CROSSHOLE RADAR TOMOGRAPHY IN AN ALLUVIAL AQUIFER NEAR BOISE, IDAHO

To determine the distribution of heterogeneities in an unconfined aquifer in Boise, ID, we compute radar tomograms for three adjacent well pairs. The fluvial deposits consist of unconsolidated cobbles and sands. We used a curved-ray, finite-difference approximation to the eikonal equation to generate the forward model. The inversion uses a linearized, iterative scheme to determine the slowness distribution from the first arrival traveltimes. The tomograms consist of a sequence of layers representing the saturated aquifer. The velocities in this saturated zone range between 0.06 to 0.10 m/ns. We use a variety of methods to assess the reliability of our velocity models. Finally, we compare our results to neutronderived porosity logs in the wells used for the tomograms. The comparison shows that the trends in porosity derived from the tomograms match well with the trends in porosity measured with the neutron probe.