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Title: Detection of tunnels by transient electromagnetic subsurface imaging
Authors: United States. Air Force. Ballistic Missile Office.
Geological Survey (U.S.)
James, Bryan A.
Keywords: Imaging systems in seismology
Military applications
Earth resistance measurements
Electromagnetic measurements
Publisher: Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Report
Abstract: This report presents the concept and initial theoretical testing of transient electromagnetic (TEM) imaging - a new method of TEM processing to directly produce a two-dimensional (2-D) subsurface resistivity image – for application to tunnel detection. The imaging method utilizes the fact that all of the induced subsurface current distribution variably contributes to the measured magnetic field at all measurement locations and at all times. These contributions may be calculated from subsurface current densities for any resistivity structure, using Biot-Savart's Law. These calculated contributions (a function of subsurface position, receiver position and time) are used as weighting coefficients to extract fractions of measured magnetic field values reflecting the influence of structure within the earth unaccounted for in the assumed earth model. Summing all of these fractions for each subsurface element, normalizing by a similar sum of synthetic data for the assumed earth, and multiplying by the assumed resistivity of the element, yields an estimate of the actual resistivity of that element. Contouring the resistivity estimates for all subsurface elements provides an apparent image of the subsurface resistivity distribution. At present there is no known way to automatically generate accurate pictures of the time-varying current density within the earth; an assumed earth model and calculation of the current density function for that model are necessary. These calculations are easily performed only for layered earth models. Practical implementation, therefore, requires an estimate of a background layered structure for the area of interest. The imaging method then yields an estimate of a 2-D or 3-D resistivity structure superimposed on the background layered structure.
Rights: Approved for public release; distribution is unlimited.
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