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<br>bedrock, locate bedrock faulting and define the stratigraphy overlying bedrock. The survey was conducted for the<br>purpose of identifying possible groundwater and contaminant migration pathways or obstructions, as well as areas<br>of depressed basement where dense non-aqueous phase liquid (DNAPL) contaminants might concentrate. The<br>survey consisted of a coarse grid of seismic lines and was designed to get coverage over a large area in an attempt to<br>get a basewide basement picture and define potential fault systems crossing the base (Figure 1). The design has the<br>disadvantage that the data are aliased because the station spacing is much shorter than the line spacing. However,<br>the design allowed us to determine four very valuable pieces of information: a) anomalous trends in groundwater<br>movement, b) depth and general structure of the basement reflector, c) areas where more detailed work may be<br>necessary and d) input for the more judicious placement of future drilling locations.<br>Previous work conducted in specific areas of the base by Tetra Tech (1993) and IT (1994) indicated that it was<br>possible to delineate these targets with the seismic reflection method, by that the data quality was dependent on<br>local site conditions and survey design had to be adjusted for these conditions. The types of cultural noise present at<br>the site during the survey included aircraft and ground traffic, with the latter posing a particular challenge, and<br>being a significant contributor to the choice of source type, as discussed in the next section. Other sources of noise<br>included electrical noise and all motors, engines and other noise associated with normal base operations.