Part of the Direct Push Methods Project
High-resolution stratigraphic characterization of unconsolidated deposits using direct-push electrical conductivity logging: A floodplain margin example
by M.K. Schulmeister, J.J. Butler, Jr., E.K. Franseen, D.A. Wysocki, and J.A. Doolittle
in Aquifer Characterization, SEPM Concepts in Hydrogeology and Environmental Geology Vol. 2, edited by J. S. Bridge, and D.W. Hyndman, SEPM, pp. 67-78, 2004.
Electrical logs of various types have been used for decades in a wide variety of geoscience applications. Except for studies within a few meters of the land surface, these logs have been obtained using existing wells or boreholes. Recently, electrical conductivity (EC) sensors have been incorporated into direct-push equipment to obtain sedimentologic information in unconsolidated deposits without the need for existing wells and at a resolution (0.02 m) that has not been possible using conventional logging tools. The high resolution of this information, coupled with the speed at which it can be obtained, makes direct-push EC logging a valuable new tool for a wide variety of hydrostratigraphic studies. We document the utility of this approach in a detailed stratigraphic evaluation of a floodplain margin in a major river valley in the United States. Throughout the central United States, unconsolidated sequences underlying floodplains are typically composed of fining-upward glaciofluvial or Holocene sediments in which silt and clay overbank deposits cap coarser materials that serve as regionally significant aquifers. EC transects at a field site on the Kansas River floodplain show that this fine-grained cap may be truncated by, or interfingered with, coarser sediments at the floodplain margin. This increased stratigraphic complexity suggests that the depositional settings assumed for the more central portions of the floodplain may not be appropriate in the margin areas. The replacement of fine-grained sediments with coarser-grained materials at the margin of the floodplain has significant implications for groundwater recharge and solute movement. Interpretations made using the EC transects are consistent with results from an electromagnetic survey, as well as head and chemistry data. This work shows that direct-push EC logging can provide information about site stratigraphy at a level of detail that would be difficult to obtain with conventional approaches. This unprecedented level of detail enables important insights to be obtained regarding stratigraphic controls on groundwater flow and solute transport.
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Kansas Geological Survey, Geohydrology Section
Updated Oct. 2005
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