Intrinsic Elastic Anisotropy of Westerly Granite Observed by Ultrasound Measurements, Microstructural Investigations, and Neutron Diffraction

Abstract

Granitic rocks attract attention because of their elastic and mechanical properties. A large number of dedicated experimental studies has been initiated due to relevance of granites to development of nuclear waste storage, geothermal projects, rock mechanics, and earthquake mechanics. Quantification of elastic properties of granites is important to determine crustal seismic velocities and stress orientation. Generally, it is assumed that granitic rocks are elastically isotropic. To determine whether rock material is isotropic or anisotropic ultrasonic methods can be applied (Babuška, 1968; Bajuk et al., 1967; Barruol and Kern. 1996; Kern & Wenk, 1990). Multidirectional (up to 132 independent directions) experiments are necessary in case of lower symmetry, unknown orientation of anisotropy or simply to diminish the effect of rock heterogeneity (e.g.,

Elastic anisotropies of deformed upper crustal rocks in the Alps

“…In all samples quartz (0001) and (11-20) show an asymmetry, which represents a sinistral motion indicating a top-to-the-north sense of shear. This is in accordance with literature and shows Zapport phase deformation in the Adula nappe (e.g., Löw, 1987;Meyre et al, 1993;Pleuger et al, 2003). Different orientation patterns of quartz pole figures (10-11) and (01-11) may be attributed to mechanical Dauphiné twinning or induced by active rhombohedral slip (e.g., Stipp and Kunze, 2008;Wenk et al, 2019).…”

“…From peak conditions to its current position within the Lepontine dome, the Adula Nappe underwent several deformation phases. The oldest, peak to post-peak deformation phase is the eclogite facies Zapport phase, which is well documented in the central part of the nappe, where it was not overprinted by younger deformation phases (e.g., Löw, 1987;Meyre et al, 1993;Pleuger et al, 2003). The Zapport phase records the earliest stages of exhumation and led to boudinage of the eclogite lenses, isoclinal folding, an ax- ial plane foliation, a N-S-trending stretching lineation and a top-to-the-north sense of shear (Meyre et al, 1993).…”

“…The central part of the Adula Nappe, where the samples for this study have been collected, exhibits a shallowly NE-dipping foliation and a NStrending lineation mainly formed during peak pressure and early stages of exhumation. The northern and southern parts of the nappe, however, have been overprinted by younger deformation (e.g., Löw, 1987;Nagel, 2008;Kossak et al, 2017).…”

Global Wave Velocity Change Measurement of Rock Material by Full-Waveform Correlation

“…Time-shift of the whole waveform is obtained by dividing the received waveform into several time windows with the length of twice the main period. When the change is large in a heterogeneous medium, the wave velocity and the natural vibration period of the medium will change according to the elastic wave theory [6,39]. Both translating and stretching need to be used to obtain time-shift.…”

“…At present, nondestructive testing (NDT) methods are commonly used for structural health monitoring (SHM) and damage assessment [5]. Wave velocity reflects the internal characteristics of a medium and can be quickly and conveniently obtained by NDT with ultrasound [6] or other methods [7,8]. For these reasons, it, as an important index, is intensively applied in damage assessment.…”