Application of Magnetic Resonance Soundings for Hydrostratigraphic Characterization of an Alluvial Aquifer for Ground-water Model Input

An integrated surface geophysical pilot study at the Texas A&M University Brazos River Hydrologic Field Research Site (BRHFRS), College Station, Texas, was done to determine the effectiveness of methods for defining the hydrostratigraphic framework and hydrogeologic properties for a ground-water availability model. Time-domain electromagnetic (TDEM) soundings and direct-current (2D–DC) resistivity imaging were used to define the lateral and vertical extent of the Brazos River alluvium aquifer, the Ships clay within the aquifer, and the Yegua Formation underlying the aquifer at the BRHFRS. Magnetic resonance soundings (MRS) were used to derive estimates of hydrogeologic properties including percentage water content, transmissivity, and hydraulic conductivity. Stratigraphically, the principal finding of this study was the relation between electrical resistivity and the depth and thickness of the subsurface geologic units at the site. Not only could thicknesses and extents of these units be defined to a greater level than previously interpreted, but lateral variations in resistivity within the alluvium aquifer also could be detected. MRS data have added supporting data to the 2D–DC resistivity and TDEM data allowing for improved understanding of the hydrostratigraphic framework. Hydrostratigraphically, individual hydraulic conductivity values derived from MRS were in close agreement with previously conducted aquifer tests. Average hydraulic conductivity values from the aquifer test are about 61 to 80 m/d, whereas, the MRS-derived hydraulic conductivity values are about 27 to 97 m/d. Results from the geophysics study demonstrated the usefulness of combined TDEM, 2D–DC resistivity, and MRS methods to reduce the need for additional boreholes and to create more accurate ground-water availability models using the acquired data.