CROSSHOLE RADAR TOMOGRAPHY IN AN ALLUVIAL AQUIFER NEAR BOISE, IDAHO

Crosshole radar tomography is used increasingly to characterize the shallow subsurface and to monitor hydrologic processes. At the Boise Hydrogeophysical Research Site (BHRS), the research group at CGISS is characterizing the hydrogeophysical parameters of a cobble-and-sand, unconfined aquifer using crosshole radar tomography. Our goal is to develop methods for mapping variations in permeability by combining non-invasive geophysical data with hydrologic measurements.We have analyzed crosshole radar data acquired between closely spaced wells with a finite-difference eikonal equation based tomographic inversion method to provide estimates of electromagnetic velocity and structure between wells. We present EM velocity distributions derived from 3 well-pairs that provide a cross section through the well-filed. Water has a very slow EM velocity (0.03 m/ns) compared to typical EM velocities of the sediments at the BHRS. Below the water table, increases in porosity probably cause the slow EM velocities. The EM velocities estimated from the tomographic inversion correlate with porosity logs derived from neutron borehole geophysical tools at the BHRS. The crosshole radar method, combined with the well control, will provide an outstanding data set to characterize the heterogeneity of the subsurface beneath this alluvial aquifer and will find ways to map permeability with geophysical information.