An Effective-Medium-Based Model for P-Wave Velocities of Saturated, Unconsolidated Saline Permafrost
A new rock physics model provides superb fits to experiment data and important insights on pore-scale distributions of ice in saline permafrost.
The Next-Generation Ecosystem Experiments (NGEE)–Arctic team developed an effective medium–based rock physics model for inferring ice content of saline permafrost from seismic P-wave velocities. Unlike many existing models that either only consider a single type of pore-scale ice distribution or rely on many tuning parameters to accounting for multiple ice distributions, the model developed in this project requires only one free parameter to achieve superb data fits.
The model provides important insights on pore-scale distributions of ice in saturated, unconsolidated saline permafrost. The modeling workflow is not only useful for permafrost, but also applicable to hydrate-bearing sediments. The team’s approach could also be generalizable to modeling cementation processes where both pore-filling and contact-cementing materials coexist in the pore space.
To better understand the relationship between P-wave velocities and ice content in saturated, unconsolidated saline permafrost, the research team constructed an effective-medium model based on ultrasonic P-wave data that were obtained from earlier laboratory studies. The model uses a two–end member mixing approach in which an ice-filled, fully frozen end member and a water-filled, fully unfrozen end member are mixed together to form the effective medium of partially frozen sediments. This mixing approach has two key advantages: (1) It does not require parameter tuning of the mixing ratios and (2) it inherently assumes mixed pore-scale distributions of ice that consist of frame-strengthening (i.e., cementing and/or load-bearing) ice and pore-filling ice. The model-predicted P-wave velocities agree well with the team’s laboratory data, demonstrating the effectiveness of the model for quantitatively inferring ice content from P-wave velocities. The modeling workflow is simple and is largely free of calibration parameters—attributes that ease its application in interpreting field data sets.
Lawrence Berkeley National Laboratory
As part of the Next-Generation Ecosystem Experiments (NGEE)–Arctic) project sponsored by the Office of Biological and Environmental Research within the U.S. Department of Energy (DOE) Office of Science, this study is supported through contract DEAC0205CH11231 to Lawrence Berkeley National Laboratory and through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory.
Dou, Shan, Seiji Nakagawa, Douglas Dreger, and Jonathan Ajo-Franklin. ”An effective-medium model for P-wave velocities of saturated, unconsolidated saline permafrost.” Geophysics 82(3), EN33–EN50 (2017). DOI:10.1190/geo2016-0474.1