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High pressure and temperature in earth crust lead to fracture and microcracks in rocks. Direct access to earth crust rocks at great depths is very costly and in most cases impossible. The study of the condition of rocks at great depths is often done using indirect methods such as seismic waves. The results of these studies are compared with the results of laboratory studies of wave velocities in different rocks and the conditions of the rocks are simulated. At high depths, hydrostatic stress is applied to the rocks of earth crust, and tectonic, earthquake and other stresses cause it to be anisotropic. The main purpose of this study is to investigate the change in compressive wave velocity due to change in compressive stress in rocks. In first, a cylindrical core of different stones with a length to diameter ratio of 2 to 2.5 is prepared according to the standard test method (ASTM D4543) and their dimensions and weight are determined. after measuring the unconfined compressive strength of cores according to standard test method (ASTM D2938), the hydrostatic pressure of 50% to 95% of it is applied to the rock samples prepared from the earth. This pressure is applied to the cores by using the Hoek cell (for lateral pressure) and the axial load machine and using ultrasonic device, determine the compressive wave velocity (ultrasonic pulse) is determined according to the standard test method (ASTM D2845) in the axial direction of the sample. Then, the wave velocity was measured during decreasing the lateral pressure (increasing deviatoric stress) in a stepwise manner, the wave velocity measured at each step. In the following, comparative diagrams of compressive wave velocity (V_p) with density (ρ_d), uniaxial compressive strength (UCS) and the effect of hydrostatic stress (σ_hyd) and deviatoric stress (σ_dev) on P-wave velocity in each sample are drawn. The results show linear relationships between compressive wave velocity and physical properties of rock samples, also the P-wave velocity at hydrostatic pressure is the highest and as the lateral pressure decreases (increasing the deviatoric stress), the velocity also decreases.

Keywords: Stress Anisotropy, Ultrasonic, Rock Triaxial Test, Compressive Wave Velocity, Rock Core Sample

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