Joint tomography of multi-cross-hole and borehole-to-surface seismic data for karst detection

Abstract

Two-dimensional (2D) cross-hole seismic tomography is widely used in karst exploration. However, this method only obtains subsurface geological structures from the profiles of two adjacent wells and cannot acquire geological information from the lateral profile direction. The tomographic imaging area and accuracy are also limited owing to sparse ray density in the near-surface area. This approach is therefore not conducive to the spatial identification and evaluation of subsurface karst structures. To address this problem, we conducted multi-hole combined seismic tomography experiments and obtained high-density and high-quality seismic spatial data using an optimized observation system and acquisition parameters. We propose a new method of joint tomography of multi-cross-hole and borehole-to-surface seismic data. We performed 2D seismic tomography inversion by combining cross-hole data with borehole and surface data based on the theory of seismic tomography, and then used the data fusing from multiple wells based on the theory of kriging method. A pseudo-three-dimensional (3D) spatial velocity field of the experimental site is established. The results show that (1) the combined borehole and surface data provide richer ray-tracing information than the cross-hole dataset alone, (2) the pseudo-3D spatial velocity field and spatial distribution of seismic anomalies (karst) can be obtained using the proposed method, and (3) a comparison with actual borehole logging data verifies the effectiveness and accuracy of the proposed method in obtaining subsurface geological structures and detecting the spatial distribution of karst, which has significant practical and application potential.