TIME-LAPSE RESISTIVITY IMAGING OF A FLUVIAL AQUIFER IN COLUMBUS, MISSISSIPPI: A COMPARISON OF DRAINED AND SATURATED CONDITIONS

An unconfined fluvial aquifer in Columbus, MS was surveyed using surface DC electrical resistivity to determine the effects that water saturation has on resistivity images of fluvial aquifer systems and to assess the validity of using studies of drained outcrops as analogs for interpretation of resistivity data collected to image buried saturated aquifers. The aquifer is composed of an upper 3 m thick flood plain facies consisting of clay and silty-clay filled channels and silty-sand, and a lower 5 m thick braided stream facies consisting of sandy-gravel. The braided stream facies is underlain by a clay-rich bed that forms the basal aquitard. The study area is located in a sand and gravel quarry, which provided good access to an outcrop located on one of the quarry walls. A survey conducted in 2002 imaged the aquifer in a drained state when quarrying was underway and the quarry pit was drained by pumping. It was re-surveyed in 2004 after quarry operations had ended and the quarry pit was filled with water. The affects of groundwater saturation on electrical resistivity are strongly dependant on the sedimentary facies. Resistivity generally decreases with water saturation in the meandering fluvial facies, which is dominated by silt and clayey-silt flood plain deposits. This is attributed to ionic currents in the pore fluid. Clay and silty-clay filled channels within this facies show very little change in resistivity with changing bulk-saturation of the aquifer. This is attributed to a lack of groundwater penetration into these relatively impermeable deposits. In the braided fluvial facies, resistivity generally increases with increasing water saturation. The cause of this is uncertain, but may be related to the development of surface charges on saturated clays, which act as capacitors and impede ionic current flow. Channels and major lithologic boundaries are well constrained in both the saturated and unsaturated states, indicating that resistivity images of drained outcrops provide a reliable analog that can be used for stratigraphic interpretation of data from buried, saturated aquifers. However, changes in resistivity with saturation are strongly dependant on clay content and degree of sorting, complicating detailed lithological interpretations in the absence of corollary data.