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

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.