Distortion of pulse signatures penetrated obliquely through tunnel in a cross-borehole pulse radar

The finite-difference time-domain method is employed to calculate the radar signatures of an obliquely penetrating tunnel. The relatively faster propagation and stronger distortion of the electromagnetic pulse are discovered due to the existence of an empty tunnel as its penetration angle becomes more oblique. To analyze the different behaviors of two arrival times, the electrical fields in 3 different tracks as transmitter side, tunnel inside and receiver side are collected around tunnel. Those data are analyzed in time and frequency domains. It renders the arrival time of the first peak to be increased relatively slower than that of the first received signal.