A HYDROGEOPHYSICAL INVESTIGATION OF LOGAN, MT USING ELECTRICAL TECHNIQUES AND SEISMIC REFRACTION TOMOGRAPHY

Logan, Montana USA is located on the Gallatin River, one of the three rivers forming the headwaters of the Missouri River. Previous hydrogeological studies have determined that the location at Logan is a possible pinch-point for the local Gallatin watershed where shallow groundwater is expected to discharge into the Gallatin River. However, groundwater monitoring wells indicate dry Tertiary sediments overlying bedrock, suggesting surface and groundwater systems are disconnected. We performed shallow seismic refraction, electrical resistivity, spontaneous potential, and electromagnetic surveys to investigate the groundwater system in the study area. Geophysical measurements were obtained near the Gallatin River and close to shallow monitoring wells. Also, we used hand samples of Mississippian aged rocks of the Madison group collected from local outcrops to help correlate geophysical results with properties of the bedrock. Interpretations of geophysical data confirm the shallow bedrock and dry sediments encountered in nearby wells. These results suggest that any pinch-point is located further upstream or that groundwater follows a network of preferential flow paths through the limestone bedrock within the study area