Downhole radar measurements were obtained in a borehole-borehole, borehole-surface, and borehole-outcrop configurations using a 60 MHz RAMAC radar system. Tomograms of subsurface velocity and attenuation structure were obtained between a number of boreholes, as well as directional and omni-directional reflections in a few deeper wells. In general, attenuation of the radar signal was relatively high, restricting maximum penetration to approximately 33 m. Tomograms from closely spaced boreholes showed flow contacts and intraflow fracture/texture variations. A series of tomograms, obtained during and after a water infiltration test located between the two boreholes, indicated the extent of water infiltration as a function of time.
Lines of GPR data were collected over many of the boreholes used for the downhole tomography. Penetration depths using 50 MHz antennas ranged from 10-15 m (two-way travel distance 20-30 m) over exposed basalt. This surface sediment layers decreased penetration depths for all antenna frequencies. As in the crosshole tomography, GPR plots showed flow contacts and textural variations within the basalt flows. Ground truth for most of the GPR lines and all borehole interpretations was obtained by comparing the radar data with geologic sections constructed with geophysical and/or video logs from the wells on or near the lines and from outcrop mapping of the surface and an adjacent cliff face.
Given the results from these tests, it is apparent that radar technologies hold promise for both near-surface and deeper characterization of the fractured basalt environments. Borehole tomography may also have a potential for fluid migration beneath sensitive facilities given a proper borehole placement and separation.