Use of Seismic Reflection Amplitude Versus Offset (AVO) Techniques to Image Dense Nonaqueous Phase Liquids (DNAPL)

Abstract

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.

Critical Review of Environmental Forensic Techniques: Part I

“…Geophysical techniques used to identify the presence of NAPLs include electrical impedance tomography (EIT) (Daily, Ramirez and Johnson, 1998), seismic re¯ection pro®ling (Waddell, Domoracki and Temples, 1996), cross radar technology, negative ion sensors (United States Environmental Protection Agency, 1998), and complex resistivity techniques. At Hill Air Force Base in Ogden, Utah, electrical impedance tomography was used with ®ber optic chemical sensors and neutron logs to verify the presence of NAPLs (United States Environmental Protection Agency, 1998).…”