A method for determining absolute ultrasonic velocities and elastic properties of experimental shear zones

Abstract

Laboratory experiments are a vital tool for assessing elastic properties of rock and determining the underlying geomechanical processes that inform large scale, predictive models. In some cases, relative values of elastic properties are sufficient, but absolute values are necessary when comparing between locations and to address upscaling from lab to field settings. However, determining absolute values of ultrasonic velocity and elastic parameters in laboratory experiments is often complex and hampered by apparatus design. Moreover, measuring the evolution of elastic properties with shear deformation has proven especially difficult. Here, we describe a method that allows measurements of P- and S-wave velocity as a function of shear deformation under stresses of 10's of MPa. The approach includes rigorous calibration experiments and accounts for the evolution of impedance contrasts at sample interfaces as a function of strain. We describe our method by applying it to sheared layers that represent simulated fault zones composed of clay-quartz mixtures and fault rocks recovered from drilling. P-wave arrival times range from 10 to 20 μs and wave speeds are 2–4 km/s during shear of layers a few mm in thickness subject to normal stress of 25 MPa and shear strains >30. Travel time data for apparatus calibration are fit with rational functions and root mean square error is used to assess uncertainty. Wave speed varies systematically with shear stress and increases with shear strain due to comminution, compaction, internal strain localization and shear fabric development.