Cross Borehole Electromagnetic Imaging

Description

In landfills containing metallic waste, the contrasts in electrical properties among contaminants enhance the effectiveness of electromagnetic method for site characterization and monitoring. Cross Borehole Electromagnetic Imaging was designed to characterize waste sites and monitor plume migration. The method is based on radio imaging. This technique measures the strength and timing of a transmitted signal from borehole-to-borehole or borehole-to-surface.

The imaging system consists of a transmitter and receiver. The transmitter and receiver are placed in separate boreholes and lowered by fiber optic cables, or the transmitter is placed in one borehole and the receiver on the ground surface. The resulting data is similar to medical tomographic imaging, which shows a two or three-dimensional image of a body structure constructed by computer from a series of flat cross-sectional images made along a certain axis. The transmitter and receiver are lowered to a station location and a measurement is made. The receiver is moved approximately 2.5 to 5 feet to the next location and another measurement is made. The receiver is again moved, and the measurements are made repeatedly until the ray path fan is completed. The resolution (smallest object imaged) is 1/20 of the distance between the transmitter and the receiver. At Sandia National Laboratories’ (SNL’s) Chemical Waste Landfill (CWL), the resolution is approximately 1.5 feet.

Limitations and Concerns

Cross Borehole Electromagnetic Imaging optimizes sampling, fills in gaps between boreholes, distinguishes between water soluble and organic contamination, minimizes drilling and sampling requirements, and does not require radioactive sources.

Applicability

This is an enhanced characterization technology used at landfills with metallic and radioactive contaminants. This technique is sensitive to changes in moisture content, permeability, and water chemistry. Therefore, it can characterize changes in the landfill system and the zone between the landfill and the water table.

Technology Development Status

Several commercial vendors have used this technology for the Department of Energy remediation projects at Fernald, Rocky Flats, and the Idaho National Engineering Laboratory based on the imaging technologies demonstrated in this project.

Web Links

http://www.sandia.gov/Subsurface/factshts/ert/cbem.pdf

Other Resources and Demonstrations

See the description of related Electromagnetic Surveys.

During 1992, the Chemical Waste Landfill at SNL was imaged. During 1993, a borehole-to-borehole survey of waste disposal pits was also completed. These surveys delineated with three-foot resolution the chromic acid plume and subsurface geology at the unlined chromic acid pit and mapped a series of disposal trenches. In 1994, a 400-foot long borehole-to-surface survey of the Kirtland Air Force Base landfill was finished. The survey defined the outline of numerous disposal trenches and buried objects. This survey also detected a storm-caused infiltration event.