Attenuation-Difference Tomography of Crosswell Radar Data Using Fresnel Theory

The traditional approach to attenuation-difference tomography employs the ray approximation where waves are assumed to propagate at infinite frequency. The ray approximation causes significant model error that generates artifacts and loss of resolution in tomographic images. In this paper, we use finite frequency (Fresnel volume) physics to represent wave propagation and propose an efficient method of computing Fresnel volume sensitivities using scattering theory. These sensitivities represent the physics of electromagnetic propagation more accurately than ray theory and thereby provide better data prediction than ray-based sensitivities. We apply this methodology to synthetic and real time-lapse radar attenuation data acquired in a mesoscale hydrogeophysical research site in Boise, Idaho. Both synthetic and field results show that Fresnel theory produces more reliable images of subsurface conductivity changes in comparison to ray theory.