Active ultrasonic imaging and interfacial characterization of stationary and evolving fractures in rock

The objective of this study is to deploy ultrasonic waves toward better understanding of preexisting and evolving fractures in rock, with the dual focus on i) reconstructing the curvilinear fracture geometry, and ii) mapping the distribution of its heterogeneous specific (shear and normal) stiffness. This is accomplished via the 3D Scanning Laser Doppler Vibrometer (SLDV) that is capable of monitoring the triaxial particle velocity at every scan point on the sample's surface. Experiments are performed on slab-like granite specimens featuring either stationary or evolving fractures where the fracturing, in the latter case, occurs in 3-point bending configuration. The rock specimens are then excited, under the plane stress condition, by a piezoelectric transducer at 20-30kHz, while the in-plane velocity response of the sample is monitored over a rectangular region covering the fracture. Thus obtained full-field data are next used to recover both the fracture geometry, and to expose its nonlinear contact behavior. The latter is then approximated point-wise in terms of the linearized contact properties i.e. specific (shear and normal) stiffness, whose recovered spatial variations for stationary and advancing fractures are found to conform with expected trends.