USE OF AZIMUTHAL ELECTRICAL RESISTIVITY TO STUDY BEDROCK FRACTURES IN THE SUBSURFACE AT RICE CREEK FIELD STATION, OSWEGO, NEW YORK

Azimuthal electrical resistivity (Wenner) is used to examine the heterogeneity of electrical conductivity around a point in the subsurface. This electrical geophysical technique can be used to approximate the strike of steeply dipping fracture. Oswego, NY is underlain by the Ordovician Oswego Formation, which consists primarily of medium to thick bedded sandstone with minor beds of siltstone and shale. The Oswego Formation is covered by ablation till and drumlins. Fracture analysis in the Oswego Formation exposures along the Lake Ontario shoreline produced two dominant sets. Both fracture sets are subvertical and strike NNW and ENE respectively. Fracture set two is associated with left-lateral en-echelon fracture zones and small left-lateral faults. Primary porosity within the sandstone units of the Oswego Formation is very low (usually less than 5% volume), therefore, groundwater in the bedrock occurs primarily in the fracture sets described above, and along fractured bedding planes.

Rice Creek Field Station (RCFS) is located approximately 1.5 miles from Lake Ontario. Ablation till and till associated with a drumlin are the dominate materials found in the shallow subsurface. Azimuthal resistivity data (a-spacing of 4 m) was collected throughout the grounds of the field station to: 1) examine bedrock fracture orientations in areas of thin till; and 2) examine the heterogeneity of electrical conductivity in the local drumlin deposit. In the valley at RCFS, the resistivity variation is consistent with the two fracture sets observed in bedrock locally, suggesting that bedrock is less than a few meters from the surface and that fracture orientations persist away from Lake Ontario. Azimuthal resistivity data collected on the drumlin produced some interesting results. Near the northern steep face of the drumlin, radial resistivity variations shows a strong WNW high trend and on the gentle southern slope the radial resistivity data showed no consistent trends. Minor reverse faults occur in some local drumlins and the consistent resistivity trend observed in the drumlin at RCFS may reflect fault surfaces in the steep face of the drumlin.