Joint inversion of surface and borehole seismic data using adjoint-state traveltime tomography for karst exploration

Karst poses a major safety risk in engineering construction within limestone areas, yet accurate and efficient detection remains challenging for conventional geophysical prospecting methods. An expressway bridge is planning to be broadened out, the argillaceous limestone bedrock covered by ~40 m thick Quaternary overburden layer is designed as the bearing stratum for pile foundation. Karst is moderately developed in the bedrock. This study employs a recently developed eikonal-equation based adjoint state traveltime tomography method to analyze karst characteristics. The advantages of this novel tomography method include its independence of ray tracing, high accuracy in complex heterogeneous media, and computational efficiency. The joint inversion of surface and borehole seismic data further enhances resolution and reliability. Totally 225 nodal geophones were deployed along a 236.4m-long two-dimensional line, intersecting a borehole that reveals a semi-filled karst cave at the depth range of 42.2-46.7 m. The seismic results demonstrate three horizontally adjacent yet disconnected karst caves within the depth range of 40-50 m below the survey line. Each cave is accompanied by a subvertical low-velocity channel extending to the base of the silty clay layer. The width of the channel appears proportional to the size of the cave, suggesting its role as a runoff passage connecting the karst cave and overlying permeable gravel layer.