Long-Wavelength Propagation in Fractured Rock Masses (3D Stress Field)

Abstract
Fractured rocks affect a wide range of natural processes and engineering systems. In most cases, the seismic characterization of fractured rock masses in the field involves wavelengths much longer than the fracture spacing; reproducing this condition in the laboratory is experimentally challenging. This experimental investigation explores the effect of fracture rock fabric and the 3D stress field on P wave propagation in the long-wavelength regime using a large-scale true triaxial device. P wave velocities increase with stress in the propagation direction and follow a power law of the form Vp=α(σ'/kPa)β; analyses and experimental results show that stress-sensitive fracture stiffness and fracture density define the α-factor and β-exponent; conversely, long-wavelength velocity versus stress data can be analyzed to identify the stress-dependent fracture stiffness. P wave velocities exhibit hysteretic behavior caused by inelastic fracture deformation and fabric changes. During deviatoric loading, the P wave velocity decreases in the two constant-stress directions due to the development of internal force chains and the ensuing three-dimensional deformation. Following a load increment, time-dependent contact deformations result in P wave velocity changes during the first several hours for the tested carbonate rocks; the asymptotic change in velocity is more pronounced for higher stress changes and stress levels. The fracture network geometry that defines the rock fabric acts as a low-pass filter to wave propagation so that wavelengths must be longer than two times the fracture spacing to propagate (Brillouin dispersion); the long-wavelength velocity and the fracture spacing determine the cutoff frequency. Fabric anisotropy contributes to anisotropic low-pass filtering effects in the rock mass.

Citation
Rached, R. M., Garcia, A. V., & Santamarina, J. C. (2022). Long-Wavelength Propagation in Fractured Rock Masses (3D Stress Field). Journal of Geophysical Research: Solid Earth. Portico. https://doi.org/10.1029/2022jb024907

Acknowledgements
Support for this research was provided by the KAUST endowment. G. Cascante and C. Lara (U. Waterloo) measured the piezo-crystals response under stress using laser measurements. G. Abelskamp edited the manuscript.

Publisher
American Geophysical Union (AGU)

Journal
Journal of Geophysical Research: Solid Earth

DOI
10.1029/2022jb024907

Additional Links
https://onlinelibrary.wiley.com/doi/10.1029/2022JB024907

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