Harbord, C; Brantut, N; David, E; Mitchell, T; (2022) A high pressure, high temperature gas medium apparatus to measure acoustic velocities during deformation of rock. Review of Scientific Instruments , 93 (5) , Article 053908. 10.1063/5.0084477.

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Abstract

A new set-up to measure acoustic wave velocities through deforming rock samples at high pressures (<1 GPa), temperatures (<1000$^\circ$C) and loads (<1.5 GPa) has been developed in a recently refurbished gas medium triaxial deformation apparatus. The conditions span a wide range of geological environments, and allow us to accurately measure stress and strains at conditions which are typically only accessible in solid medium apparatus. Calibrations of our newly constructed internal furnace up to 1 GPa and 400$^\circ$C demonstrate that hot zone position decreases with increasing pressure, so that sample position is adjusted to minimise temperature gradients. Ultrasonic velocity measurements are conducted in the direction of compression by the pulse-transmission method. Arrival times are corrected from delays in the sample assembly and relative measurements are obtained by cross-correlation. Delays in reflected load are nearly linearly dependent on applied load due to load dependent stiffness of interfaces between the spacers and the sample. Measurements of such delays can be used to infer sample load internally. We illustrate the working of the apparatus by conducting experiments on limestone at 200 MPa confining pressure and room temperature and 400$^\circ$C. Ultrasonic data clearly show that deformation is dominated by microcracking at low temperature and by intracrystalline plasticity at high temperature.

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