Imaging a shallow aquifer in temperate glacial sediments using seismic reflection profiling with DMO processing

We acquired a seismic reflection profile to image a shallow (<100 m) aquifer system on a small island in Puget Sound, north of Seattle, Washington. The aquifer system is comprised of temperate glacial sedimentary strata, with the primary aquifer lying approximately 45 m below the surface. We chose the site because there are water-well boring descriptions available and a sea cliff parallels the profile location, providing stratigraphic control. A 20-lb (9-kg) weight drop was used as an energy source to acquire a 30-fold common-midpoint (CMP) reflection profile along a 400-m line. Analysis of the recorded wavefield was not straightforward because of complex stratigraphy and the presence of a laterally variable thin bed (similar to 5-15 m) of high-impedance lodgement till within a few meters of the surface. We used finite difference modeling to determine that conventional CMP processing would provide a reasonable approximation for imaging strata in the primary target zone, roughly 20 m below the high velocity till. For CMP processing, we analyzed the velocity structure using iterative dip moveout (DMO) velocity analysis. Use of this method results in a velocity held that dramatically improves the poststack depth-migrated section.