Sonar Detection for Karst Cavities and Fracture Under Bored Pile by Generalized S-Transform

The existing of karst cavities under the bored-pile bottom is of great danger to the quality and stability of the project. Sonar detection is a new technology to ensure the quality of bedrock in construction. Because of the complexity of pile hole materials, detecting the fractures and cavities is still a technical challenge which needs to be studied further. The generalized S-transform was adopted to extract the arrival times of the reflection from the karst cavities at a certain receiver, which can be used to calculate the depth and size of karst cave. Seven models with karst cavities and fractures with different depths were used to verify the feasibility and the effect of the depth of fracture to the detecting karst cavity with the generalized S-Transform. A high-orders staggered-grid finite-difference method was adopted in wave field modelling to obtain the synthetic test signal. The results show that it is feasible and effective to detect karst cavities and fracture under bored pile using the generalized S-transform. The changes caused by the reflected wave significantly appear as peaks in the time-frequency spectrum calculated by generalized S-Transform, which has great advantage in identification for weak reflections.