CHARACTERIZING HYDROSTRATIGRAPHIC COMMUNICATION OF THE SEMI-CONSOLIDATED SEDIMENT AQUIFERS OF THE FLATHEAD VALLEY IN NORTHWESTERN MONTANA THROUGH HYDROGEOPHYSICAL SURVEYS

To accurately forecast the cascading effects of increased stress to a hydrologic system, characterization of the continuity and permeability of the primary confining layer (PCL) separating the shallow and deep intermontane alluvial aquifers is required. Geophysical methods provide a faster cost-effective alternative to drilling to acquire additional information on the changes of hydrostratigraphy with depth. Geoelectric resistivity models recovered through inversion of TEM central loop sounding data to delineate changes in geoelectric properties with depth, providing information on the depth, thickness and resistivity of the hydrostratigraphy. Comparison of geoelectric resistivity models with well completion report lithologies yield information about the permeability of the hydrostratigraphy and can infer the potential for occurring hydrostratigraphic communication. The geological history of the Flathead Valley created a complex stratigraphic sequence of glacial sediments comprising the primary confining layer (PCL). Glacial sediments include glaciolacustrine, glaciotectonite tills, subglacial traction tills and melt-out tills. Characterization of the PCL is the primary target for geophysical investigation as a critical element in understanding the hydrostratigraphic communication. The geoelectrical resistivity of glacial sediments is highly variable. Whether the PCL of the Flathead Valley, Montana presents geoelectrical property distinctions that are targetable by electromagnetic surveys is unknown. To assess the targetability of the glacial deposits comprising the PCL, a series of central loop soundings were completed. Geoelectrical models recovered through inversion and compared to well completion report lithology indicate the PCL presents a resistivity target that can be imaged using electromagnetic methods. The PCL appears to be variable throughout the Flathead Valley with predictable geoelectric resistivity ranges