MAPPING BURIED KARST CONDUITS: WENNER ARRAY PROFILING VS 2D MULTI-ELECTRODE EARTH RESISTIVITY SECTIONS

The purpose of this investigation was to test whether a line of resistivity soundings could delineate changes in bedrock depth and identifies positions of solutionally-enlarged fractures, without recourse to much more sophisticated and expensive 2D methods. This is a useful test since many karst investigators, particularly in the developing world, only have access to simple resistivity sounding equipment.

The test involved making an 80 m long Wenner array vertical electrical sounding (VES) profile across a filled sinkhole at the Perry Farm Park near Kankakee in northeastern Illinois. Approximately 3-5 m of loess and till mantle karstic Silurian dolomite beneath the Park. This profile line coincides with the location of an automated 2D multi-electrode dipole-dipole resistivity survey. Each of the Wenner array soundings was inverted for a 2-layer model. The Wenner sounding profile did not show deepening of the fresh bedrock interface over the fracture zone, but did indicate an anomalous low resistivity zone. Borehole core correlates well with the predicted depth and width of this zone, which contains fractured and altered dolomite. The inverted 2D model from the dipole-dipole pseudosection shows similar bedrock topography and the location of the same anomalous low resistivity zone. The dipole-dipole profile also depicts a gently dipping feature in this anomalous zone. Dolomite bedrock resistivities along this profile vary with depth and lateral position, probably indicating various degrees of weathering and/or varying moisture content within the bedrock.