Using high resolution borehole geophysics for DNAPL-detection and environmental site characterization

This dissertation investigates the applicability of high-resolution borehole geophysical measurements for the in situ detection of DNAPLs. In pursuit of this goal, a series of crosswell seismic and crosswell radar datasets were acquired at a site of known DNAPL contamination, the US DOE Pinellas facility. The combination of an extensive crosswell dataset, a large suite of well logs, and continuous core sampling allowed exploration of the possible petrophysical signatures of DNAPL saturation on both the laboratory and field scale.; To process the Pinellas datasets, a new class of adaptive traveltime tomography algorithms were developed, capable of flexibly accommodating regions of poor ray coverage and highly irregular source/receiver geometries. These techniques, based on greedy mesh refinement, guarantee that the resulting inverse problem fulfills a priori model resolution constraints. The method improves on previously developed algorithms through the use of a new refinement heuristic which results in higher quality tomographic meshes. Using this novel approach to traveltime tomography, several multiwell seismic and radar profiles acquired at the Pinellas site were simultaneously inverted to generate high-resolution velocity models. The tomographic imaging results were integrated with logging data to generate a detailed map of the primary lithologic and hydrogeologic units at the site. Although non-aqueous contaminants were not conclusively identified at this facility, several anomalous regions with high radar velocities and high levels of seismic attenuation were detected below the water table; these signatures are consistent with soils partially saturated with either gas or DNAPLs.; The second component of the site investigation was an extensive laboratory study examining the dielectric and acoustic properties of Pinellas core materials partially saturated with trichloroethylene (TCE). High frequency dielectric measurements were acquired using the time-domain reflectometry method; these experiments quantified the relationship between TCE saturation and dielectric constant for several soil samples. A series of ultrasonic transmission measurements explored similar relationships between DNAPL levels and P-wave velocity at in situ stress conditions. The combined results of these laboratory experiments provide insight into the possible joint seismic/radar signatures of subsurface DNAPLs and partial confirmation of existing effective medium models used to describe such systems.