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A new set-up to measure acoustic wave velocities through deforming rocksamples at high pressures (up to 1000 MPa), temperatures (up to 700$^\circ$C)and differential stress (up to 1500 MPa) has been developed in a recentlyrefurbished gas medium triaxial deformation apparatus. The conditions span awide range of geological environments, and allow us to accurately measuredifferential stress and strains at conditions which are typically onlyaccessible in solid medium apparatus. Calibrations of our newly constructedinternal furnace up to 1000 MPa confining pressure and temperatures of up to400$^\circ$C demonstrate that the hot zone is displaced downwards withincreasing confining pressure, resulting in temperature gradients that areminimised by adequately adjusting the sample position. Ultrasonic velocitymeasurements are conducted in the direction of compression by thepulse-transmission method. Arrival times are corrected for delays resultingfrom the geometry of the sample assembly and high-precision relativemeasurements are obtained by cross-correlation. Delays for waves reflected atthe interface between the loading piston and sample are nearly linearlydependent on differential applied load due to the load dependence of interfacestiffness. Measurements of such delays can be used to infer sample loadinternally. We illustrate the working of the apparatus by conductingexperiments on limestone at 200 MPa confining pressure and room temperature and400$^\circ$C. Ultrasonic data clearly show that deformation is dominated bymicrocracking at low temperature and by intracrystalline plasticity at hightemperature.

A high pressure, high temperature gas medium apparatus to measure acoustic velocities during deformation of rock