Saturation-Dependent Velocity Anisotropy In Borehole Radar Data

Layered geological materials can potentially exhibit significant velocity anisotropy when the<br>thickness of the layers is small compared to the propagating wavelength. For electromagnetic (EM)<br>waves in the ground-penetrating radar (GPR) frequency range, velocity anisotropy due to fine-scale<br>layering can often be ignored in the saturated zone because changes in velocity between layers are<br>relatively small. In the vadose zone, however, significant velocity anisotropy can result in a layered<br>system due to the strong dependence of dielectric properties on saturation, and the pronounced<br>saturation heterogeneity that can exist. As the overall saturation in the vadose zone decreases, finegrained<br>layers preferentially retain water while coarse-grained layers preferentially drain; this process<br>enhances the dielectric contrast between the layers. If anisotropy is present in the subsurface and not<br>accounted for in a crosswell tomographic inversion, significant artifacts can appear in the resulting<br>tomogram and important subsurface features can thus be obscured. Analysis of crosswell GPR data<br>collected above and below the water table at a site near Abbotsford, British Columbia shows that, in the<br>vadose zone, anisotropy is more significant than in the saturated zone.