High-resoultion shallow anomaly characterization using cross-hole P- and S-wave tomography

Cross-hole seismic tomography is a high-precision method that can obtain the velocity structure between boreholes. At present, single-component geophones are mainly used for P-wave velocity estimation. However, S waves have a shorter wavelength than P waves, incurring a high-resolution tomography image. In this paper, we apply three-component cross-hole seismic tomography to shallow geological survey. First, three-component geophone is used to collect cross-hole seismic data. Secondly, the apparent velocity polarization method is used to separate the P and S wavefields from the recorded three-component waveform data. Then, a damped least-squares traveltime tomography is used to calculate the cross-hole P- and S-wave velocity structures. Finally, the distribution of Poisson's ratio between the boreholes is derived from the inverted P- and S-wave velocities. Numerical model experiments show that this method can solve some typical shallow subsurface problems, such as the detection of karst features and boulders. The proposed method is used to detect an underground air-raid shelter located in Hangzhou, Zhejiang Province, China, and the P- and S-wave velocities and the Poisson's ratio are obtained with high precision. The tomography image of the air-raid shelter agrees well with its known location. and the calculated Poisson's ratio distribution shows that the air-raid shelter may be partially filled with mucky clay. Three-component cross-hole seismic tomography achieves a high data acquisition efficiency and can simultaneously obtain shallow subsurface stratum velocities and Poisson's ratio with high precision. Thus, this technique can be used for shallow-subsurface surveys.