Geophysical Investigations Of Earthen Dams: An Overview

The Sacramento District U.S. Army Corps of Engineers (USACE) controls 16 dams in the state of California. Three of these dams were classified in the 2006 USACE Screening for Portfolio Risk Analysis as being in Class I or Class II, where the hazards associated with potential failure are considered either urgent and compelling, or urgent, respectively. These dams are Isabella and Martis Creek, both Class I, and Success Dam, identified as Class II. All three dams are earthen structures with either an impervious core or blanket on the upstream toe. Starting in 2005, the USACE teamed with the U.S. Geological Survey in acquiring geophysical data on these dams to provide geotechnical information to be used in assessing the dams and their foundations. The goal of this paper is to provide a review on the status of these surveys, one of which is complete (Success Dam), another underway (Isabella Dam), and a third (Martis Creek) where detailed surveys are planned for the spring of 2008. The techniques being applied include direct current (DC) resistivity, compressional (P) and shear (S) wave seismic reflection and refraction tomography, audio-magnetotellurics (AMT), frequency- and time-domain electromagnetics (EM and TDEM), and self potential (SP). Other techniques are continually being evaluated as project demands change.