Evaluating the Hydrogeologic Framework of the Mississippi Alluvial Plain Using Geophysical Methods

Author(s): Minsley, B.; Kress, W.; Johnson, C.; Lane, J.; Bloss, B.; Thayer, D.

A geophysical framework model of the Mississippi Alluvial Plain is being developed using a diverse suite of geophysical methods applied over multiple scales. Measurement techniques include terrestrial and waterborne continuous resistivity profiling (CRP), electrical resistivity tomography (ERT), time-domain electromagnetics (TDEM), and surface and borehole nuclear magnetic resonance (NMR). CRP methods have been used to map approximately 68 km over land and 1,200 km of streams within the study area to characterize the near-surface (<15 m) lithology of major geomorphologic units that control recharge and groundwater/surface-water exchange to the alluvial aquifer. CRP surveys have identified the boundaries of individual geomorphic features and indicate that these features have distinct ranges of resistivities. The deeper subsurface structure in the region (5-200 m) is being measured using ground-based TDEM measurements, which are capable of measuring the electrical resistivity variations within the alluvial aquifer and can be used to identify subcropping hydrogeologic units. Six east-west regional-scale profiles of time-domain electromagnetic (TDEM) measurements, each comprising 10-20 TDEM soundings and spanning 100-200 km, were conducted in the MAP study area. These profiles are approximately traverse to the synclinal axis of the Mississippi embayment. The profiles were spaced north to south at about 100 km intervals and represent a total area of nearly 100,000 sq. km. These regional-scale profiles are being used to refine the 3D aquifer structure the MAP study area and will be used to guide the survey design and planning of a large airborne electromagnetic survey of the MAP region that will begin in 2018. Borehole and surface NMR data were collected to estimate hydraulic properties and characterize subsurface hydrostratigraphy. NMR methods are used to measure hydraulic properties in the formation including total-, mobile-, and bound-water content, estimates of pore-size distribution, and hydraulic conductivity with depth. The interpreted hydrostratigraphic layers from the surface NMR measurements were consistent with the presence and thickness of a confining unit overlying a more coarse-grained aquifer and were validated by observations from nearby boreholes and TEM surveys. The goal of the comparison is to establish a relation between resistivity and NMR results and facilitate development of a petrophysical relationship between the resistivity and hydraulic conductivity. Resistivity values may then be used as a cost-effective way approximate aquifer hydraulic conductivity distributions that will be input into regional groundwater models.

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