Seismic imaging directs an intense sound source into the ground to evaluate subsurface conditions and to possibly detect high concentrations of contamination. Receivers called geophones, analogous to microphones, pick up “echoes” that come back up through the ground and record the intensity and time of the “echo” on computers. Data processing turns these signals into images of the geologic structure. This technology is similar in principle to active electromagnetic survey technology.
There are two types of seismic images produced as the sound waves travel into the ground. Reflected waves travel downward, bounce off a layer or object in the soil or rock, and return to the surface. Refracted waves are those that travel downward, then turn at a geologic boundary (such as the surface of a rock layer) and travel along it before returning back to the surface. Reflected waves generally show more subsurface detail. However, multiple “echoes” can make reflections very difficult to interpret. Refracted waves are typically used to profile shallow bedrock (i.e., rock less than 100 feet below the ground).
During the survey process, the reflections provide a three-dimensional digital model of the subsurface. This information can be used to identify preferential flow paths, determine the placement and screening of wells, and help select a remediation technology. In addition to providing information about subsurface formations, the indirect detection of dense contaminants including dense non-aqueous phase liquids (DNAPLs) may be possible from the seismic data.
Seismic imaging using cross-well surveys is another means of providing subsurface characterization and monitoring information. It is often advantageous to site the seismic source below the surface (down hole), below the highly attenuating near-surface materials. Based on this need, Sandia National Laboratory (SNL) initiated a program to develop a “magnetostrictive” seismic source specifically for down hole applications.
Limitations and Concerns
While this technique may provide images of subsurface geology, demonstrations have shown that it does not directly locate and define contaminant plumes. Unless pooled by some geologic feature, the overall density contrast of DNAPL may be imperceptible. Seismic reflection does not work well in formations that are geologically heterogeneous.
Seismic imagery is a tool used to characterize subsurface geology. It may be useful in helping to identify dense nonaqueous phase liquids (DNAPLs).
Technology Development Status
High resolution, three-dimensional seismic reflection imaging has been used in exploration for oil and gas, as well as for subsurface fresh water, since the 1950s. It is still being field tested for use at contaminated sites. Recent technological advances have made it possible to generate high-resolution images of formations up to 3,000 feet deep. The down hole applications to detect contaminant plumes are still in development.
Other Resources and Demonstrations.
See http://www.serdp-estcp.org/content/download/3259/54751/file/ER-199601-CP.pdf for cost and performance data for seismic reflection surveys used to map subsurface geologic, subsurface hydro-geologic, and subsurface DNAPL contaminant source areas at the Letterkenny Army Depot near Chambersburg, Pennsylvania; Alameda Naval Air Station, Alameda, California; Tinker Air Force Base, Oklahoma City, Oklahoma; and Allegany Ballistics Laboratory, Mineral County, West Virginia. The primary objective of the project was to verify that seismic reflection is a viable technique for delineating DNAPL sources. The results, however, show that seismic surveys are not effective at directly detecting DNAPL. Still, this technology appears to be a useful tool for imaging subsurface conditions for site characterization and for determining the most likely locations for DNAPL source zone migration and accumulation. The California Department of Toxic Substances Control has accepted the use of seismic imaging at a number of sites for migration pathway analysis. For example, there have been five surveys at the Stringfellow National Priorities List site near Riverside, California.
See http://www.sandia.gov/Subsurface/factshts/geophysical/magneto.pdf for a description of the Department of Energy’s demonstration of the magnetostrictive source at Hanford, Washington.
See http://www.clu-in.org/download/char/542r04017.pdf (p.79) of U.S. EPA’s 2004 report on Site Characterization Technologies for DNAPL Investigations.
See http://costperformance.org/monitoring/pdf/9_nhplati.pdf for a description of case study of subsurface imagery in sediments.