The Application Of Very Low Frequency Electromagnetics And Downhole Geophysical Techniques To An Island Hydrogeologic Study: Rhode Island Formation, Narragansett Basin

Very Low Frequency (VLF) geophysics has been utilized to supplement data collected using<br>downhole geophysical techniques for the purpose of studying a fractured bedrock aquifer on<br>Conanicut Island in Narragansett Bay, Rhode Island. Other components of a larger ongoing study<br>include the modeling of groundwater flow, the study of the aquifer’s natural hydrogeochemistry,<br>and the anthropogenic impact on the aquifer. The aquifer is comprised of a portion of the Rhode<br>Island Formation, which ranges from a micaceous schist to a meta-sandstone and<br>meta-conglomerate. Geophysical and outcrop measurements reveal five fracture orientations<br>through which groundwater is expected to flow. These correspond to the bedrock’s dominant<br>foliation as well as numerous fracture surfaces apparently related to post-Carboniferous tectonics.<br>The groundwater model is being developed to evaluate the suitability of treating the aquifer as<br>an equivalent porous medium. Downhole geophysical investigations conducted on Conanicut<br>Island by the United States Geological Survey (USGS) yielded data which appear to suggest a<br>predictable trend in the flow of groundwater into a bedrock well with depth. Based on data<br>collected at the time of this publication, groundwater flow to the well is observed to decrease<br>exponentially with depth. The location of these fractures and their orientations provided the<br>impetus for the VLF application. Linear filtering of VLF measurements (after Karous and Hjelt,<br>1983) has provided for the modeling of the positions and orientations of large water-bearing<br>fracture zones. Fracture orientations based on VLF geophysical interpretations appear to<br>correspond to borehole televiewer images and orientations measured at formation outcrops on the<br>island.