Assessing Seepage Flow Conditions through a Dam Quickly and Accurately without Drilling or Drawing down the Reservoir

Abstract

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.