Airborne geophysical characterization of critical zone structure in a mountain headwater system, upper East River, Colorado

Abstract:

Geologic controls on groundwater flow, particularly in tectonically and topographically complex mountainous terrain, can be difficult to quantify without a detailed understanding of the regional subsurface geologic structure. This structure can influence the magnitude of groundwater flow through the mountain block, which in turn impacts groundwater composition and the flux of metals and nutrients to the near-surface ecosystem. In support of several ongoing studies in the upper East River and surrounding watersheds in central Colorado, regional-scale airborne electromagnetic (AEM), magnetic, and radiometric surveys were conducted in late 2017 over an area of nearly 500 square kilometers. These data give a new view of the geologic structure underlying the region that is unprecedented in both resolution and spatial coverage. Inversions of the AEM data indicate good correlation with known geology and help to extend interpretations of geological model structure in this complex mountain watershed. Resistivity values exhibit a large dynamic range over the entire survey area—spanning more than four orders of magnitude—and suggest that AEM data will be useful in distinguishing important geologic features in the study area. Portions of the dataset exhibit strong airborne induced polarization (AIP) effects, including double sign changes, which provide additional constraints on areas that may contain polarizable materials such as disseminated metals, fine grained lithology, or geochemical alterations. Modelling AIP allows extracting maximum information from the dataset, producing at the same time corrected resistivity models and chargeability models that display more realistic geological features. We will discuss how these airborne geophysical datasets, along with existing ground-based observations, is being used to add a new dimension in mapping critical zone structure with high spatial resolution.