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Borehole Geophysics and Geology of Test and Monitor Wells in Eastern Nevada
As part of its Clark, Lincoln, and White Pine Counties Groundwater Development Project, the Southern Nevada Water Authority (SNWA) has implemented an exploratory well-drilling and hydraulic-testing program to better define the hydrologic parameters and extents of the regional and intermediate aquifers and to provide sites for long-term monitoring of ground-water conditions. To achieve the objectives of the program, various activities have been performed, including literature review, geologic mapping, surface geophysical surveying, test and monitor well drilling, borehole geophysical logging, aquifer testing, and ground-water sampling. The program has targeted basin-fill, volcanic-rock, and regional carbonate-rock aquifers within the project basins. Since the implementation of the exploratory program, 7 test wells and 19 monitor wells have been installed in 4 project basins and 2 nearby basins.
Lithologic and geophysical logs for each of the monitor and test wells were developed. These logs were correlated to identify potentially productive zones composed of porous gravels or fractured rock and nonproductive zones containing clay or healed fractures having low permeability. This paper discusses two of the well sites—a basin-fill site and a carbonate-rock site—chosen for geophysical log analysis and correlation with lithology. The requisite logs to substantively interpret and differentiate zones of potentially productive porosity and higher permeability are also discussed. The geophysical analysis of the basin-fill wells shows that sand and gravel-rich material identified in the lithologic log correlates with potentially productive zones within the aquifer and that clay-rich horizons tend to have significantly lower production potential. The geophysical analysis of the carbonate–rock wells indicates fracture zones that are potentially productive and zones where fractures are healed and thus less likely to be productive. Insight into the lateral extent of potentially productive zones within the aquifer can be determined by comparing the geophysical analysis of both the test well and monitor well at a given site.
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