In 2018, the Mississippi Alluvial Plain water availability project began a multiple-year airborne geophysical mapping initiative, incorporating both regional and high-resolution airborne electromagnetic (AEM), magnetic, and radiometric surveys. The initial high-resolution survey comprised approximately 2,500 line-km of airborne geophysical data over a 1000 sq. km survey block near Shellmound, Mississippi, using the CGG Resolve helicopter system in March 2018. Later that year, the first phase of the regional AEM survey began- also with the Resolve AEM instrument- and acquired nearly 17,000 line-km of data mainly along west-east flight lines separated by 12 km in the northern and southern portions of the study area, with 6 km-spaced lines in the central portion of the study area. As part of the regional survey, approximately 2,000 line-km were acquired along several rivers in order to better characterize the connectivity between surface and groundwater. Each year, additional flight lines will be flown to increase the resolution of the regional hydrogeologic framework. The target resolution for the completed regional survey is 3 km-spaced flight lines, with the next phase of work in 2019 using the Tempest fixed wing instrument to increase the depth of investigation.

Comparison of preliminary resistivity data to the existing base of Mississippi River Valley alluvial (MRVA) aquifer maps and borehole data indicate that the EM sensor was was able to fully penetrate the Quaternary alluvium, exposing the spatial extent of underlying Mississippi Embayment units. Resistivity data below the MRVA base indicate several subcropping aquifer (high resistivity) and confining (low resistivity) units from the Mississippi Embayment sequence. Resistivity data also show that the Quaternary-Tertiary contact may have more topographic relief than previously observed from boreholes. I will illustrate the tradeoffs between the high-resolution survey block that is able to map a relatively small area with great detail, with the more widely spaced regional survey grid that captures regional details of the entire alluvial plain. Interpretations of the resistivity models will be used to refine digital surfaces of the base of aquifer, subcropping units, and shallow confining layers that can be incorporated into groundwater models. By better quantifying and reducing uncertainty in the geologic framework, we hope to produce better estimates of hydrologic uncertainty that can help to inform management decisions.

All are welcome to attend, coffee and cookies as usual.