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Groundwater Exploration Stratagem Ground Water Survey in California The purpose of the survey was for groundwater exploration, to identify one or more favorable well locations for a high-yield irrigation well. Hydrogeologic units in the area consist of younger and older alluvium, Paso Robles Formation  (clay, sand, and gravel), and older units consisting of the Careaga Sandstone and Monterey Formation (shale) of Pliocene and Miocene age.  Tar sands and salty or sulfurous water are commonly found in these older units.  Therefore, one of the objectives of the survey was to define the depths at which this poor-quality ground water might occur. Line A is orthogonal to Line B, so that the NW end of Line A joins the SW  end of Line B.  Line A is roughly aligned across the presumed electrical strike of the formations and Line B is roughly parallel to the strike.  Data from existing wells and some vertical electrical soundings  (VES) completed in the area were used to constrain the geoelectrical model. A test hole was completed at the proposed well location, and pre-drilling interpretation of the profile (image) compares favorably with the  results.  The base of the saturated younger and older alluvium was found at about the 150-foot depth. Clay and sand of the Paso Robles Formation were logged to a depth of 200-250 feet.  Below 250 feet decreasing sand and increasing amounts of siltstone (material of low electrical resistivity) were noted to the total depth explored (about 300 feet) in the test hole. The test hole was not carried deeper because the Stratagem profile shows that only materials of relatively low resistivity (either siltstone or materials having poor quality water) are present to at least 600 feet.