Using elastic-wave seismic data to image an ultra shallow buried paleochannel

Abstract

This thesis is a case study concerning the application of reflection seismic methods to a very specific instance: mapping an extremely shallow underground feature in the framework of groundwater contamination and remediation ongoing at Operable Unit 2, Hill Air Force Base (UT). The target is the bottom of a paleo-channel eroded in a clay layer at a depth of 10-15 m from the topography and filled with a mix of sands, clays and gravels, which have different compaction and water saturation. The channel acts as contaminant trap for dense non-aqueous phase liquids, therefore a detailed map of its geometry, lateral boundaries and depth is crucial to the remediation effort. The clay-rich sediments that cover much of the study area preclude any effective use of the most commonly utilized ground-penetrating radar. Here I present the results of the processing of a 3-D reflection dataset acquired in 2000 over the buried channel, showing that it is possible to image and delineate the geometry and depth of the very shallow structure using reflection seismic techniques. The resulting contour map of the buried channel can be directly used to plan injection/extraction well placements in the ongoing ground water remediation program. The outcome is in good accordance with the available well data in the deepest, critical part of the buried paleo-channel, where the contaminant is assumed to be concentrated. The map of the channel obtained from the reflection seismic data, although taking advantage of the available borehole information eliminates the need for many wells. In addition, I designed and performed a 2-D elastic-wave seismic survey at the same site in 2000, using three-component sources and receivers. The processing effort resulted in horizontally polarized shear-wave and P-wave stack sections across and along the channel. Despite the less efficient seismic source, the S-wave images are superior to the correspondent P-wave images. Finally, after inverting the first arrival traveltime of both P- and S-wave data, I obtained maps of the VP/VS ratio and equivalent Poisson's ratio along the acquisition lines. These maps highlight areas with different sand and shale content, allowing distinguishing lithology changes in the subsurface.