Laser ultrasonic measurements to estimate the elastic properties of rock samples under in situ conditions

We present a new noncontact methodology to excite and detect ultrasonic waves in rocks under in situ pressure and temperature conditions. Optical windows in the side of a pressure vessel allow the passage of a laser source and a receiver for noncontact laser ultrasonic measurements. A heating mantle controls the temperature, and a rotational stage inside the vessel makes it possible to obtain measurements as a function of angle. This methodology is the first to combine the advantages of laser ultrasonics (LUS) over traditional transducer methods with measurements under in situ pressure and temperature conditions. These advantages include the absence of mechanical coupling, small sampling area, and broadband recordings of absolute displacement. After describing the experimental setup, we present control experiments to validate the accuracy of this new system for acquiring rock physics data. Densely sampled rotational scans performed on an Alpine Fault ultramylonite rock reveal a decrease in P-wave anisotropy from 62% at atmospheric pressure to 36% at 16 MPa. This result highlights the importance of performing rock physics measurements under in situ confining stress and demonstrates the advantages of the methodology for investigating anisotropy. In addition, a 5.6% decrease in the P-wave velocity of the ultramylonite sample between 20 °C and 100 °C at a constant 10 MPa confining stress demonstrates the capability of this new methodology for acquiring data under both in situ pressure and temperature conditions. This new methodology opens the door for probing the pressure and temperature dependence of the elastic properties of rocks and other materials using LUS techniques.