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.