INVESTIGATING EARLY INDICATIONS OF A POTENTIAL SINKHOLE IN AN AGRICULTURAL AREA OF ROCKINGHAM COUNTY, VIRGINIA, USING ELECTRICAL RESISTIVITY IMAGING

A farmer in Bridgewater, Virginia, consistently observed a small patch of his land where snow would not accumulate, and crops matured at an accelerated rate for extended periods. Curious about its implications and cautious of a potential developing sinkhole, posing a danger to heavy equipment, the farmer contacted the county to investigate further.

Electrical resistivity, a non-destructive geophysical method, provided 2D images of the conductivity distribution within the subsurface. This study analyzes outcomes from merged electrical resistivity tomography (ERT) lines of dipole-dipole and Schlumberger arrays via a 2D inversion modeling software. These inversion models employed a Cholesky Decomposition, least-squares method with smoothness constraints and differing noise thresholds.

Two 56-electrode resistivity lines were placed with 6.25 m spacing between electrodes, crossing through the area of concern and an additional thermally variant patch (detected by thermal drone readings conducted in conjunction with this work). The results revealed a conductive pathway of approximately 60-300 Ohm-m, suggesting a connection between the water-saturated near-surfaces of both anomalies to what appears to be a conduit in the subsurface linking the two thermal anomalies at a depth of 20-30 meters and continuing beyond them at greater depth--which is consistent with known processes for sinkhole formation.

Nevertheless, several questions remain outstanding that will constrain the final interpretation. While the accelerated growth within these anomalous patches suggests that radiation of heat (e.g., a warm spring), it has yet to be determined if the saturated ground absorbs heat from the atmosphere. The ERT surveys do not indicate whether the development of a sinkhole poses an immediate danger or if the concern is more long-term, though the persistence of the thermal anomaly through sixteen years of historical imagery suggests the latter. The ninety-meter distance between the two connected patches does signify that the developing sinkhole could eventually be of significant size.