Borehole radar tomography and dielectric logging at the savannah river site

摘要

The subsurface sediments near the M-Area Settling Basin at the Savannah River Site (SRS), South Carolina, have been contaminated with dense nonaqueous phase liquids (DNAPLs). As part of an evaluation of geophysical techniques that could be used to monitor remediation of DNAPLs, we tested two borehole-to-borehole radar systems and two dielectric logging tools. With the first radar system the waves did not penetrate the clay layers, making conventional tomographic processing impossible. In June, 1997, we tested a more powerful, lower frequency, radar system in the same wells and penetrated the clay layers. The travel times ere processed with tomographic methods, and the variations in the velocities correlate with known geology. Numerical simulations were run to test for effects of wave-guiding between the clay layers. Although we did not find wave-guiding to be strong enough to invalidate the tomographic processing, we did find the tomographic inversions to be quite sensitive to errors in picked travel times for closely spaced wells. We also tested two dielectric logging tools. We conclude that dielectric logging and radar tomography are both useful for mapping stratigraphy, that dielectric logging should be useful for remediation monitoring, and that crosswell radar may also be useful for remediation monitoring, if appropriately done.