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 delineating groundwater pathways

Computerized axial tomography (CAT) has been successfully used in medical diagnostics for nearly a decade. Doctors routinely use CAT images to locate and evaluate tumors or other abnormalities in the head and abdomen of patients. In the mid-1970s, an adaptation of the CAT concept was developed at the Lawrence Livermore National Laboratory to investigate rock below the ground surface [Fig. 1]. This geophysical exploration technique has been employed in a wide range of applications including characterizing a prospective urban transit station prior to excavation.