Detection of Clandestine Tunnels using Geophysical Techniques
Detection of Clandestine Tunnels using Geophysical Techniques
Dr. Sanjay Rana, Director, PARSAN
Forensic geophysics is a branch of forensic science and is the study, the search, the localization and the mapping of buried objects or elements beneath the soil or the water, using geophysics tools for legal purposes. There are various geophysical techniques for forensic investigations in which the targets are buried and have different dimensions (from weapons or metallic barrels to human burials and bunkers). Geophysical methods have the potential to aid the search and the recovery of these targets because they can non-destructively and rapidly investigate large areas where a suspect, illegal burial or, in general, a forensic target is hidden in the subsoil. When in the subsurface there is a contrast of physical properties between a target and the material in which it is buried, it is possible to individuate and define precisely the concealing place of the searched target. It is also possible to recognize evidences of human soil occupation or excavation, both recent and older. Forensic geophysics is an evolving technique that is gaining popularity and prestige in law enforcement.
Searched for objects obviously include clandestine graves of murder victims, but also include unmarked burials in graveyards and cemeteries, weapons used in criminal activities and environmental crime i.e. illegally dumping material.
Clandestine tunnels present an excellent contrast with surrounding soil in various physical properties like dielectric constant, resistivity, seismic wave velocity, density etc., thus becoming favorable targets to be detected by geophysical methods.
GEOPHYSICS FOR CLANDESTINE TUNNEL DETECTION:
There are various techniques to investigate the subsurface like magnetic, gravity, Ground Penetrating Radar, EM, seismic etc. Geophysical methods are inherently non-destructive and this very nature of geophysics makes it an ideal tool for tunnel detection. Following sections present appropriate geophysical methods with examples:
Ground Penetrating Radar:
Ground penetrating radar is a low cost tool for quick site investigations to locate clandestine tunnels under favorable site conditions. The GPR functions by sending high-frequency electromagnetic waves into the ground from a transmitter antenna. Some of these waves are reflected back to the surface as they encounter changes in the dielectric permittivity of the matrix through which they are traveling and are detected by a receiver antenna. The amplitude and two-way travel time of these reflections is recorded on a portable computer. This information is then used to construct a two-dimensional plot of horizontal distance versus travel time. Data collected in the field are stored on a portable computer for later analysis.
The antennas can be ground coupled (antenna directly in contact with the ground, suitable for smooth surfaces) or air couples (antenna held at a constant height from the surface, suitable for rugged terrains).
The dielectric contrast between host and target determines the reflected amplitude. Air filled tunnel has a relative permittivity of 1, whereas normal soils have relative permittivity ranging from 5-12, therefore providing an excellent contrast for reflections. Water has relative permittivity of 81, and hence water filled tunnels and voids can also be detected using GPR.
The key limitation of the method comes from high conductivity of ground, which limits the penetration of EM signals, reducing the ability of GPR to locate deep targets.
GPR is a strong investigative tool under favorable site conditions. GPR may not be appropriate in all cases; in clay-rich soils, for example. Following is a typical example of tunnel detection using GPR:
Electrical Resistivity Imaging
Electrical resistance surveys introduce an electrical current into the soil and measure the ease (or difficulty) with which this current flows within the soil. 2D electrical resistivity surveys involve multiple electrodes (typically 64) planed at a regular interval along a line, and taking measurements to build a 2D profile along the profile. Computerized equipment have made it possible to complete observations along a profile in less than 20 minutes, and hence making the method suitable for quick investigations.
Tunnels, whether air filled or water filled, have a significant contrast in electrical resistivity from surrounding soil, making these a favorable target to be detected by electrical resistivity imaging method.
Presented below are various examples obtained from electrical resistivity imaging over caves, burial chamber and tunnels.
Multichannel Analysis of Surface Waves (MASW)
First introduced in GEOPHYSICS (1999), the multichannel analysis of surface waves (MASW) method is one of the seismic survey methods evaluating the elastic condition (stiffness) of the ground. MASW first measures seismic surface waves generated from various types of seismic sources such as sledge hammer analyses the propagation velocities of those surface waves, and then finally deduces shear-wave velocity (Vs) variations below the surveyed area that is most responsible for the analyzed propagation velocity pattern of surface waves.
Tunnels have a significant contrast in shear wave velocities compared to surrounding soil, making MASW a good method to detect voids and tunnels, as shown in example hereunder:
Micro-gravity surveys measure difference in density of subsurface, and are used regularly to detect low or high density materials in subsurface. An air filled tunnel or void is an excellent target for micro gravity due to its low density.
The survey is performed in a grid pattern, and required careful data acquisition, processing and interpretation. Due to slow process of acquisition, the method should be used only in cases where other methods fail to deliver due to site conditions and geology restraints. Following is an example of 3D perspective image showing only the low density values, delineating the direction of the tunnel.
Geophysical methods can be used effectively by law enforcement and defense personnel for locating clandestine tunnels. Geophysical methods are most successful as part of an integrated and flexible research design. The tools should be chosen based on geology of the area and site conditions. It is always recommended to use more than just a single technique to uniquely resolve the issue.
This document has been prepared to provide a detailed insight to reader on the potential of geophysical methods to investigate sites for clandestine tunnels. Various pictures and case studies have been used from different resources, and acknowledgement has been given wherever possible. The usage is for information dissemination purpose only, and not to claim copyright on any such material, and without any copyright violation.
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