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Seismic and Gravity Imaging for Dam Design in a Complex Geologic Setting

Description: 

Major infrastructure construction typically relies on boreholes to characterize geotechnical properties. In complex geologic settings, however, drilling may be insufficient to capture the full range of subsurface conditions and geohazards. Here we present perspectives from a project to develop a 96,000 acre-foot reservoir with four dams totaling 10,000 linear feet in Colorado’s Rocky Mountains. Challenges at the site include dozens of polyphase faults concealed by 100+ meters of local volcaniclastics, gypsum karst sinkholes, nearly 50 different geologic units with differing geotechnical properties, and rich cultural resources that preclude intrusive investigations in much of the area. Proactively building a comprehensive, 3D geologic/geotechnical model by integrating borehole data, geologic mapping, and geophysics has been key to adapting to geologic complexity while optimizing engineering design and construction planning with respect to local geology. Site-wide gravity surveys and 3D gravimetric tomography efficiently initialized the model, guiding dam layout to avoid dissolution-prone and poorly lithified units while taking advantage of antiformal bedrock highs. Boreholes then focused on gathering samples from dam foundations and other key areas. Active-source seismic imaging, including full-waveform, reflection, and 3D tomography, constrained structure between boreholes. In turn, the seamless gravity model plus geologic mapping allowed robust constraints on 3D structure beyond borehole and seismic footprints. The 3D geologic model was then populated with physical parameters determined from lab tests on borehole samples, allowing the subsurface to be treated as an engineering material for dam design. The 3D modeling reduced project cost by allowing design to avoid problematic units, take advantage of favorable structures (e.g., bedrock highs), estimate foundation excavation depths, and identify onsite quarry material. We conclude that comprehensive 3D geotechnical models can optimize planning and design, and that geophysics provides a strong base for these models, even in the most challenging geologic settings.

Session: Geophysics in a Changing World: Monitoring Applications from Seismology and Beyond - I

Type: Oral

Date: 4/15/2025

Presentation Time: 05:15 PM (local time)

Presenting Author: Will

Student Presenter: No

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