SUBSURFACE VOID EVALUATION AND BEDROCK TOPOGRAPHY OF THE CEDARVILLE DOLOMITE (SILURIAN), WEST-CENTRAL OHIO, USING ELECTRICAL RESISTIVITY GROUND IMAGING (ERGI)

The Silurian Cedarville Dolomite forms much of the bedrock in west-central Ohio and is commonly quarried for use as aggregate. The presence of sizable voids is of interest to quarry operators because blasting rock with voids increases the hazards associated with ejecta. Additionally, quarry operators are interested in bedrock topography and the depth to bedrock, which is proportional to the cost to remove overburden. The Cedarville exhibits extensive submillimeter- to cm-scale secondary porosity. Larger decimeter- to meter-scale voids within some quarry walls, as well as in outcrops of Cedarville elsewhere, suggest the potential for voids in areas yet to be quarried. This study used electrical resistivity ground imaging (ERGI) to evaluate the presence of subsurface voids, image bedrock topography, and determine the depth to bedrock of the Cedarville Dolomite adjacent to an active quarry in Darke County, west-central Ohio.

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 and rock properties such as water content, grain size, and composition. Previous studies by Zaleha and other researchers elsewhere have shown that ERGI is particularly useful for detecting air- and water-filled voids. Four ERGI surveys were conducted using a Super Sting R1 IP resistivity meter and an array of 28 electrodes. Survey lengths ranged from 81-162 m with depths of 17-34 m.

Results show that the dolomitic bedrock (resistivities of >100-650 Ohm-m) is readily differentiated from the overlying glacial till (resistivities of <100 Ohm-m). Resistivities of the bedrock are generally comparable to those of the Cedarville elsewhere. However, in one part of the study area, bedrock resistivities are lower, attributable to higher water content associated with a locally high, perched water table. Results also indicated that no sizable voids are present beneath the surveyed profiles. On a large scale throughout the study area, the general depth to bedrock on the ERGI profiles varies little. However, the surveys do show local erosional relief on the bedrock surface on the order of 4-9 m over distances of meters to tens of meters.