Remote sensing of buried resistive inhomogeneities by electromagnetic transmission measurements between the ground surface and a borehole—Theory and experiment

A theoretical solution for the electromagnetic response of a three‐dimensional inhomogeneity in a conductive half space has been developed for horizontal electric dipole excitation. We assume that the transmitting frequency is sufficiently low that we may use the quasi‐static approximation. The solution for the subsurface electric field is obtained in the form of an integral equation system which is reduced to a matrix equation and solved numerically. This theoretical model is applied to determine the feasibility of remote sensing of underground quarries by transmission measurements between the ground surface and a borehole. The first experimental results are also given and they show that it is possible to find a cavity (of about 100 m 3 ) situated at a depth of 15 m and at a distance of 5 m from the drillhole.