Use of geophysical logs to estimate water-quality trends in carbonate aquifers

The water quality in carbonate aquifers can be determined by analysis of resistivity and porosity logs. When supporting data from water analyses are available, the value of the cementation exponent m can be determined more precisely. Data for this study were taken from logs of oil-test wells, Amstrat sample studies, drill-stem tests and water test wells in parts of Montana, North and South Dakota, and Wyoming. The preferred resistivity curves for apparent water resistivity (Rwa) analyses are the deeply focused laterolog and the induction log. The standard electric log can be used if the drilling mud is not saturated with salt. The preferred porosity logs are the sonic, sidewall neutron, compensated neutron, and the density logs. Older, uncalibrated neutron curves can be empirically calibrated in some instances, however, resulting porosities are frequently anomalous when compared to those determined from core or modern logs. When apparent water resistivity is determined for many wells, the data can be plotted and contoured to outline areas of recharge, direction of probable ground-water movement, and location and salinity of brine areas. (USGS)