survivor testimonies are a traditional way of grave 'detection' at a different scale used nowadays.

13.4.1.1.2 Basic Observation

Basic observation of vegetation (foliage, plant growth) and soil changes (coloration, sinking or compaction) can also be taken into account for investigators. Rodriguez and Bass (1985) reported that greater burial depths (1.2 m; 0.6 m) caused deeper depression than shallower burials (0.3 m). They observed that because of the redeposition of deeper layer of soil at the surface, coloration of topsoil was different above the grave than in the surroundings. Such method needs trained staff and favourable weather to improve efficiency of search operations.

13.4.1.1.3 Foot Search

In order not to forget any evidences, a common progression pattern can be realised. The search area can be divided into codified zones in which different teams may work. The foot search process must be performed from the outside toward the inside in order to minimize disturbance within the zone. The searcher can thus move in decreasing concentric circles within the zone. The line or strip search consists of a kind of zigzagging between limits of the search area. Search can be conducted by a team of searchers in right line separated at 2 m from one extremity of the delimited area to the other. Another variation consists of separating people in two teams facing each other (interlocking lines), each of them starting from one boundary (natural or man-made) and starting again perpendicularly (Killam 1990). For more efficiency a mix of strip and interlocking lines can be used.

13.4.1.1.4 Cadaver Dogs

Air scent dogs are trained to detect different kinds of elements: drugs, explosives, human scent of live humans. Decaying bodies produce gas, carbon dioxide, hydrogen sulphide and methane that reach the soil surface and are carried by wind. Such gases are water-soluble. Thus rain increases the soil moisture, favouring gas detection by dogs. Each step of the decomposition process thus produces a distinct scent and is discriminated by the dogs trained for it. They are trained to locate not only buried or concealed human remains, but also body fluids. Such a technique is employed for more than 30 years. Cadaver dogs are adaptable to any area without causing any damage or disturbance and can smell a cadaver from 2 weeks until several years after (12-15 years). They may also be efficient for early PMI (>3 h) (Oesterhelweg et al. 2008). This possibility however, needs further studies.

They can be used either in cases of crime investigation or in search of disaster victims. After the 2004 Tsunami in South East Asia, an air scent dog team from the French Gendarmerie worked in the debris of hotels in Phuket Island to find bodies (Fig. 13.1).This team is more commonly required to work through the French territory. Cadaver dogs can be a good help in the location and recovery of scattered body elements in a large area, reducing time and manpower (Komar 1999).

13.4.1.1.5 Aerial Observation/Photography

Aerial observation from helicopter or airplane of relief due to modification of soil compaction can also be useful. Digging modifies the layers of soil disposition and causes differences in colour contrast of soil, as in the case of older graves that show colour contrast of vegetation (type, difference of growth, etc.). Prosecution can also use satellite photography and photographs provided by government secret agencies. Aerial photography may help to cover a large search area but needs aircraft (expensive) and specialists.

Fig. 13.1 Cadaver dog and handler team from the French Gendarmerie (CNICG, Gramat) in the process of detecting buried human corpses after Tsunami in 2004 (Khao-Lak, Thailand) (photography: E. Gaudry, personal collection, France)

13.4.1.2 Intrusive Ground Search Methods

Probing with a soil-sampling stick can be used to detect differences in soil compaction causing softness. Holes made by insertion of probes in soil may not only be used to measure temperature and pH, but also for soil analysis and combustible gas vapour detection (Killam 1990). This method causes minimal damage, but is time-consuming. Excavations with heavy engines (bulldozers) can also be performed but includes risk of damage.

13.4.1.2.1 Passive Geophysical Prospecting Methods

The following methods listed below are based on the measurement of physical and chemical modification of soil after burying. They are more sophisticated and not commonly used although they do not necessarily cause deterioration to the site.

13.4.1.2.2 Soil Stratigraphy

This discipline takes into account the distinct layers in a non-disturbed soil in comparison with grave backfill mixing the different layers all together and modifying chemical and physical characteristics. It is time-consuming and its efficiency is questionable.

13.4.1.2.3 Detection of Cadaver Scent

Decaying cadavers emanate gases during a defined period (mainly ammonia, hydrogen, methane, sulphur and carbon dioxide). Such scent can be detected to localise the body in soil when external conditions enable it.

13.4.1.2.4 Magnetic Surveying

This technique is commonly used in petroleum search, detecting with a gravimeter micro-variations in the earth's magnetic field caused by burial and backfilling (difference of density). This technique can easily detect metallic objects but can be disturbed by interferences leading to false positives. This passive prospecting method requires trained personnel who are able to interpret signals and is sometimes not adapted to locate a grave.

13.4.1.3 Active Geophysical Prospecting Methods

13.4.1.3.1 Geophysical Resistivity

This sophisticated method is based on the use of conductance of an electrical current through different soil layers. Discrimination between surrounding soils of graves and disturbed soil due to digging and refilling operations can be highlighted by this technique.

13.4.1.3.2 The Ground Penetrating Radar or GPR

The ground-penetrating radar (GPR) detecting changes in soil (Kalacska and Bell 2006) provides data to search and rescue team for locating and measuring distance to trapped victims (disaster situation) but also buried objects prior to digging (Perrot et al. 2007). This material is very sensitive but expensive and requires trained people to use it and interpret signals. Such a tool is regularly used by the Signal, Images and Voices Processing Unit (IRCGN, France) under the aegis of the Criminal Investigation National Unit (Voillot et al. 2006), a crime scene investigation team belonging to the IRCGN (Fig. 13.2). Complementation by other techniques is necessary to increase significantly the efficiency of search.

Fig. 13.2 Use of Ground-Penetrating Radar (GPR) to locate illegal graves (photography, IRCGN, France)

13.4.1.3.3 Metal Detector

This material can detect conductive metal by generation of an electromagnetic field transmitted to the soil surface. When the field meets a metallic object, an eddy current is created, thus generating a loss of the power signalled by the detector. Such material is easy to use and convenient whatever the condition. But it requires trained people and can detect a metallic object at shallow depth only.

13.4.1.3.4 Remote Sensing

When a search covers a larger area, recent studies show that optical remote sensing applying the process of interaction of light with a material can be a powerful tool. Hyperspectral imagery acquired from aircraft is a powerful detection material that can provide specialists analysing such data accurate information regarding the location of a burial site simultaneously or retrospectively (Kalacska and Bell 2006).

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