Applications of borehole geophysical logs in groundwater studies

Abstract

Over the past 5-year program cycle, the Geological Survey of Canada (GSC) has completed downhole geophysical logging to i) support regional stratigraphic studies, ii) conduct velocity analyses for seismic reflection surveys, iii) support hydrogeophysics studies, iv) adopt new methodologies, and v) maintain a national calibration facility. The geophysical logs provide in situ information on physical and chemical properties of sediment and rock, and of borehole fluid parameters to support groundwater studies through improved lithological characterization. Using examples from different project settings across southern and eastern Ontario, the benefits of an integrated approach to geophysical, geological, geochemical, and hydrogeological datasets will be discussed. In collaboration with the Ontario Geological Survey's 3D Quaternary sediment mapping projects in Southern Ontario, the GSC collected geophysical logs in 20 cored boreholes in the Niagara and central Simcoe County regions. The integration of sediment core examination with geophysical logs supported an enhanced understanding of local complex geological processes. While significant changes in log responses at key stratigraphic boundaries helped to distinguish potentially water-bearing coarse-grained units from fine-grained aquitards, logs yielded equally useful information about subtle changes in grain sizes, mineralogy, and velocity structure related to depositional conditions. At two sites east of Toronto, geophysical logs were interpreted together with chemostratigraphic data to identify geochemical changes in the sediment column. Elemental variations were well aligned with geophysical log responses and changes in grain size. Geophysical logs also identified non-chemical changes in sediment, such as variations in porosity/density, or pore fluid chemistry, providing highly complementary datasets for detailed litho-stratigraphic analyses. Over the past few years, new technologies have been applied in GSC bedrock and sediment calibration boreholes. The use of a GSC-developed controlled frequency vibratory source is allowing for selection of source parameters fine-tuned for different geological settings. This is showing great potential for high-resolution downhole velocity testing, supporting optimized reflection seismic velocity conversions. An experimental deployment of slim-hole borehole magnetic resonance (BMR) tools in calibration holes around Ottawa was conducted in bedrock and post-glacial sediments. These instruments are designed to measure formation porosity and pore size distribution without the use of radioactive sources. Although the influence of magnetite-bearing minerals from Shield-derived sediments is a topic of ongoing study, early qualitative results from the tests are encouraging. The evaluation of new tools using calibration datasets is an important driver for the ongoing work to update geophysical calibration facilities.