Experiment Study Of Third-order Elastic Modulus Of Anisotropic Rock And Acoustic Velocity Around Hole

Acoustic elasticity phenomenon is that the acoustic velocity of ultrasonic wave through elastic medium which under stress is different from the acoustic velocity of the rock whithout stress.A number of studies on acoustic elasticity experiments show that the nonlinear effects of rocks are quite obviously,through the experiment,we can measure the velocity of acoustic wave through the rock and the velocity around borehole under different uniaxial stress,and then find the third-order elastic modulus of the rock from the acoustic velocity.In the practical application,the third-order elastic modulus of the stratum medium is known,the magnitude and direction of in-situ stress can be obtained by measuring the velocity of the acoustic wave through the stratum.So it is of great significance to study the theory and method of inversion in-situ stress,developing the experiment investigation about the third-order elastic modulus of the rock and the velocity distribution of borehole.This paper is giving priority to experimental exploration and based on acoustic elasticity theory,making experiment on three kinds of rock materials and drilling media of rocks.The third-order elastic modulus of the rock is measured by experimentally measuring the longitudinal and two transverse wave velocities through the rock.Firstly measures the acoustic velocity of the acoustic wave through each plane of medium in the reference state.Through three coordinate direction of compressional wave and shear wave velocity measurements found rock samples have weak anisotropy and calculate the velocity anisotropy parameters with the reference state of the rock,from the parameters of Thomson to determine the rock sample is transversely isotropic media.Changing the direction of shear wave polarization to measure the shear wave velocity which along the direction of perpendicular to the plane,the results show that under the same reference state along the different polarization direction of shear wave velocity have obvious change.We consider the anisotropy in the reference state of rock samples in the process of solving the third-order elastic modulus.Inversion of the theoretical values of velocity in different stress states and compare the velocity measurements with the theoretical values.It is found that the experimental value of the velocity which through the rock center is consistent with the theoretical value about the different stress state,it is indicated that the measurement of acoustic elastic effect can be used to detect in-situ stress.The experimental study on the acoustic velocity which distribution around rock's borehole is mainly focused on the P-wave velocity distribution in borehole,there is little experiments of the shear wave velocity.Diretly hypothesis that the sample is isotropic in the course of theoretical calculation.In this paper,considering the intrinsic anisotropy of the rock sample,the single axis force in the vertical well axis direction is applied to the 654 sample of the borehole,Transverse wave and the longitudinal wave propagating along the well axis at different azimuth angles of different stress states is measured.The results show that the maximum wave velocity of the hole circle is consistent with the minimum principal stress direction and the minimum wave velocity direction corresponds to the maximum principal stress direction of the loaded sample respectively.So we can determine the direction of the horizontal principal stress by method.