Measuring Groundwater in the Snowy Range with Nuclear Magnetic Resonance and Airborne Electromagnetic Mapping

The rivers that supply water to most of the West rise in the Rocky Mountains. As drought increases across the country, understanding the hydrology of these alpine regions becomes important to assuring water supplies in the future. The geophysical technique nuclear magnetic resonance (NMR) can make a significant contribution to this effort because it directly measures water content and flow properties of an aquifer, which are typically difficult hydrologic variables to quantify. In this project, conductivity data from an airborne electromagnetic survey in the Snowy Range was analyzed together with NMR soundings. Conductivity is relatively fast to measure and covers a large geographic area, but it detects bedrock changes or conductive anomalies like ore concentrations as well as the presence of groundwater. NMR soundings on conductive areas provided ground truthing by testing whether water was present, at what depth, and how much. This makes it possible to tell what water levels were detected by the conductivity measurement, which theoretically allows the groundwater map to be extended beyond the point NMR measurements and throughout the airborne survey area. NMR data is highly susceptible to electromagnetic noise, and few existing studies have concentrated on this issue. Fractured rock aquifers such as those in this study have even lower signal to noise ratios than most NMR data; therefore, a key part of the project was determining how best to filter and invert the data to get valid results.