New York Water Science Center
QUESTIONSCURRENT CONDITIONSCurrent streamflow conditions in New York.
DATA CENTER
ABOUT THE New York
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 ADVANCES IN BOREHOLE GEOPHYSICS FOR GROUND-WATER INVESTIGATIONSDetailed information on subsurface conditions is essential for the development and management of ground-water resources and the characterization and remediation of contaminated sites. Borehole geophysical techniques provide 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 borehole geophysical techniques. Portable geophysical loggers that are specifically designed for ground-water applications are available. The geophysical loggers are PC-based and have menu-driven software for the collection, display, and analysis of digital log data. Drawworks for shallow investigations are highly portable, and some have plastic-coated logging cables for easy decontamination. Many of the logging probes can be used in boreholes with a diameter as small as 5 centimeters. Many probes are capable of collecting multiple geophysical parameters with a single logging run, thereby greatly increasing the efficiency of the logging operation. Electromagnetic-induction logging replaced normal-resistivity logging in the oil industry many years ago. Induction probes have been designed specifically 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 combination with gamma logs, are used to identify lithology and zones of electrically conductive contamination such as landfill leachate and saltwater intrusion. High-resolution flowmeters that use heat-pulse and electromagnetic methods can measure extremely low vertical flow rates in boreholes. Conventional impeller flowmeters that are widely used in ground-water studies have a lower measurement limit of about 2 meters per minute, whereas the high-resolution flowmeters have lower measurement limits of less than 0.03 meters per minute. Flowmeters can be used to measure borehole flow under ambient as well as pumped conditions. Borehole-flow measurements made under ambient conditions can help to delineate transmissive fractures and other permeable zones and to indicate the direction of vertical hydraulic gradients; they also are useful in interpreting fluid-conductivity logs and borehole water-quality data. Borehole-flow measurements made under pumped conditions can be used to develop hydraulic-conductivity profiles of aquifers. Television cameras commonly are used in ground-water studies to inspect the condition of well casing and screens; they also can be used to directly view (1) lithologic texture, grain size, and color; (2) water levels and cascading water; and (3) bedrock fractures. Television logs can be obtained in clear water and above the water level. The most sophisticated television systems are magnetically oriented and provide a 360-degree digital image of the borehole wall. Acoustic televiewers provide a magnetically oriented, 360-degree, photographlike image of the acoustic reflectivity of the borehole wall. Televiewers have been used in the oil industry for many years and are being used increasingly in ground-water applications. Televiewer logs, which indicate acoustic transit time and reflected amplitude, can be obtained from water- or mud-filled holes. The newest digital televiewer systems allow interactive determination of fracture orientation. Borehole radar provides a method to detect fracture zones at distances as far as 30 meters or more from the borehole in electrically resistive rocks. Radar measurements can be made in a single borehole (transmitter and receiver in same borehole) or by cross-hole tomography (transmitter and receiver in separate boreholes). Single-hole, directional radar can be used to identify the location and orientation of fracture zones, and cross-hole tomography can be used to delineate fracture zones between boreholes. The movement of a saline tracer through fracture zones can be monitored by borehole radar. REFERENCESJohnson, C.D., 1994, Use of a borehole color video camera to identify lithologies, fractures, and borehole conditions in bedrock wells in the Mirror Lake Area, Grafton County, New Hampshire, in U.S. Geological Survey Toxic Substance Hydrology Program - Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4015, p. 89-93. Lane, J.W., Haeni, F.P., and Williams, J.H., 1994, Detection of bedrock fractures and lithologic changes using borehole radar at selected sites in Proceedings of the Fifth International Conference on Ground Penetrating Radar, Kitchener, Ontario, Canada, June 12-16, 1994: Waterloo, Ontario, Waterloo Centre for Groundwater Research, p. 557-592. Lane, J.W., Haeni, F.P., Placzek, G., and Wright, D.L., 1996, Use of borehole-radar methods to detect a saline tracer in fractured crystalline bedrock, Mirror Lake, Grafton County, New Hampshire, in Proceedings of the 6th Annual International Conference on Ground Penetrating Radar, Sendai, Japan, September 30-October 3, 1996, p. 185-190. Lane, J.W., Haeni, F.P., Soloyanis, S., Placzek, G., Williams, J.H., and others, 1996, Geophysical characterization of a fractured-bedrock aquifer and blast-fractured contaminant-recovery trench, in Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Keystone, Colorado, April 28-May 2, 1996: Wheat Ridge, Colo., Environmental and Engineering Geophysical Society, p. 429-442. Paillet, F.L., Crowder, R.E., and Hess, A.E., 1996, High-resolution flowmeter logging applications with the heat-pulse flowmeter: Journal of Environmental Engineering Geophysics, v. 1, no. 1, p. 1-11. Williams, J.H. and Conger, R.W., 1990, Preliminary delineation of contaminated water-bearing fractures intersected by open-hole bedrock wells: Ground Water Monitoring Review, v. 10, no. 3, p. 118-126. Williams, J.H., Lapham, W.W., and Barringer, T.H., 1993, Application of electromagnetic logging to contamination investigations in glacial sand-and-gravel aquifers: Ground Water Monitoring and Remediation Review, v. 13, no. 3, p. 129-138. Williams, J.H. and Lane, J.W., 1998, Advances in borehole geophysics for ground-water investigations: U.S. Geological Survey Fact Sheet 002-98, 4 p. Young, S.C. and Pearson, H.S., 1995, The electromagnetic borehole flowmeter - description and application: Ground Water Monitoring and Remediation Review, v. 15, no. 4, p. 138-147. For further information, contact:
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