Measuring water content heterogeneity using multifold GPR with reflection tomography

Continuous multioffset acquisition of ground penetrating radar (GPR) data provides the capability to measure the lateral and vertical distribution of soil moisture. Multioffset data enable measurement of radar velocity, which in turn allows the estimation of soil moisture through an appropriate petrophysical relationship. Although rarely used in GPR investigations, reflection tomography coupled with prestack depth migration has the ability to measure lateral velocity variations with much greater resolution and accuracy than conventional methods of velocity analysis. I used reflection tomography in the post-migration domain to estimate radar velocity and the Topp equation to estimate subsurface moisture distribution in two and three dimensions. At a contaminated site near a former refinery I identified a near-vertical boundary separating coarse-grained sands and gravels from a unit containing a high fraction of silts and clays. At a chlorinated solvent waste site, I found significant heterogeneity in the moisture content distribution despite apparent homogeneity indicated by direct push methods.