High Resolution Seismic Refection to Determine Thickness of Alluvium and Faulting in Bedrock

A seismic reflection survey was conducted at March Air Force Base (MAFB) with the objective to map the depth to bedrock, locate bedrock faulting and define the stratigraphy overlying bedrock. The survey was conducted for the purpose of identifying possible groundwater and contaminant migration pathways or obstructions, as well as areas of depressed basement where dense non-aqueous phase liquid (DNAPL) contaminants might concentrate. The survey consisted of a coarse grid of seismic lines and was designed to get coverage over a large area in an attempt to get a basewide basement picture and define potential fault systems crossing the base. The design has the disadvantage that the data are aliased because the station spacing is much shorter than the line spacing. However, the design allowed us to determine four very valuable pieces of information: (a) anomalous trends in groundwater movement, (b) depth and general structure of the basement reflector, (c) areas where more detailed work may be necessary and (d) input for the more judicious placement of future drilling locations. Previous work conducted in specific areas of the base by Tetra Tech and IT indicated that it was possible to delineate these targets with the seismic reflection method, by thatmore » the data quality was dependent on local site conditions and survey design had to be adjusted for these conditions. The types of cultural noise present at the site during the survey included aircraft and ground traffic, with the latter posing a particular challenge, and being a significant contributor to the choice of source type, as discussed in the next section. Other sources of noise included electrical noise and all motors, engines and other noise associated with normal base operations.« less