Differentiation of Structurally Controlled Seepage in a Dam and Dike Complex by Integrated Geophysical Surveys

ABSTRACT Persistent seepage from an earthfill dam and associated structures has been observed for decades at Lake Capote, a small, spring-fed, off-stream, man-made lake in southwestern Colorado. Extensive engineering measures in 2006 did not fully remediate seepage. Therefore, we employed electrical resistivity tomography (ERT), induced polarization (IP), and self-potential (SP) methods to determine the location of seepage and its relationships with local geologic conditions. ERT revealed an approximately north–south striking horizontal, high-resistivity feature extending underneath both the northern part of the dam and the adjacent south dike. IP results indicated that this interpretive feature exhibits low chargeability (i.e., a low ability to store electrical charges), and SP results identified localized high positive magnitudes in it. In the context of borehole data and published literature, ERT and IP results suggest that this feature is a rare sandstone bed in the local shale-dominated bedrock (Lewis Shale) and also that it is the location of persistent seepage. Seepage appears to be occurring under the dam along faults in the interpretive sandstone (similar faults are observable in a nearby outcrop) and also within highly fractured shale under the south dike. In contrast, the middle part of the dam shows evidence, in both ERT and IP results, for seepage through its embankment rather than in its foundation. Our results demonstrate the value added to forensic investigations of dams by the integration of different geophysical methods, and they also portend the crucial importance and cost-saving potential of preconstruction geophysical site characterization of damsites.