Geophysical Logging in Carbonate Aquifers

Some logging methods are inherently superior to others for the analysis of limestone and dolomite aquifers. Three such systems are the density, neutron, and acousticvelocity logs.

Relative percentages of limestone and dolomite, average matrix (grain) densities of the rock mixtures, and porosity of the rock mass can be calculated from density, neutron, and acoustic-velocity logs. Wells drilled before the 1960's generally were logged with an uncalibrated neutron tool. In some instances, the resultant curve can be empirically calibrated.

With supporting data from resistivity logs, apparent water resistivity (Rwa) can be estimated. Depending on rock and mud resistivities, the two most useful resistivity curves for water-quality studies are the deeply-focused laterolog and the induction log. For older wells, the standard electric log may be used, if the drilling mud is not salt-saturated.

Rwa contour maps derived from geophysical data are useful to outline areas of recharge, direction of probable ground-water flow, and location and salinity of brine areas. Another useful application of well logs is estimation of ground-water temperature. These temperatures are obtained from maximum-reading thermometer data that generally are recorded on each logging run. Maps of porosity can be used to outline potential areas for completing large-yield wells.