Val O. Kofoed

Unique applications of MMR to track preferential groundwater flow paths in dams, mines, environmental sites, and leach fields

Groundwater systems have been notoriously difficult to map with high degrees of accuracy. As a result, not only have traditional geophysical methods proven inaccurate for groundwater characterization work, but they are often costly in terms of time, money, and environmental trauma. This paper describes a unique application of magnetometric resistivity or MMR (Edwards and Nabighian, 1991) for groundwater mapping and modeling, which is high-speed, accurate, minimally invasive, and cost effective. This method has now been deployed at many different sites all over the United States and in other countries like Canada, England, Peru, Sri Lanka, and Argentina. In 2007, the method was employed at 17 dams; some are large well-known structures in the United States. Through two case histories, this paper will assess the effectiveness of this methodology.

ASSESSING SEEPAGE FLOW CONDITIONS THROUGH A DAM QUICKLY AND ACCURATELY WITHOUT DRILLING OR DRAWING DOWN THE RESERVOIR

This paper examines the application of a proven geophysical technology that is highly effective in assessing seepage flow conditions through, beneath and around earthen embankments. The technology works rapidly with minimal disruptions to dam operations—it requires no drilling or draining of the reservoir. It has become a competitive solution to many of the complex challenges faced with dam safety diagnostics and monitoring and the technology has consistently and repeatedly providing intelligence to dam operators in identifying problems with their dams and pointing to appropriate costeffective methods for resolving them. At the 2009 SAGEEP conference in Austin, TX, a seepage related case study of the Method was presented. It showed that the Method found a two-meter sinkhole at the bottom of a reservoir, which was later independently verified. Now, three years later, the method has improved and results are significantly more detailed, accurate and descriptive in defining seepage problems. For instance, data reduction now involves predicting the magnetic field generated from electric current flow in a homogenous space and comparing that to the measured magnetic field. This comparison enhances detection of subtle flow paths in a more electrically homogeneous environment. In addition, a sophisticated inversion algorithm has been developed to quantify the distribution of electric current flow beneath the study area. With these upgrades, the Method is proving to be even more accurate at finding the zones of highest transport porosity related to seepage flow through dams. Although electrical and hydraulic conduction are governed by very different principles, the distribution of electric current flow can qualitatively infer a general distribution of hydraulic conductance, thus helping to appraise the integrity of a dam’s embankment, abutments and foundation. To illustrate how the Method in its current form works, an actual case study and physical modeling experiments will be presented that demonstrate the Method’s application and accuracy with regard to characterizing preferential seepage flow through earthen embankments.

Controlled source audio frequency domain magnetics for seepage diagnosis of Laurel Bed Dam in southwest Virginia, USA

This paper examines both the theoretical basis and the practical implications of a minimally invasive groundwater mapping method as applied to seepage detection in earthen embankments. The method involves inducing a low voltage, low amperage, audio frequency electrical current into the groundwater system. This electric current naturally gathers in areas of highest conductivity—which include high porosity regions within the saturated zone. Per the Biot-Savart law—which relates magnetic fields to their source electric currents—the technology can reveal vital information about the location, character and preferential flow paths of the groundwater system through which it is passing. When properly captured, measured, filtered, and reduced, the data derived from that magnetic field can be used to create both twodimensional maps and three-dimensional models of the subsurface electric current distribution some of which can be interpreted as seepage flowpaths. This method can be applied to a host of seepage related issues, especially tracking and pinpointing the leak locations in a dam’s embankment, abutments, foundation or outlet works. This paper will convey the findings of one case study in which the efficacy of this method has been demonstrated at Laurel Bed Dam, Virginia, USA.