This paper presents a case study using various geophysical techniques in unconsolidated sediments where shallow faulting is probable and most likely affects ground‐water flow. Wenner and dipole‐dipole resistivity, ground penetrating radar (GPR), and high resolution reflection seismic data are compared with cone penetrometer (CPT) information, existing monitoring well data and models of anticipated fault styles. The results of this study suggest that (1) the CPT study, combined with monitoring well data may suggest that discontinuities in corrrelatable zones indicate that faulting is present; (2) the addition of Wenner and dipole‐dipole data may further suggest that offset zones exist in the shallow subsurface but make it difficult to map specific faults; (3) the high resolution seismic data will image faults to within a few meters of the surface but cannot identify faulting on the scale of our models; (4) offset 100 MHz and 200 MHz common mid‐point (CMP) GPR will image features that may be fault planes and strands similar to the models; and (5) the combination of all the tools in this study, particularly the GPR and seismic, may allow for the mapping of small‐scale, shallow faults in unconsolidated sediments that affect ground‐water flow.

As part of a multiscale hydrogeophysical and modeling study, a pseudo three-dimensional (3-D) seismic survey was conducted over a contaminant plume at P area, Savannah River site (South Carolina), to enhance the existing geologic model by resolving uncertainties in the lithostratigraphic sequence. The geometry of the dissolved phase trichloroethylene plume, based on initial site characterization, appears to be confined to a narrow corridor within the Eocene sand overlying a clay unit approximately 25 m (82 ft) below land surface. Processing the seismic data as a 3-D data volume instead of a series of closely spaced two-dimensional lines allowed for better interpretation of the target horizons, the lower clay, and the sand above the clay. Calibrating the seismic data with existing borehole geophysical logs, core data as well as vertical seismic profiling (VSP) data allowed the seismic data to be inverted from two-way traveltime to depth, thereby facilitating full integration of the seismic data into a solid earth model that is the basic part of a site conceptual model. The outcome was the production of realistic horizon surface maps that show that two channel complexes are located on the section, which are not present in the conceptual model, and that the upper and middle clays are not laterally continuous as previously thought. The geometry of the primary channel has been transposed over the map view of the plume to investigate potential relationships between the shape of the plume and the presence of the channel.

Borehole dilution tests have been used for characterization of aquifer hydrogeologic properties for several decades. Based on the principles of borehole dilution tests, we conducted what more appropriately may be considered a wellbore fluid displacement test in a limestone aquifer in South Carolina. Our study area is a quarry in the coastal plain of South Carolina. Using a solution of reagent grade NaCl and deionized H(2)O as a tracer, a brine slug was introduced into a 5 cm (2 in.) diameter Schedule 40 PVC well with a 6-m slotted screen at the bottom. Immediately following addition of the brine, a recording electrical conductivity (EC) sensor was placed in the well opposite the screen and set to record EC in 2-min intervals for 5 days. An alternative to previous methods for analyzing data from wellbore brine displacement tests was developed. Results were analyzed using SEAWAT-2000 to account for the density dependency of brine flow and transport. The high spatial resolution, three-dimensional numerical simulation enabled direct incorporation of well construction peculiarities, including the sand pack and length of screen, in the data analysis. Hydraulic conductivity, effective porosity, and longitudinal dispersivity were adjusted in the simulation model until the best match of simulated wellbore fluid concentrations to observed concentrations was achieved. Using this procedure, we were able to obtain a very close agreement between observed and simulated concentrations and, hence, reliable estimates of the hydrogeologic properties of the aquifer in the vicinity of the test well.

Recent advances in seismic reflection amplitude analysis (e.g., amplitude versus offset-AVO, bright spot mapping) technology to directly detect the presence of subsurface DNAPL (e.g., CCl4) were applied to 216-Z-9 crib, 200 West Area, DOE Hanford Site, Washington. Modeling to determine what type of anomaly might be present was performed. Model results were incorporated in the interpretation of the seismic data to determine the location of any seismic amplitude anomalies associated with the presence of high concentrations of CCl4. Seismic reflection profiles were collected and analyzed for the presence of DNAPL. Structure contour maps of the contact between the Hanford fine unit and the Plio/Pleistocene unit and between the Plio/Pleistocene unit and the caliche layer were interpreted to determine potential DNAPL flow direction. Models indicate that the contact between the Plio/Pleistocene unit and the caliche should have a positive reflection coefficient. When high concentrations of CCl4 are present, the reflection coefficient of this interface displays a noticeable positive increase in the seismic amplitude (i.e., bright spot). Amplitude data contoured on the Plio/Pleistocene-caliche boundary display high values indicating the presence of DNAPL to the north and east of the crib area. The seismic data agree well with the well control in areas of high concentrations of CCl4.

Field experiments were conducted to determine the location and distribution of subsurface DNAPL contamination at two DOE sites by use of two-dimensional, high-resolution seismic reflection surveys and borehole geophysical data. These studies make use of seismic reflection amplitude versus offset (AVO) technology to directly detect the presence of subsurface free-phase DNAPL.The research was conducted at Savannah River Site, SC and Hanford Site, WA. At each of these sites research consisted of site evaluation, seismic model studies, seismic acquisition, processing, and interpretation. The seismic model studies were undertaken prior to field acquisition to determine the likelihood of an AVO response from the DNAPL. The full Zoeppritz equations were used to create the model responses.At M-Area, Savannah River Site the model data show that at the base of the DNAPL plume the seismic amplitude should flip polarity on the far offsets (Class 2 AVO). The 2D seismic reflection field data processed into near and far offset stacks indicate such anomalies occur at the depth where the known DNAPL occurs. At the 200 West Area, Hanford Site the DNAPL is suspected to pool at two different stratigraphic intervals. The model data show that at the upper interval, the Hanford Fine/Plio-Pleistocene boundary, amplitude should decrease along the boundary if DNAPL is present (Class 1 AVO anomaly dim-out). At the lower boundary the models show that, if DNAPL is present, the seismic amplitude should become very negative (Class 3 AVO bright spot). A series of 2D seismic lines were acquired and processed. In the areas where DNAPL was suspected to occur (high concentrations measured in wells) seismic amplitude was found to decrease dramatically along the Hanford Fine/Plio-Pleistocene contact and along the Plio-Pleistocene/caliche contact the seismic amplitudes became even more negative consistent with the model study.