Final report on passive and active low-frequency electromagnetic spectroscopy for airborne detection of underground facilities

Abstract

DISUAIMERThe vertical component of the magnetic field generated by a thee-phase Executive SummaryElectromagnetic (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 fkom the scattering of the primary field emitted fiom the active EM transmitter by the anomalous zones, fiom 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. The conductivity and permeability distribution may be associated with man-made structures that include metal and voids which, in turn, may be an element of a UGF. Passive EM provides a means of directly sensing the operation of electrical equipment or of power lines feeding them,with the potential to both locate the equipment or power lines fiom spatially sampled data and characterize the equipment &om the spectral content of the EM signal.The objective'of the research performed under this project was to advance EM induction sensor technology and software for modeling and interpreting EM induction data, and perform experiments testing EM methodology pertaining to UGF.Hardware development primarily focused on improvements to the GEM-2 instruments, including changes that have been implemented in operational units that have been built, as well as design and module prototyping for the next generation. We also have experimented with new sensor calibration techniques. 'An undertaking to derive more rigorous imaging algorithms for active source EM data has been the focus of software research efforts. In an attempt to achieve a practical compromise between the simple, crude Born approximation and the very complex rigorous solution of Maxwell's equations for the general three-dimensional scattering problem, an inversion algorithm based on the "extended Born" approximation has been developed. This scheme accounts for the field alteration generated by the conductivity anomaly in an approximate way that eliminates the need to solve a large matrix equation. The approximation is based on the assumption that the variation of the electric field in the anomalous region is small, which, unfortunately, is not always the case. Efforts to improve the approxi...