Cross-Plotting Conventional Geophysical Well Logs To Identify Permeable Pathways In A Fractured Bedrock Aquifer

Fractures in the Galena-Platteville aquifer serve as conduits for the migration of<br>groundwater contaminants. Conventional geophysical well-log data from the Parson’s Casket<br>Hardware Superfund site, Beividere, IL were cross-plotted to test the hypothesis that permeable<br>pathways in Galena Group dolomite produce diagnostic clustering of log values. Bedding-plane<br>fractures, subvertical fractures, and zones of vuggy porosity were identified in several wells and<br>then geophysical well logs, including natural gamma, short-normal resistivity (SNR), and caliper<br>were examined for the same intervals. Log values were cross-plotted and visually inspected for<br>clustering in the vicinity of permeable intervals. A bedding-plane fracture produced two highresistivity,<br>low-gamma clusters and a low resistivity, high gamma “tail”. The two clusters<br>represent a difference in lithology on either side of the fracture and the “tail” represents a claychoked<br>bedding-plane fracture. The fracture produced clustering of log values in several adjacent<br>wells. Inconsistent clustering is observed for SNR versus caliper plots in subvertical fracture<br>zones. Low resistivities and enlarged borehole diameter appear to correlate with subvertical<br>fracturing; however, clustering signatures were not repeated between adjacent wells cut by steeply<br>dipping fractures. Cross-plots of SNR versus gamma and SNR versus caliper exhibited clustering<br>of log values for intervals of vuggy porosity. Logs from several wells produced bimodal clusters<br>which suggest lithology and/or porosity variations throughout the vuggy interval. This study<br>demonstrates that typical water-well logs exhibit clustering of log values associated with<br>permeable fractures and horizons; however, only those clusters associated with bedding-plane<br>fractures proved to be repeatable between wells.