HR: 0830h
AN: H31B-0446    [PDF]
TI: Improving Estimates of Subsurface Water Content by Accounting for Saturation-Related Anisotropy
AU: * Irving, J
EM: jdirving@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Room 360, Mitchell Building, Stanford, CA 94305-2215 United States
AU: Knight, R
EM: rknight@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Room 360, Mitchell Building, Stanford, CA 94305-2215 United States
AB: Ground-penetrating radar tomographic methods are commonly used to obtain EM wave velocities in the subsurface, which are then transformed into estimates of soil water content using rock physics relationships. In most cases, velocity anisotropy is not accounted for in this process. That is, the earth is usually represented as a collection of isotropic, constant-velocity cells during the tomographic inversion of radar travel time data, and anisotropy information is not used when transforming velocities to water content. In cases where velocity anisotropy in the subsurface is significant, however, the inversion of travel time data under an isotropic assumption can result in serious artifacts; false heterogeneity can be produced because we attempt to fit an anisotropic medium with a series of isotropic cells. Further, not providing anisotropy information in the transformation from velocity to water content results in greater uncertainty in the water content values obtained. By accounting for velocity anisotropy in both the tomographic inversion and rock physics steps, we can improve estimates of subsurface water content obtained from radar tomographic data. A material consisting of thin isotropic layers is equivalent, in the long wavelength limit, to a homogeneous but anisotropic medium. Numerical modeling of 1-D coarse/fine layered systems under the assumptions of effective medium theory and capillary equilibrium indicates that, in the saturated zone, velocity anisotropy is likely unimportant because changes in velocity between layers result largely from porosity differences, which are minor. In the vadose zone, however, significant velocity anisotropy can result in layered systems due to the strong dependence of dielectric properties on saturation, and the pronounced saturation heterogeneity that can exist. As the overall saturation in the vadose zone decreases, fine-grained layers preferentially retain water while coarse-grained layers preferentially drain; this enhances the velocity contrast between the layers. Here, we investigate these results in the field through the anisotropic inversion of crosswell radar data collected near Abbotsford, British Columbia. We wish to examine under which circumstances it is important to account for velocity anisotropy in the tomographic determination of water content. Using an anisotropic, 1.5-D inversion code that incorporates ray tracing to determine ray paths from source to receiver, we compare velocity anisotropy in the vadose zone with that in the saturated zone.
DE: 0689 Wave propagation (4275)
DE: 1719 Hydrology
DE: 1866 Soil moisture
SC: Hydrology [H]
MN: 2003 Fall Meeting