USING GPR TO ESTIMATE WATER CONTENT AT THE HANFORD SITE: ACQUISITION AND PROCESSING METHODOLOGIES

Ground-penetrating radar (GPR) velocity in sediments is strongly controlled by water content. This is due to the large velocity contrast that exists between the sediment matrix (~0.07 - 0.16 m/ns) and the water (0.034 m/ns) occupying some or all of the pore space. Yet the potential for measuring and monitoring water content using GPR is not, in practice, fully realized. Several empirical and theoretical mixing equations have been formulated to provide a quantitative relationship between radar velocity and water content (e.g. Topp equation, CRIM equation, Hanai-Bruggeman mixing formula). In sandy sediments we can use these equations to estimate water content to within a few percent, and with site specific calibration the accuracy can be improved. Thus, if we can accurately measure GPR velocity, we can estimate subsurface water content.

We investigate GPR methodologies for estimating near-surface ( < 20 m) water content at four sites located throughout the Hanford complex. This includes time-lapse monitoring using rapid common-offset acquisition, and procedures for acquiring and processing continuous multi-fold data in two and three dimensions. We compute continuous water content profiles both laterally and in depth with an emphasis on accurate velocity analysis of GPR data using travel-time inversion, pre-stack migration techniques and conventional normal-moveout analysis.