Constructing Geological Models in Mountainous Watersheds by Integrating Airborne Geophysics and Geological Maps for Improved Hydrological Understanding

Abstract:

Developing accurate hydrological models is paramount for predicting water quality and availability. Quantifying subsurface heterogeneity in mountainous watersheds– particularly bedrock structure and hydraulic parameters –is a significant challenge; limiting our ability to quantify deeper groundwater flow that are increasingly recognized as an important component of streamflow. Often, bedrock information is sparse and direct knowledge (if any) comes from a solitary well or limited access to sporadic rock outcrops that does not accurately depict the geologic variability controlling subsurface fluid movement. At the same time, although the surface geologic maps are readily available in many locations, such information along with geologists’ expert knowledge has not been effectively used for parameterizing subsurface properties.

In this study, we develop innovative methodology to construct a 3D geological model for watershed hydrological models as well as geophysical data/interpretation, digital elevation models, borehole data, and field observations. In particular, we focus on integrating surface and airborne geophysical surveys, which provide critical information on the geological unit interface depths as well as the variability in bedrock properties (e.g., fracture densities, effective porosity) within a single geologic formation. A robust geomodelling software (SKUA-GOCAD) enables us to incorporate the publicly available geological maps to build a first-pass geological model using the Structure and Stratigraphy Workflow. Geophysical data are then incorporated to refine the initial model horizon boundaries and to develop a geochronologic structure. Identified fault systems can be modeled as 3D planes, while intrusive rock bodies can be modeled based on airborne geophysical results. This workflow results in a structurally coherent model that honors the geometric relationships and geologic boundaries of the subsurface, resulting in an accurate geological model that fits various data interpretations. We demonstrated this approach using the datasets in the East River Watershed, Crested Butte, CO and discuss implementation of these datasets into watershed-scale hydrologic models.