Geophysical Characterization Of A Levee With Dc Resistivity And Electromagnetic Measurements

A geophysical characterization of a portion of American River levees in Sacramento, California was conducted in May 2007. Targets of interest included sand lenses that<br>underlay the levees and the depth to a clay unit that underlies the sand. The concern is that the erosion of these sand lenses can lead to levee failure in highly populated areas of<br>Sacramento. DC resistivity and electromagnetic surveys were conducted over a 6 mile length of the levee on roads and bicycle and horse trails. 2-D inversions were conducted<br>on all the geophysical data. The OhmMapper and SuperSting surveys produced consistent inversion results that characterized the targets of interest. GEM-2 apparent resistivity data were consistent with the DC inversion results. However, the GEM-2 data could not be inverted due to large system drifts. While this would not be as large a problem in conductive terrains, it is a problem for a small induction number electromagnetic profiling system such as the GEM-2 in a resistive terrain (the sand lenses). Despite issues with the GEM-2 inversion, this geophysical investigation was successful in detection of the sand lenses and the depth to the clay zone of interest.