Collaborative Research: DAMPED - Developing Advanced Methods for Profiling Energy Dissipation of Seismic Waves In Situ

The safe and cost-effective design of civil infrastructure depends on accurately predicting how the ground responds to earthquakes and vibrations generated by human activities. This is especially important for critical facilities, such as nuclear power plants, that must withstand strong ground motions. A key factor controlling this response is damping, the natural ability of soil and rock to dissipate the energy of traveling seismic waves. Although engineers can readily measure how fast seismic waves travel through the ground, measuring how much energy the ground dissipates remains difficult and often unreliable. Consequently, engineers frequently rely on laboratory tests of small soil samples that may not represent the much larger volume of soil and rock beneath a structure. This research will develop and validate new field methods for directly measuring energy dissipation in the ground. More reliable measurements will improve safety and reduce the cost of critical infrastructure, including nuclear power plants and small modular reactors, which are increasingly being considered to help meet the nation’s growing demand for electricity. The project will also broaden participation in science and engineering through educational activities serving students from pre-college through graduate school, as well as workshops for practicing engineers and geoscientists. This research will investigate the small-strain damping ratio of geomaterials by addressing four fundamental questions. First, what are the relative contributions of material damping, geometric spreading, and apparent damping from wave scattering to the in-situ wavefield attenuation at scales relevant to both seismic site response (hundreds of meters) and anthropogenic dynamic loading (tens of meters)? Second, is the energy dissipation associated with each of these three mechanisms frequency-dependent or frequency-independent within the frequency bands relevant to these problems? Third, how can small-strain damping be measured in situ more reliably across these varied spatial scales and frequency bands? Fourth, to what extent do in-situ measurements of damping improve the predictive accuracy of multidimensional ground response analyses? To address these questions, the research team will pursue five interconnected research tasks. High-fidelity viscoelastic wave-propagation simulations will be used to isolate the individual attenuation mechanisms. Invasive testing methods will provide high-resolution damping profiles within the shallow subsurface, while noninvasive methods will extend the measurements to depths beyond the practical reach of invasive testing. Laboratory studies will compare laboratory- and field-based damping estimates. Finally, ground-response analyses at a well-instrumented downhole-array test site will evaluate whether field-based damping measurements improve predictions of recorded ground motions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.