Geophysical tools in hydrogeological studies of shallow unconfined aquifers: Gravity for reconnaissance survey and ground-penetrating radar for high-resolution applications

Shallow unconfined aquifers of glacial origin are a characteristic feature of the hydrogeological landscape of northeastern United States. They form the most productive chain of aquifer systems in this region, and it is essential to develop appropriate geophysical tools to characterize and inventory them. Gravity method, despite its proven potential for geophysical studies of aquifers, was found to be quite underutilized in this region. In order to examine the effectiveness of this method in characterizing this class of aquifers, a pilot study was undertaken to delineate the bedrock topography across the Palmer river basin, a member aquifer of this system. Various refinements were made in the method to make proper use of the advantages as well as to address the challenges characteristic of this type of aquifers, and the bedrock was delineated within a reasonable confidence interval despite sizable uncertainties in the gravity data.; After a successful gravity study, we looked for other geophysical tools that could provide complementary information on such aquifers. We used seismic refraction and found it quite effective in mapping the vital constraints of an aquifer like water table and bedrock in detail, to a resolution of a few meters. For still finer resolution, we decided to make an in-depth study of GPR method for its high-resolution applications to shallow subsurface imaging.; Keeping in view the rich information content of GPR data, we felt that the key strategy for a successful GPR study lay in the proper diagnosis and identification of the various phases in the image. We tried to accomplish this through extensive field study and the use of basic laws of electromagnetic wave propagation.; The surficial geology of northeastern United States provided an ideal setup for a possible refraction study in GPR. We identified refraction phases in a number of field observations, and used them to map the respective refracting interfaces by employing certain standard seismic refraction techniques. The results, verified through corresponding reflection surveys and even through direct measurement using hammer-driven probes in some cases, showed close agreement, thereby illustrating the potential of the refraction method in GPR studies.