Use of Integrated Geophysics to Characterize Paleo-Fluvial Deposits

Near-surface fluvial depositional environments at two field sites were characterized using integrated geophysical surveying techniques to determine the location and extent of paleo-fluvial deposits. The integrated surveys consisted of ground conductivity and electrical dipole-dipole resistivity measurements. The two techniques were used in combination to map the geoelectrical structure of the subsurface from which correlations to geologic structures were made. The correlations were subsequently confirmed by exploratory drilling and test trench data. Subsurface zones exhibiting high electrical resistivity values were correlated to coarse-grained fluvial deposits (i.e., sands and gravels) and zones exhibiting low electrical resistivity values were correlated with fine-grained fluvial deposits (i.e., clays and silts). Ground conductivity surveys were conducted to generate a map of the near-surface expression of the fluvial depositional environment. Dipole-dipole resistivity surveys were subsequently performed to generate vertical images of the subsurface structure and to map the vertical and lateral extent of the paleo-fluvial depositional environments. The integrated geophysical survey method successfully discriminated between fine-grained and coarse-grained fluvial deposits due to the significant electrical contrast that exists between these types of sediments and allowed for greater insight of the subsurface geologic structure and the nature of the depositional environments.