Seismic Reflection: Upstream, Downstream, And On Earthen Dams And Dikes

High-resolution seismic reflection has been used successfully to characterize material and investigate a variety of problems associated with earthen dams and abutments. Limitations and challenges of seismic reflection when interrogating these structures and lithologies are nontrivial and require very critical thinking. Seismic reflection has proved an effective tool for mapping confining units for integration into the cutoff wall at the Keechelus Dam in Cle Elum, Washington; mapping lithologies and bedrock structures for earthquake retrofitting at Bend, Oregon; delineating karst in bedrock beneath the dam core responsible for subsidence on the upstream side of a major flood control structure at Clearwater Dam, Missouri; and detecting high permeability zones within a glacial outwash embankment of a water retention dam near Enumclaw, Washington. Extreme geometries and material variability associated with any man-made structure are the most formidable challenge to seismically imaging. Inconsistent source wavelets, out-of-the-plane energy, extreme statics (topography and velocity based), and source noise (disproportionately high percentage of surface waves) are all problems that are not unique to earthen dams, dikes, and levees, but they are certainly more prevalent with those types of structures. Success of the technique in these settings is source characteristics and spatial oversampling.