HR: 0800h
AN: NG31B-0875    [Abstracts]
TI: Fresnel Zone Georadar Attenuation Difference Tomography
AU: * Johnson, T C
EM: tcj@cgiss.boisestate.edu
AF: Center for Geophysical Investigation of the Shallow Subsurface Department of Geosciences Boise State University, 1910 University Drive, Boise, ID 83725
AU: Routh, P
EM: routh@cgiss.boisestate.edu
AF: Center for Geophysical Investigation of the Shallow Subsurface Department of Geosciences Boise State University, 1910 University Drive, Boise, ID 83725
AU: Knoll, M
EM: mknoll@cgiss.boisestate.edu
AF: Center for Geophysical Investigation of the Shallow Subsurface Department of Geosciences Boise State University, 1910 University Drive, Boise, ID 83725
AB: Georadar attenuation difference tomography is useful tool for imaging temporal and spatial changes in bulk electrical conductivity due to fluid flow. The most common method of attenuation difference tomography employs the ray approximation where waves are assumed to propagate at infinite frequency. This approximation causes significant model error that generates artifacts and loss of resolution in the inverse estimates. In this paper we propose an efficient method of computing Fresnel volume sensitivities using scattering theory. These sensitivities account for finite frequency propagation and represent the physics of electromagnetic propagation more accurately than ray theory. Consequently the Fresnel theory sensitivity matrices predict data more accurately than the ray theory counterpart. We use singular value decomposition (SVD) analysis to show how and why this physical improvement allows Fresnel volume inversions to localize targets and resolve bulk conductivity changes better than ray-based inversions. By decomposing the sensitivity kernels using SVD we analyze the nature of the basis functions used to construct the inverse estimates for a synthetic case. The basis functions corresponding to the Fresnel and ray theory sensitivity kernels display a significantly different character. The Fresnel basis functions are more localized and are less oscillatory than the ray theory counterparts. The ray-based basis functions quickly become oscillatory and are dominated by the well know X-pattern. The slowly varying and localized composition of the Fresnel based basis function allows Fresnel volume inversions to resolve localized targets more accurately than ray based inversions. In addition,more higher order basis functions are required to appropriately fit the data in the ray based case. Thus we require fewer basis functions from the Fresnel operator to achieve the same misfit level. Computational efficiency of the scattering approach compared to the finite difference solution of Fresnel sensitivity will be presented in this work. Fresnel theory increases the utility of attenuation difference tomography in providing valuable flow and transport information for monitoring or estimating hydrogeologic parameters.
DE: 5144 Wave attenuation
DE: 6982 Tomography and imaging
DE: 3260 Inverse theory
DE: 0629 Inverse scattering
DE: 0689 Wave propagation (4275)
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting