Tunnel signature prediction for a cross‐borehole seismic survey

Abstract

Seismic location of tunnels or voids with a cross‐borehole survey is examined with field data and theory. The field data were taken at a site with a 2.2-m high by 2.7-m wide, roughly rectangular cross‐section tunnel, using a newly developed 1 to 5 kHz system employing a P‐wave sparker source. The synthetic records were obtained using a 2.5-D boundary‐valued solution for an explosive point source near a cylindrical void, and the solution was evaluated with the method of steepest descent. The synthetic waveforms compared well to the field data; both showed a maximum reduction of amplitude in the tunnel shadow of 8 dB and a maximum first arrival delay of 0.1 ms. Additional theoretical modeling was used to examine the variations of the received signals with tunnel size and frequency and showed amplitude reduction increased with frequency and tunnel size. Calculations showed that S‐waves scattered from the tunnel are more than 20 dB smaller than the primary P‐wave on hydrophones and more than 12 dB smaller on particle velocity sensors and so could be difficult to see in field data. The close comparison of synthetic waveforms to the field data indicate that the cylindrical model can be used to model data for roughly square cross‐section tunnels or voids, as well as for circular cross‐section tunnels, and thus is useful for data interpretation and survey planning.

Optimization of Array Configurations and Panel Combinations for the Detection and Imaging of Abandoned Mineshafts using 3D Cross-Hole Electrical Resistivity Tomography

“…However cross-borehole tomography would seem to be an ideal technique to determine whether a particular region, for example directly beneath a building, contained hidden void spaces. There are various geophysical methods that can be employed in a cross-hole configuration, but only seismic tomography has been extensively used and demonstrated in this context (Nath, 2004;Orlowsky et al, 2003;Misiek et al, 2001;Rechtien et al, 1995). We were able to find just one study that describes the use of cross-hole electrical resistivity tomography (ERT) to detect uncharted mines (Maillol et al, 1999).…”

A current look at geophysical detection of illicit tunnels

“…Rechtien et al (1995) demonstrate the use of cross-borehole seismic methods to detect a tunnel 2.7 m × 2.2 m in South Korea by using field and synthetic data. Greenfield et al (1994) use synthetic data to interpret and validate a cross-borehole radar anomaly from a tunnel in South Korea.…”

In-theater seismic acquisition: Operational examples from a tunnel detection team

“…Photograph of the inside of the tunnel discovered at the Sarposa prison in southern Afghanistan in 2011 that facilitated the escape of more than 480 prisoners. There are numerous examples of both general void detection (Branham and Steeples, 1988;Dobecki, 1988;Inazaki et al, 1998;Peterie et al, 2009) and tunnel detection (Belfer et al, 1988;Rechtien et al, 1995;Llopis et al, 2005;Tucker et al, 2007;Walters et al, 2007;Walters et al, 2009) using shallow seismic methods. Seismic techniques intuitively make sense based on the large contrast in material properties between an air-filled cavity and the surrounding geologic medium.…”