Amphibolites Study from Various Borehole Depth

ABSTRACT In this study the elastic properties of crystalline rocks – amphibolites from different boreholes were studied. We studied the depth effect on the elastic properties of mineralogically quite homogenous rocks – amphibolites. The samples were taken from three different crustal levels – shallow (tens of meters) Stařechov (Czech Republic), medium deep (first thousands of meters) KTB (Germany), and deep crustal depths (up to 12 km) KSDB3 (Russia). The elastic properties were first determined experimentally using high-pressure apparatus and 3D ultrasonic testing. The effect of main rock fabric components were evaluated using the method of inverse calculation from experimental data. There was found an increase of P-wave velocity, P-wave stiffness elastic constants with the depth for 19 samples, collected from different depth, and all loaded to 400 MPa of hydrostatic pressure. This fact can be explained by the stress memory effect. Stiffness constants and Young modulus were determined for primary material orientation INTRODUCTION Elastic anisotropy is a crucial physical property of crustal rocks namely when reflecting deformational history of the rock. Knowledge of the anisotropy of deep crustal rocks (e.g., Kern et al. 1991) is of special interest for studies focused on the evolution of the crust. The relationship between amphibolite elastic properties and mineral composition and/or rock fabric has been extensively studied in the past (Siegesmund et al. 1989). The influence of microcracks on the elastic wave velocity and its anisotropy has been noted in many previous studies (Crampin 1977, Babuška and Pros 1984 among others). Numerous studies are focused on the systematic study of elastic properties of rocks extracted from boreholes e.g., KTB (Berckhemer et al. 1997) or KSDB3 (Nikitin et al., 2001, Saikia et al. 2021). In all of these studies, there is missing comparative research concentrated on the relationship between the actual depth of rock occurrence and its elastic properties.