Seismic Reflectlon/F?Efractlon Survey To Characterize The Subsurface At An Npl Site In The Mojave Desert

In July and August 1991, a seismic reflection and refraction survey was performed at the Marine Corps<br>Logistic Base (MCLB) in Barstow, California. The purpose of the survey was to determine the<br>configuration of: 1) bedrock, 2) the water table, 3) confining layers, and to precisely locate faults; mostly<br>the Waterman, that may affect contaminant pathways.<br>The site is on the National Priority List (NPL) with known contamination from solvents and petroleum<br>hydrocarbons. The MCLB is located within an alluvial basin in the Mojave block, a large wedge shaped<br>fault block bounded by the San Andreas and Garlock Faults.<br>The seismic survey was performed in support of the Remedial Investigation/Feasibility Study being<br>conducted by the Navy. it was designed to be innovative in response to the depth and nature of various<br>geologic formations, especially in light of difficufties experienced in collecting data in this desert region.<br>The Bison Model 9048 seismograph was used to record data generated by the Bison Elastic Wave<br>Generator Ill. The data was processed by using Seistrix 2386 for reflection and Firstpix and Gremix for<br>refraction. A split-spread array allowed for the extraction of both CDP reflection data and forward and<br>reverse refraction data. A total of ten miles of data were shot within a ten day period. Vertical seismic<br>profiling (VSP) was performed in four wells to assist in interpreting the surface information.<br>Combining data from the two techniques suggest that there is a mounding of water at the en echelon<br>faults which cut across the highlands into the alluvium. Faults not only displace the bedrocks but,<br>extend into sediments near the surface. The seismic survey provided a picture of the subsurface which<br>will assist in planning future work.