Detecting Karst Voids Based on Dominant Frequencies of Seismic Profiles

The presence of near-surface karst voids is an extremely difficult issue in the construction of a high-speed rail (HSR) foundation. Seismic constant-offset profile (COP) method is one of the shallow geophysics technologies which may be used for the detection of karst voids. Although a COP image does not directly reveal the characters related to the anomalies in a karst terrain, the dominant frequency of the COP image in a karst terrain is significantly lower than the dominant frequency over the background without karstification or voids. This dominant-frequency anomaly is due to the strong attenuation effect when seismic waves propagate through any karst voids. Thus, we propose using the dominant-frequency anomalies of the COP image to directly detect near-surface karst voids in a karst terrain. First, we generate a high-resolution time–frequency spectrum for each COP trace, using the modified Wigner-Ville distribution (WVD) algorithm which combines WVD with a multichannel maximum entropy method. Second, we estimate a high-precision dominant-frequency function which varies along the reflection time, based on the corresponding high-resolution time–frequency spectrum. Finally, we detect the geological anomalies by analyzing low-frequency distributions in the dominant-frequency image for all traces. We demonstrate this procedure with a case study for the detection of karst voids within the high-speed rail foundation in a karst terrain, and verify the interpretation of hidden voids, cavities, clays and peats directly with drilling cores.