Lab-based X-ray imaging for accurate seismic velocity measurements at high pressure

Nicholas Backhouse, Andrew R Thomson , Craig Wilson, Billy Rayner, Ian Wood, Neil Hughes, Mehdi Zahir, John Peter Brodholt

A novel approach for accurately measuring the compressional (P) and shear-wave (S) velocities of materials at high pressure is demonstrated up to ~6 GPa using a Paris-Edinburgh press. Typically, accurate ultrasonic velocity measurements at high pressure require synchrotron facilities to image the sample length in-situ and to determine the internal cell pressure using X-ray diffraction from an internal standard. Here, replacing synchrotron-based imaging, sample lengths are directly measured in-situ using a custom-built divergent-beam X-ray imaging system in a university laboratory. The imaging system combines a microfocus source, an X-ray fluorescent scintillator, magnification optics and a high-sensitivity CMOS camera to obtain radiographic X-ray images of comparable quality to those obtained at synchrotron facilities in exposures of ~ 10 seconds. We demonstrate the capabilities of the system with in-situ velocity measurements of a natural novaculite (quartz) sample throughout compression. Measured velocities are accurate to ± 0.3% on average. In place of a diffraction-based pressure calibrant, we demonstrate that pressures can be determined from the observed velocities of the novaculite sample by using literature elasticity data of quartz. Experimental pressures were shown to extend to 5.8 GPa, with all reported pressure conditions accurate to better than ± 0.3 GPa. Finally, the compressional wave travel time of the alumina buffer rod has been calibrated as a pressure marker for use in future compression experiments.