Active Detection of Tunnels by Induced Seismic Spectra

A series of crosshole tests were conducted at the Colorado School of Mines Test Facility located near Idaho Springs, Colo. The primary objective of the study was to determine the modifications imposed on a seismic signal originating from a repeatable source caused by presence of a tunnel. The seismic source was a modified air gun, designed for generation of midfrequency range P-waves. In this study, 50- to 500 Hz frequencies were used. As tests were made in crosshole fashion at different elevations between the boreholes, alterations in the frequency spectrum were attributed either to material changes or the presence of anomalies. The major conclusions in order of priority were as follows: (a) The complete seismic signature is affected by presence of a tunnel. Both frequency and time domain procedures can be used to locate the tunnel. (b) Crosshole seismic tests can be used in several ways to detect the presence of a tunnel: (1) Delays in first arrival times, (2) High frequency signal loss, (3) Signal attenuation, (4) Reflected arrivals, (5) Diffracted in phase signals; (c) Both P- and S-waves can be used for tunnel detection; (d) Stresses caused by a tunnel's construction effectively increase the size of the target by approximately one radius.