Investigating Dam Seepage using Geophysical Methods: Providing Hands-On Geophysical Experience to Geological Sciences and Engineering Students

An electrical resistivity survey was carried out at an existing leaking dam site in Salem, Missouri. The purpose of the study was to investigate possible seepage pathways and to provide students at the University of Missouri-Rolla hands-on experience in the application of geophysics to engineering problems. The electrical resistivity data were acquired using the dipole-dipole and pole-dipole arrays. The data suggest two geoelectric layers: an upper conductive layer interpreted to be surficial clay deposits and a lower more resistive layer interpreted to be the dolomite bedrock characteristic of this part of Missouri. The results further suggest that the bedrock is laterally discontinuous, which we suggest to be the result of fracturing and solution activity. We also observed an anomalously sub-circular conductive zone within the bedrock, which we interpret as a subsurface solution cavity filled with water or finer materials such as silt or clay. This anomalous area was laterally extensive and occurred beneath the spillway of the dam. We suggest this subsurface cavity may have provided potential pathways for the water seepage which emptied the dam. This study demonstrates the usefulness of electrical resistivity measurements in detecting seepage zones under dams. In addition, the study also provided students at the University of Missouri Rolla hands-on experience and an appreciation of geophysical methods as an investigative tool for engineering problems