RESOLVING GRAVEL POINT-BAR, OVERBANK, AND GLACIOFLUVIAL DEPOSITS USING ELECTRICAL RESISTIVITY GROUND IMAGING (ERGI), MAD RIVER, WEST-CENTRAL OHIO, USA

The gravel-bed Mad River in west-central Ohio flows through a low-relief valley that is filled with 80 m of glaciofluvial outwash and a meters-thick veneer of Holocene fluvial deposits. In the study reach, bankfull channel width is 50 m to 70 m and bankfull depth is 3 m to 4 m. Grain sizes of channel-bar deposits are dominantly coarse to very coarse gravel and small cobbles. Overbank deposits are dominantly clay to coarse sand. Outwash deposits are typically gravel.

Electrical resistivity ground imaging (ERGI) surveys were conducted to test the effectiveness of ERGI at resolving the various deposits, as well as their water content. ERGI is a geophysical technique that provides information on the electrical properties of materials at depth, specifically the resistance to current flow. ERGI is sensitive to sediment properties such as water content, grain size, and composition. Three surveys were conducted across a gravel point bar in both the across-stream and along-stream directions, and from an inner-bar chute channel up onto the adjacent floodplain. Resultant profiles were 14 m and 27 m long with depths of 3 m and 6 m, respectively.

Results show that well-drained surface gravels (resistivities of >400-1700 Ohm-m) are readily differentiated from associated gravel-bar deposits below the water table (resistivities dominantly <100 Ohm-m). Additionally, the fill of a cross-bar channel was discernable from other bar deposits. In all surveys, underlying glaciofluvial gravels (resistivities of <200 Ohm-m) were readily differentiated from the recent bar and overbank deposits. The position of the imaged contact between the recent deposits and the glaciofluvial outwash correlated with bankfull channel depth. Resistivities of overbank deposits varied primarily with moisture content related, in part, to topography (from a few tens of Ohm-m up to 150 Ohm-m). Nearly saturated deposits of a chute channel near the inner bank exhibited low resistivities (a few tens of Ohm-m) and were readily discernable from adjacent overbank and subjacent glaciofluvial deposits. Additionally, the along-stream survey, from the upstream portion of the point bar to the downstream portion, appears to show differences in the amount and distribution of water attributable to the nature of groundwater flow through the bar and differences in hydraulic head.