Abstract:
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.
Introduction
Geometric and interfacial properties of fractures and related features (e.g. faults) in rock and other like materials are the subject of critical importance to a wide spectrum of scientific and technological facets of our society including energy production from natural gas and geothermal resources (Baird et al. 2013, Verdon and Wustefeld 2013, Taron and Elsworth 2010), seismology (McLaskey et al. 2012), hydrogeology (Cook 1992), environmental protection (Place et al. 2014), and mining (Gu et al. 1993). Unfortunately, a direct access to fracture surfaces in rock is, in most field situations, either non-existent or extremely limited (e.g. via isolated boreholes, shafts, or adits), which necessitate the use of remote sensing techniques where the contact law at the boundary of rock discontinuities is often assumed to be linear and represented in a parametric fashion via e.g. the so-called (shear and normal) specific stiffness, relating the contact traction to the jump in displacements across the interface (Schoenberg 1980). Despite its heuristic and simplistic nature, the fracture's interfacial stiffness matrix not only is proven to be immediately relevant to the stress and thus stability analyses in rock masses (Eberhardt et al. 2004), but also bears an intimate connection to the fracture's hydraulic properties (Pyrak-Nolte and Nolte 2016, Pyrak-Nolte and Morris 2000), and may serve as a precursor of progressive shear failure along rock discontinuities (Hedayat et al 2014).
