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Radar attenuation tomography using the centroid frequency downshift method

Lanbo Liu
Department of Geology and Geophysics, 354 Mansfield Road, U-45, University of Connecticut, Storrs, CT 06269-2045, USA

John W. Lane,
Branch of Geophysical Applications and Support, U.S. Geological Survey, 11 Sherman Place, U-5015, Storrs, CT 06269, USA

Youli Quan
Department of Geophysics, Stanford University, Stanford, CA 94305, USA

Abstract

A method for tomographically estimating electromagnetic EM wave attenuation based on analysis of centroid frequency downshift CFDS of impulse radar signals is described and applied to cross-hole radar data. The method is based on a constant-Q model, which assumes a linear frequency dependence of attenuation for EM wave propagation above the transition frequency. The method uses the CFDS to construct the projection function. In comparison with other methods for estimating attenuation, the CFDS method is relatively insensitive to the effects of geometric spreading, instrument response, and antenna coupling and radiation pattern, but requires the data to be broadband so that the frequency shift and variance can be easily measured. The method is well-suited for difference tomography experiments using electrically conductive tracers. The CFDS method was tested using cross-hole radar data collected at the U.S. Geological Survey Fractured Rock Research Site at Mirror Lake, New Hampshire NH during a saline-tracer injection experiment. The attenuation-difference tomogram created with the CFDS method outlines the spatial distribution of saline tracer within the tomography plane.


Final copy as submitted to Journal of Applied Geophysics for publication as: Liu, Lanbo, Lane, J.W., Jr., and Quan, Youli, 1998, Radar attenuation tomography using the centroid frequency downshift method: Journal of Applied Geophysics, v.40, p.105-116.


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