Coupled synchrotron x-ray imaging and static/dynamic acoustoelastic testing to understand the nonlinear elasticity of fractured rock under stress

Seismic characterization of fractures requires quantitative mapping between in-situ fracture characteristics and the corresponding elastodynamic properties. Laboratory experiments coupling X-ray imaging and ultrasonic testing are conducted to quantify how bulk/fracture deformation, porosity, and contact area/distribution influence the elastodynamic response of stressed fractured rock. The stress-dependency of the ultrasonic wave speed, amplitude, and frequency are analyzed in relation to the changes in true contact area, contact size distribution, and fracture aperture gleaned from high resolution in situ 3-D and 2-D synchrotron X-ray imaging during quasi-static and dynamic loading of Westerly granite and Berea sandstone samples with saw-cut fractures normal to the loading axis. A subset of fractured Westerly granite samples is thermally damaged to decouple the influence of bulk damage and fault on the ultrasonic signatures. We confirm connections between ultrasonic transmission and contact area and find a correspondence between the transmitted wave frequency and fracture aperture. We observe the lowest acoustic nonlinearity parameter ß for the sample with the largest estimated contact area; the two samples with similar contact areas but different contact size distributions yield similar ß irrespective of the bulk stiffness. These findings have implications in improving the interpretability of seismic attributes in relation to subsurface fracture properties.