Final Report on Passive and Active Low-Frequency Electromagnetic Spectroscopy for Airborne Detection of Underground Facilities

摘要

Electromagnetic (EM) induction techniques can potentially provide information that will contribute to the delineation and characterization of Underground Facilities (UGF) which produce weapons of mass destruction or perform other covert activities deemed threatening to our national security. EM geophysical techniques, used widely in mineral exploration, geohydrology, engineering, and environmental assessment, consists of measurements of fluctuating magnetic fields with a loop antenna at the surface of the earth above or in a bore hole near the subsurkce target area. The fluctuating fields may be natural (magnetotellurics), excited by a controlled source which is part of the EM instrumentation (active EM), or emitted by an electrically active target (passive EM). Active EM provides information about the conductivity andor permeability distribution in the subsurface resulting from the scattering of the primary field emitted from the active EM transmitter by the anomalous zones, from induced eddy currents in highly conductive targets, magnetic polarition of permeable targets, perturbation of background current flow in a conductive earth, or a combination of these phenomena.