An Emerging Method Using Electromagnetic Wave Computed Tomography for the Detection of Karst Caves

Electromagnetic wave CT technology, serving as an emerging geophysical method, is an effective way in detecting underground karst caves. In this paper, a numerical model for electromagnetic wave CT detection was established using Finite Difference Time Domain algorithm to interpret the propagation mechanism of electromagnetic wave in karst cave detection. The saturated sands with high porosity are adopted as the filling medium of the cave while the limestone is selected as the surrounding rock in this simulation. Propagation rules obtained from numerical simulation are drawn as follows: (1) when electromagnetic wave propagates inside the karst cave, refraction, reflection, diffraction, and obstacle gain behavior will occur. (2) The refraction phenomenon has relatively more influence on the energy absorption of electromagnetic wave while the disturbance of the waveform is principally attributed to the reflection phenomenon. (3) The influence of the factors such as the type of filling material and water content should be taken into account if the absorption coefficient of karst caves to electromagnetic wave is inconsistent. Furthermore, the tomographic images gained from experiment are interpreted according to the simulation conclusions. It is found that the analysis of tomographic images show general agreement with the previous exploration results, so it is feasible to use electromagnetic wave CT technology in karst cave exploration. The research presented herein aimed to serve as a based theory for analyzing tomography results in the field of karst caves exploration.