Use of Electrical Resistivity and Self‐Potential Surveys for Dam Seepage Investigations

Abstract

Electrical resistivity imaging (ERI) and self‐potential (SP) surveys provide an effective means of delineating seepage conditions within earth embankment dams, levees, and other structures. Recent advances in resistivity tomography inversion allow reconstructing the resistivity structure at the embankment/foundation contact, implying that internal embankment erosion can be effectively inferred from surface measurements. ERI and SP measurements presented here were collected at two different reservoir pool elevations over portions of the upstream and downstream embankment faces. In addition, offshore resistivity and SP measurements were collected over portions of the reservoir area immediately upstream of the dam. These data were interpreted as individual data sets and also as a differenced data set. Due to the embankment construction method used, the embankment soils are observed to be fairly uniform in their geoelectric structure. This lack of a complex internal dam shell structure allowed fairly direct resistivity imaging of what is believed to be resistivity variations at the embankment foundation contact. Comparison with nearby drill sample results leads to the conclusion that possible internal erosion has occurred at this location. SP variations observed over similar locations as the resistivity results indicates that a pre‐construction alluvial river channel is likely conveying seepage flows beneath the embankment dam. Portions of this alluvial channel were not removed prior to dam construction, allowing a preferential foundation seepage path. Planned future ERI and SP monitoring of this site should allow for much more detailed images of seepage and possible internal erosion as monitoring measurements are conducted with varying reservoir pool elevation, and therefore varying hydraulic head gradients within and beneath the embankment.