This ESTCP project evaluated High Resolution 3-D Seismic Reflection at four Department of Defense (DoD) sites for its ability to detect dense non-aqueous phase liquid (DNAPL) - primarily TCE, PCE, and DCE. Seismic waves generated at the soil surface travel downward, encountering materials with different densities (e.g., DNAPL). Subsequently, these waves are reflected back to the surface where geophones detect and record the seismic waves, in particular, those attenuated by the DNAPL and marked by differing seismic velocities. The wave travel times determine the depth and thickness of the stratigraphic profile. The data is interpreted and a 3-D model of subsurface features is generated. In theory, it was expected that the DNAPL would provide a unique "seismic fingerprint" allowing investigators to determine its subsurface location.
The 3-D seismic surveys consisted of general site research, Vertical Seismic Profiles (VSP) to determine subsurface material seismic velocities, site surface feature surveys, seismic reflection surveys, subsurface attribute analysis to delineate anomalies, confirmation borehole drilling, and groundwater sample analyses by EPA Method 8260. The study concluded that seismic waves could not differentiate DNAPL from other geologic features due to inadequate seismic resolution, geologic variations and interferences, and limited capabilities of the equipment. However, 3-D seismic reflection along with fracture trace analysis successfully identified potential contaminant migration pathways (e.g., limestone and bedrock fractures).
3-D seismic surveys are conducted at an average cost of $20,000 per acre, with up to five orders of magnitude more data than traditional vertical borehole drilling. Costs can vary significantly due to the size of the site, the type of subsurface being drilled to complete the VSPs, the depth of drilling, and the amount of preexisting data. Under ideal conditions, this technology can reduce the number of boreholes needed to characterize subsurface geological features. 3-D seismic surveys may provide data on vertical fracture systems that are not usually detected by vertical exploration boreholes. A detailed 3-D illustration of subsurface features can be generated with seismic data to identify preferential contaminant migration pathways.
Typical geological variations can account for the anomalies in the seismic survey data. Peat, organic sands, humus, and landfill debris reduce the effectiveness of the seismic survey. Quality control of seismic resolution, a few feet under ideal conditions, is difficult to maintain between differing sites, operators, hardware and software. VSPs and confirmation sampling require the installation of conventional boreholes, thus requiring some intrusive exploratory efforts. Currently, this technology is not deemed capable of locating subsurface DNAPL source zones. (Project Completed - 1999)