We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
Electromagnetic (EM) Induction logging can be an efficient and versatile method for tracking seawater intrusion. Both water chemistry and lithology affect EM logs. EM conductivities of Southern California coastal silts and clays often reach 200 millisiemens per meter (mS/m ). Coarse sand and gravel zones intruded by seawater reach EM conductivities greater than 800 mS/m. As part of a United States Geological Survey (USGS) seawater intrusion study, EM log, chloride concentrations and specific conductance data from 9 monitoring wells along the coastal Oxnard Plain in Ventura County, California were used to correlate EM conductivity and water quality and to characterize seawater intrusion. EM logging and water chemistry sampling were done in 1994 and again in 1996. Specific conductance and chloride concentration of water samples analyzed in this study ranged from 100 to 2,000 mS/m and 26 to 6,500 milligrams per liter (mg/1) respectively. Large changes in conductance and chloride concentrations occurred during the 1994 -1996 period. These large changes are directly related to an increase in rainfall after 1994 and the resulting decrease in agricultural pumping within the Oxnard Plain. Linear regression analysis of chloride concentration versus specific conductance, chloride concentration versus EM log conductivity and change in chloride concentration versus change in EM log conductivity show correlation coefficients of 0.89, 0.72, and 0.87, respectively. Contour mapping of EM log conductivity and chloride concentration as well as changes in both parameters show common trends. EM logging can be used to track seawater intrusion along the coastal Oxnard Plain. However, water chemistry sampling techniques must be refined and standardized to get representative samples, which can then be statistically correlated to EM conductivity. Regression analysis indicates a correlation between chloride concentration and EM log conductivity even with the interfering effects of lithology and with improper sampling techniques.
