oa Water Table Depth Estimations Using Electrical Sounding And Refraction Seismic Measurements

Schlumberger electrical soundings and refraction seismic<br>measurements were taken to determine the depth to the water<br>table in the Salinas Valley, California.<br>In this area steeply dipping siliceous shales outcrop and<br>form significant topographic variations. Only a thin layer<br>of surface soils covers these rocks.<br>The electrical soundings show that resistivity decreases with<br>depth, from 100 ohm meters at the surface to 20 ohm meters at<br>about 20 feet. The seismic refraction data shows velocity<br>increasing from about 1300 feet/second at the surface to<br>about 5000 feet/second at depths of 20 feet.<br>Three local wells with known water table depths were used as<br>a check on the interpretation and show that both methods<br>provided reasonably correct depth interpretations. More<br>detailed comparison of the two techniques shows that the<br>refraction seismic data provides a more accurate depth to the<br>water table at these three sites.