DETECTION OF UNDERGROUND CAVITIES WITH MONOFREQUENCY ELECTROMAGNETIC TOMOGRAPHY BETWEEN BOREHOLES IN THE FREQUENCY-RANGE 100MHZ TO 1 GHZ

In order to detect underground cavities, we have designed a geophysica l method based on the interaction of a monochromatic electromagnetic w ave in the frequency band 100 MHz to 1 GHz with the ground situated be tween two boreholes. Three different approaches are involved in the de sign of this EM tomographic method. 1. The application of an 'exact' t heory is used to calculate artificial data, based on an integral equat ion method. These data are inverted using a tomographic algorithm whos e basic assumptions are rather coarse. The results show that, however, cavities can very well be detected and their position recovered. 2. D ata are obtained with a physical reduced-scale model in a water tank, in which all parameters are known in advance. The inversion process co nfirms the validity of the method. 3. Underground cavities are actuall y detected. The above approaches are described and discussed and resul ts are shown. The equipment involved and its operation is quite simple : the surface devices are commercially available and only the transmit ter and receiver antennae must be specially built. The method is shown to be quite efficient and its cost should be reasonably low.