USGS science for a changing world Advances in Borehole Geophysics for Ground-Water Investigations

Detailed information on subsurface conditions is essential for the development and management of ground-water resources and the characterization and remediation of contaminated sites. Borehole geophysics provides a highly efficient means for the collection of such information. Recent advances in methods and equipment have greatly increased the ability of geoscientists to obtain subsurface information in ground-water investigations through the use of borehole-geophysical techniques. Portable geophysical loggers that are specifically designed for ground-water applications (fig. 1) are now available. The geophysical loggers are PC-based and have menu-driven software for the collection, real-time display, and subsequent analysis of digital log data. Drawworks for shallow investigations are highly portable, and some have plastic-coated logging cables for easy decontamination. Logging probes are available that can be used in boreholes with a diameter as small as 5 centimeters. Many probes are capable of collecting multiple parameters with a single logging run, thereby greatly increasing the efficiency of the logging operation and taking full advantage of the synergistic nature of geophysical log data. Figure 1. A PC-based geophysical logger with portable drawworks and plastic-coated logging cable in use at a contaminated ground-water site. Electromagnetic-induction logging replaced normal-resistivity logging in the oil industry many years ago. Induction probes recently have been designed for small-diameter monitoring wells. Induction logs can be collected in water-, air-, and mud-filled holes and through PVC casing. Major factors that affect induction-log response in sand-and-gravel aquifers are the concentration of dissolved solids in the ground water and the silt and clay content of the aquifer. Induction logs, which are commonly run in combinal ion with gamma logs, can be used to identify lithology and zones of electrically conductive contamination such as landfill leachate (fig. 2) and saltwater intrusion