The Use of Common-Offset and Multiple-Offset GPR methods for Forensic Investigations
F.S. Hansen (TU Delft - Civil Engineering & Geosciences)
Dominique J.M. Tillard – Mentor (TU Delft - Geo-engineering)
D. Draganov – Graduation committee member (TU Delft - Applied Geophysics and Petrophysics)
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Abstract
This thesis is part of a longer series of research into the use of geophysical methods for inves-tigation for forensic purposes, a collaboration between the Delft University of Technology (TU Delft) and the Netherlands Forensic Institute (NFI). In this investigation, the difference between a grave containing a human body and a refilled empty pit is explored at the Amsterdam Research Initiative for Subsurface Taphonomy and Anthropology (ARISTA) facility, using common-offset GPR data collection. In addition, common-offset data is gathered at a test site at TU Delft, in order to redefine the locations of previously buried targets. Multiple-offset GPR datasets are also col-lected at both sites, one of which is processed using electromagnetic interferometry (EMI) and adaptive subtraction (AS) in an attempt to remove direct waves.
The positions of the targets at the TU Delft site were redefined, but with some questions as to whether the site has been altered in the past year without the knowledge of the author. High lev-els of interference in the ARISTA facility data due to close proximity to various metal and plastic objects makes it difficult to determine the true differences caused by the presence of the cadaver. The author suggests using a 500-MHz antenna for further investigations at the site due to high wave velocity which leads to a low resolution when using a 250-MHz antenna, and due to more homogeneous soil at the ARISTA facility. The optimal procedure for EMI+AS is discussed, and sug-gested to be the use of a bandpass filter to remove very high and low frequencies from the raw data prior to EMI. The method is shown to be reasonably effective, especially when the data is strongly impacted by the presence of direct waves, where simply topmuting the data would re-move too much information. A script was prepared in MATLAB which has been optimised for the application of EMI to GPR data, and further scripts were prepared for use in Seismic Unix for the purpose of AS, in the hopes that others may find these a useful beginning to further applications of this method.