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Application of Geophysics in the Delineation of the Freshwater/Saline-Water Interface in the Michigan Basin, 1989

By: Westjohn, D.B.


The presence of shallow saline groundwater (< 30 M deep) in the Michigan Basin is known from chemical analyses of groundwater collected in some areas of the basin. However, the position of the freshwater/saline-water interface is unknown for most of the State. Data from borehole geophysical logs are used to delineate the altitude of the base of freshwater within the area where Mississippian and younger rocks underlie Pleistocene glacial deposits. The character and thickness of the transition zone and the location of the top of the first brine-bearing unit also are interpreted from electric (resistivity) and porosity logs. In areas where geophysical logs are sparse, vertical electrical-resistivity soundings are used to estimate the depth to the first brine-bearing sandstone and to map the vertical extent of freshwater. Geophysical logs and vertical electrical-resistivity soundings indicate that freshwater transects the glacial-deposits/bedrock interface. Areas are defined where the freshwater section exceeds 300 meters in thickness. These areas are predominated by glaciofluvial deposits that overlie freshwater-bearing Pennsylvania sandstones. The thickness of the transition zone ranges from a few meters to greater than 150 meters, and it consists of Pennsylvanian sandstones and shaly sands that contain saline water. The shallowest brine-bearing sandstones generally are confined to the Upper-Mississippian rock sequence, but Pennsylvanian sandstones also contain brine in some areas. (See also W90-08400) (Author's abstract)

RECORD ID: 9008405

F&G CODE: 02f; 07b; 02k

Westjohn, D.B., Application of Geophysics in the Delineation of the Freshwater/Saline-Water Interface in the Michigan Basin, Regional Aquifer Systems of the United States: Aquifers of the Midwestern Area. Papers Presented at 24th Annual AWRA Conference and Symposium; November 6-11, 1988, Milwaukee, WI. AWRA Monograph Series No. 13, 1989. American Water Resources Association, Bethesda, Maryland. p 111-134, 22 fig, 1 tab, 13 ref.

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