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Description
In the fractured crystalline rock terrains of eastern San Diego County, where residents are dependent on groundwater, wells with low or even insufficient yields are common. Most known high yield wells in this area are located on or near lineaments identified from air-photographs. It is a well-established technique to site wells on these lineaments in order to benefit from the increased permeability commonly associated with these features. The VLF method has the potential to verify the presence of subsurface conductive zones associated with saturated fracture systems. In addition, there is the potential to identify conductive features where no mapped air-photo lineaments exist. If further characterization of the location and subsurface orientation of subsurface fracture zones is possible with the VLF method, improvement might be made in siting wells for increased yields. 40 VLF surveys were conducted in Lee Valley California, where a detailed map of air-photo lineaments was previously produced 19 VLF surveys crossed mapped air-photo lineaments. VLF anomalies were observed in 18 of these locations. Anomalies were also observed in 3 of 4 locations where lineaments can be extrapolated into the basin from areas where they are better exposed. VLF anomalies were also identified in areas where no mapped or extrapolated air-photo lineaments exist. Two dipole-dipole resistivity surveys in these areas indicate that VLF anomalies result from shallow conductivity contrasts as well as deeper structural features. Surveys were conducted in the vicinity of three high yield wells in different areas of eastern San Diego County in order to evaluate VLF response in areas with known hydraulic characteristics. In all three locations VLF anomalies were observed. Estimation of dip directions at these locations corresponded to the expected direction based on the position of the well and the mapped lineaments. A well was sited in Descanso California based on VLF anomaly positions. This well appears to have intersected a shallow conductive zone and exhibits a relatively high yield. The data collected for this study indicate that the VLF method is a useful tool in the location of structures associated with increased permeability in a fractured crystalline rock environment. The sensitivity to small conductivity contrasts suggests that this technique is best applied in areas where additional control, such as mapped air-photo lineaments, exists.
