A high-resolution seismic-reflection investigation mapped reflectors and identified characteristics influencing the hydrogeology underlying a portion of the Oxnard Plain in Ventura County, California. Design and implementation of this study was heavily influenced by high levels of cultural noise from vehicles, power lines, roads, manufacturing facilities, and underground utilities/vaults. An abundance of very high quality reflections within the optimum window on shot gathers allowed accurate velocity analysis and statics corrections, which resulted in consistent and coherent common-midpoint (CMP)-stacked reflections with minimal wavelet degradation. Acquisition and processing flows were tailored to this noisy environment and relatively shallow target interval. Layering within both upper and lower aquifer systems was delineated at a vertical resolution potential of about 2.5 m at 350 m depth. Discontinuities in bedding with significant impact on the groundwater flow, particularly related to saltwater intrusion from agricultural pumping near the coast, were delineated on CMP-stacked sections and correlated to widely separated boring data, thereby providing continuity and realism to hydrologic models.
Rectangular Schlumberger arrays can be used for 2-dimensional lateral profiling of apparent resistivity at a unique current electrode separation, hence single depth of penetration. Numerous apparent resistivity measurements are collected moving the potential electrodes (fixed MN spacing) within a rectangle of defined dimensions. The method provides a fast, cost-effective means for the collection of dense resistivity data to provide high-resolution information on subsurface hydeogeologic conditions. Several rectangular Schlumberger resistivity arrays were employed at Los Alamos National Laboratory (IANL) from 1989 through 1995 in an area adjacent to and downhill from an outfall pipe, septic tank, septic drainfield, and sump. Six rectangular arrays with 2 AB spacings were used to delineate lateral low resistivity anomalies that may be related to fractures that contain clay and/or vadose zone water. Duplicate arrays collected over a three year time period exhibited very good data repeatability.The properties of tritium make it an excellent groundwater tracer. Because triiium was present in discharged water from all of the anthropogenic sources in the vicinity it was used for this purpose. One major low resistivity anomaly correlates with relatively high tritium concentrations in the tuff. This was determined from borehole samples collected within and outside of the anomalous zone. The anomaly is interpreted to be due to fractures that contain clay from the soil profile. The clay was deposited in the fractures by aeolian processes and by surface water infiltration. The fractures likely served as a shallow vadose zone groundwater pathway.
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