Underground phenomena imaging using electromagnetic geophysical tomography: Electromagnetic geophysical tomography is a new technique used to characterize prospective urban subways, locating tunnels and natural caverns, mapping fractures in rock, and in d

Electromagnetic geophysical tomography, an adaptation of the CAT (computerized axial tomography) concept, was developed at the Lawrence Livermore National Laboratory to investigate rock below the ground surface. Electromagnetic geophysical tomography (EGT) can be used to characterize prospective urban subways, locating tunnels and natural caverns, mapping fractures in rock, and delineating groundwater pathways. In most applications where EGT systems are used, the sensors (antennas) are placed within two boreholes. The transmission characteristics of radio waves are measured between numerous transmitter and receiver locations. Since the electrical properties of rock mass affect radio waves passing through it, it is possible to calculate the electromagnetic properties of the rock sampled by the waves. By moving the antennas up and down the boreholes, multiple views of the rock are obtained. The EGT image is reconstructed using a computer and is then displayed on a CRT and analyzed. The image provides a detailed map of the electrical properties of the rock. These images enable an interpreter to understand the geologic structures and materials present.