DETECTION OF UNDERGROUND CAVITIES BY ELECTRO-MAGNETIC METHOD BETWEEN THE SOIL SURFACE AND THE BOREHOLE

The method described led to the detection of a cavity which, because of its depth and size, could not be investigated by other known geophysical techniques. This method carries out an electrical investigation of the ground round a borehole by means of two electrodes which are located on the surface and are transmitting an alternative current of constant amplitude; they are placed at a variable distance from the axis of the borehole and from 2 electrodes measuring the vertical component of the resulting electric field; they are lowered in the borehole at a variable depth. Details are given of the results of a small-scale model study and of in-situ measurements. Data interpretation is outlined. The method appears to be a reliable low-cost technique for use in the investigation of limited areas such as foundations or for detecting a cavity in the vicinity of a control borehole after investigations have been conducted with other methods. /TRRL/