Forensic Geophysics

Abstract

Forensic geophysics involves the study, search, localisation and mapping of buried objects or elements within soil, buildings, or water using geophysics tools for legal purposes. Various geophysical techniques can be used for forensic investigations in which the targets are buried and have different dimensions. Geophysical methods have the potential to aid in the search and recovery of these targets because they can non-destructively and rapidly examine large areas where a suspect, an illegal burial, landfill or some other forensic target is hidden.

Appendix: How Does GPR Work

In principle, Ground penetrating radar consists of sending into the ground high-frequency electromagnetic pulses (10–3000 MHz) and measuring the time taken for the signal emitted from the transmitting antenna to return to the receiver after reflection and/or diffraction from any discontinuities present in the material under investigation. The round-trip time (TWT), expressed in nanoseconds – ns, allows for the measurement of the distance in time between the antennas and the “target “; this distance may be transformed into depth (meters) in the subsoil once the speed of propagation of the pulses in the medium under investigation is known.

The attenuation of these pulses in the subsoil is related to two factors: the presence of moisture in the soil and the chosen frequency. As regards the presence of moisture, a high water level in the soil makes it very conductive and this significantly attenuate the electromagnetic signal with the risk that it does not penetrate (or only partially penetrates).

The choice of frequency to be used depends on whether the transmitter is connected to an antenna (Tx) which produces a very short electromagnetic pulse (on the order of 1–10 ns). The chosen pulse duration, in turn, is linked to the antenna frequency and the required vertical resolution, or the ability to distinguish between two layers or objects close to each other. In other words, the higher the frequency of the antenna, the shorter the pulse, which results in a low signal penetration (because the attenuation also depends on the frequency) but in a higher vertical resolution.

GPR instrumentation typically has two possible configurations: the so-called bistatic configuration, in which the transmitting antenna is physically separate from the receiver; and the monostatic configuration in which the transmitting and receiving antenna coincide.

The graphical representation of GPR data is a fundamental step in the understanding and interpretation of the results. These results are presented as greyscale radargrams (or stratigraphies) of the sub-soil and modern software allows for very high visual resolution and definition. Furthermore, if the ground scans envisaged parallel profiles within a grid, then maps (or plans) may be obtained and displayed of the investigated region that represent, at various depths, not only the geometries of the buried objects but also their size, normally using a medium envelope algorithm, also known as an average amplitude envelope (Fig. 8.7).

Fig. 8.7

These are examples of how the same anomaly (in this case a wall structure) can be displayed in three different ways: a radargram or em subsoil stratigraphy (top, left), a map (or plan) at different depths in the subsurface (at the center) and a 3D reconstruction (bottom, right) acquired with the GPR

To correctly interpret a radargram , you must know how the section was acquired. The transmitted pulse from the radar antenna is not propagated in the soil or in a linear manner material like a laser, but rather it behaves as a so-called radiation cone “illuminating” the buried target also before being perpendicularly over the target itself (like a lamp burning in the darkness of a room). The diameter of the cone increases with the depth of the ground penetrating radar signal . Moreover, its dimensions also depend on the acquisition surface conditions of the antennas frequency used (for example, high frequencies constrict the diameter of the cone) (Fig. 8.8).

Fig. 8.8

The transmitting antenna (Tx) emits a signal that travels not vertically in depth as a laser, but it creates a radiation cone that ‘illuminates’ the target and is reflected towards the receiving antenna (Rx). To the right, the analogy with an illumination cone, for example purposes only

The presence in the subsoil of a void or of any more or less point-wise object produces a characteristic electromagnetic response: the diffraction hyperbola. The hyperbolic anomaly comes from the reflection of the point-source (buried target ) and occurs, as we have seen, because the energy is emitted in the form of a cone that ‘illuminates’ an area larger than the target itself. Consequently, the signal is reflected not only in a perpendicular direction to the target directly below the antennas, but also just before and just after, thanks to the additional transmission of oblique waves. Only the hyperbola peak corresponds to the actual position of the source (Fig. 8.9).

Fig. 8.9

The diffraction hyperbola, visible in this figure, derives from reflection of the intercepted target . It should be noted that only the apex of the hyperbola identifies the correct position of the buried object

The maximum horizontal resolution approximately corresponds to the footprint of the radiation cone (or illuminated area). The signal round trip time, and consequently of the depth estimation, may be calculated using the so-called calibration of the hyperbolic traces resulting from an abnormality. It is important to note, however, that it is possible to determine the depth of a target only if the speed of signal penetration through the material or materials is known.

With the exception of buried conductive materials (for example, metal, which has a high conductivity and magnetic permittivity), electromagnetic waves pass through the buried target , continuing their penetration and producing different reflections at different depths. In some cases, this effect allows us to estimate not only the depth of the upper part (top) of the object, but also its vertical dimensions (for example, in the presence of an underground tunnel, it is possible to identify not only the top of the tunnel, but also its bottom).