The relatively fast propagation of electromagnetic signals through empty man-made tunnels has played a key role in detecting deep underground tunnels using a short-pulse cross-borehole radar system. Our cross-borehole radar system measured the pulse signatures of an obliquely penetrating tunnel using eight different borehole pairs at a test site in Korea. Compared to the arrival times of the first peaks, the arrival times of the first received signals at an appropriate amplitude level provided an increasingly clear indication of the empty tunnel as its penetration angle became more oblique. A quadratic relationship between the arrival time of the first received signal and the oblique angle of the empty tunnel was obtained in pure granite.

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