Integrated geophysical and geological investigation of a heterogeneous fluvial aquifer in Columbus Mississippi

Abstract

A heterogeneous fluvial aquifer system exposed in a sand and gravel quarry outcrop in northeastern Mississippi was examined to quantify the relationship between geophysical attributes and lithostratigraphic properties. Two dimensional seismic reflection, ground penetrating radar, and DC resistivity profiles collected along the top of the quarry wall were compared with outcrop descriptions, photographs, and laboratory resistivity measurements and grain-size analysis conducted on a core collected near the center of the geophysical profiles. The aquifer is composed of an upper laterally continuous sandy-clay meandering fluvial system and a lower cross-bedded gravelly-sand braided fluvial system separated by a terrace. Seismic reflection data image the terrace, but depositional structures within the aquifer are below seismic resolution. These structures, including meter-scale channels and decimeter-scale channel-fill facies are imaged with ground penetrating radar , which correlates well with the outcrop and with the DC resistivity profile. The meandering fluvial facies is characterized by relatively low resistivity values (500–1250 Ω-m) and laterally continuous high-amplitude radar reflections. Lateral reflection terminations in the radar data indicate onlap at channel boundaries, which are filled with silty clay and clayey silt with very low resistivity values (50–400 Ω-m). The braided fluvial facies is characterized by higher resistivity values (1600–2600 Ω-m) and low-amplitude short bidirectional dipping radar reflections. Laboratory measurements of resistivity from a core collected along the quarry wall show a linear correlation to clay + silt content in all units within the meandering facies that have greater than 2% clay fraction, allowing surface DC resistivity data to be used as a predictor of clay + silt content in this facies. This is confirmed by comparison of predictions of clay + silt fraction based on the resistivity data to outcrop lithology. The combination of all three geophysical datasets is essential to fully characterize compartmentalization within this heterogeneous aquifer system.