Developing a robust geologic conceptual model using pseudo 3-D P-wave seismic reflection data

Abstract

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.

Structural and stratigraphic control on the migration of a contaminant plume at the P Reactor area, Savannah River site, South Carolina

“…The three paleochannels were interpreted on the seismic section and they appear to incise the top of the Tobacco Road Formation (cyan, brown, and violet thick dashed lines in Figure 6b). Moreover, these paleochannel structures coincide with those imaged and documented by Addison et al (2009). Based on the cross section derived from the CPT and natural gamma logs (Figure 4), the hydrogeologic facies appear to be continuous laterally across the study area.…”

“…An uninterpreted (a) and interpreted (b) part of the shallow seismic reflection profile trending northwest-southeast in the study site confirming the presence of paleochannel complexes in the first 30+ m (98+ ft) depicted by the cyan, brown, and violet thick dashed lines (from Addison et al, 2009) and suggesting deeper faulting. Electrical resistivity (CPT-L3) and natural gamma (POS-2) logs used to calibrate the radar reflective horizons are also shown.…”

“…However, the hydraulic gradient calculated from potentiometric surface data between the piezometers (mapped in Figure 2) indicates a convergence of groundwater flow near the area west of the POS wells (Figure 16). This area of convergence appears to be within the paleochannel 2 imaged in the SSR section and by Addison et al (2009). The groundwater flow, in a local spatial scale, could possibly have a preferred pathway trough paleochannel 2, hence, the plume narrowing down away from the source and flow updip stratigraphic bedding.…”

“…Addison et al (2009) indicated that the paleochannel complexes at the study site are acting as conduits for the plume migration, thus, the slight turn of the plume to the south-southwest near the POS wells. However, the hydraulic gradient calculated from potentiometric surface data between the piezometers (mapped in Figure 2) indicates a convergence of groundwater flow near the area west of the POS wells (Figure 16).…”

“…Structural and Stratigraphic Control on the Migration of a Contaminant Plume Figure 4. Cone penetrometer tests (CPT) and natural gamma log correlation with hydrostratigraphic units and hydrogeologic facies for our study area (modified from Addison et al, 2009). The water table is approximately at 13-m (43-ft) depth dividing the upper Three Runs aquifer into seven hydrogeologic facies.…”