Mississippi Alluvial Plain (MAP): MRVA Properties

Electrical resistivity results from two regional airborne electromagnetic (AEM) surveys (Minsley et al. 2021, and Burton et al. 2021) over the Mississippi Alluvial Plain (MAP) were combined by the U.S. Geological Survey to produce three-dimensional (3D) gridded models and derivative hydrogeologic products. First, the base of the Mississippi River Valley Alluvial aquifer (MRVA) was updated using the AEM resistivity data, both borehole and manual picks, and a supervised machine learning algorithm. The 3D resistivity elevation grid was then intersected with the 2018 potentiometric surface and the new base of MRVA surface to isolate the saturated MRVA extent and generate estimates of the hydrogeologic framework and properties. The saturated aquifer thickness was calculated as the difference between the potentiometric surface elevation and the MRVA base elevation. The average electrical resistivity and facies classification of the saturated aquifer material were calculated for each 1 kilometer (km) x 1 km grid cell. See child item “Mississippi Alluvial Plain: Electrical Resistivity & Facies Classification Grids” for more details on the facies classes. Lastly, the degree of connectivity across the base of the MRVA, i.e. how likely the MRVA is hydraulically connected to deeper subcropping Tertiary units, was estimated through the vertically integrated electrical conductance (VIC) between different vertical offsets (+/- 5 meter (m), 10 m, 25 m) from the aquifer base. For example, for every 1 km x 1 km cell, the VIC for +/- 25 m is the result of integrating the electrical conductance values from all 5 m elevation layers between 25 above the MRVA base and 25 m below the MRVA base. Areas with high VIC values suggest there is low or minimal hydraulic connection across the MRVA base, while low VIC values indicate areas of high potential connection. All products were exported as raster images in Georeferenced Tagged Image File Format (GeoTIFF) files. Burton, B.L., Minsley, B.J., Bloss, B.R., and Kress, W.H., 2021, Airborne electromagnetic, magnetic, and radiometric survey of the Mississippi Alluvial Plain, November 2018 - February 2019: U.S. Geological Survey data release, https://doi.org/10.5066/P9XBBBUU. Minsley, B.J., James, S.R., Bedrosian, P.A., Pace, M.D., Hoogenboom, B.E., and Burton, B.L., 2021, Airborne electromagnetic, magnetic, and radiometric survey of the Mississippi Alluvial Plain, November 2019 - March 2020: U.S. Geological Survey data release, https://doi.org/10.5066/P9E44CTQ.

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