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Analysis of dispersion of pulse signals in underground tunnels using finite-difference time domain and short-time Fourier transform

The dispersion of pulse signals of a cross borehole radar simulator for tunnel detection was analyzed using FDTD and STFT. Two simulation cases with extremely different dispersion properties were simulated by the FDTD method. In the two cases, the trace path between the transmitter and receiver penetrated an air-filled tunnel perpendicularly and obliquely, respectively. The recorded fields in the cross sectional plane during FDTD computation were decomposed into several frequency components by STFT. By analyzing the propagation behaviors of frequency components, the different dispersion characteristics according to the tunnel's oblique angle variation are visually rendered in detail.