Formatted Title
Use of High-Resolution Seismic Reflection Data to Assess Geologic Heterogeneity and Effect on Groundwater Flow
Background/Objectives
Historically, a simplifying assumption of relatively homogeneous and isotropic permeability distribution in aquifers has been used in the design and implementation of remedies for sites with groundwater contaminated with dissolved plumes of chlorinated solvents. However, many remedies have fallen short of their predicted outcomes due to the inherent heterogeneity in the permeability architecture in aquifers. At a site in southern California, historic release of chlorinated solvents has resulted in a groundwater plume. While the plume behaves relatively predictably in the shallower aquifer, with increasing depth the plume migration and distribution is more uncertain. Historical interpretations of the cause of irregular migration of the plume at deeper zones have included growth faulting and salinity-induced density effects.
Approach/Activities
In order to further assess the nature of the aquifer at the site, a two-dimensional, high-resolution seismic line was acquired along a road roughly parallel to the direction of plume migration in the shallow aquifer. The seismic reflection data for our study was acquired along one linear geophone spread using five geometrics geode signal enhancing seismographs and 14-Hz vertical component geophones. The geophone spread consisted of 120 geophones spaced 10 feet apart, which resulted in a line length of 1,190 feet. The seismic source (shot) consisted of a combination of impacting a rubber mat and HDPE plastic plate with a 20-pound sledgehammer and impacting an aluminum plate with an 80-pound propelled energy generator. Additionally, shot point locations were conducted off the ends of the geophone array where access was permitted. The shot data was independently acquired five times at each source point. Only data of high quality was vertically stacked (i.e., the records at each source point were stacked together) during processing although for most source points that included each record. Each geophone location and elevation along the line were recorded. Vintage CPT and well data were integrated with the seismic stacking velocities to determine the time/depth conversion and calibrate the seismic response to the lithology.
Results/Lessons Learned
The seismic data collected are of excellent quality, showing both parallel and chaotic seismic facies in the zone of interest. Parallel, high-amplitude reflectors are interpreted as interdistributary splays and estuarine fine-grained strata. Chaotic, low amplitude seismic facies are sand-rich, and interpreted as distributary channel deposits. A series of slump faults is also interpreted from offset reflectors. The structural and stratigraphic heterogeneity is inferred to impact groundwater flow directions, enhanced by salinity-related density flow effects.