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 m3) situated at a depth of 15 m and at a distance of 5 m from the drillhole.