The Acoustoelastic Experiment Study Of Saturated Rocks With Uniaxial Stress

A lot of experiments have proved the existence of acoustoelasticity in rock,the acoustoelasticity is that acoustic velocity of elastic wave changes with the stress.The acoustoelasticity of rock provides the possibility of nondestructive testing about ground stress.At present,many researchers measured the stress and the third-order elastic moduli of the medium by the acoustoelastic theory.At the same time,measured the acoustic velocity around hole also predict the direction of the main stress in rock.In many media of sound propagation,the nonlinear effect of rock is more obvious than other media,so the studies that the acoustic elastic effect of dry rock and its acoustic velocity around hole are more.However,the acoustic elasticity of fluid-saturated porous media is more complicated than that of single-phase medium.The nonlinear term of fluid and solid coupling is difficult for the measurement of third-order elastic modulus in the experiment.At present,there are few studies on the acoustic elastic effect of water saturated rock and the sound velocity around the hole.It is a significant to study the experimental of third-order elastic moludi of the dry and water saturated rocks and acoustic velocity around hole.In this paper,on the basis of acoustoelastic theory,on two different rocks were acoustic velocities were measured in natural state,dry state and water saturated state under uniaxial stress.Based on acoustoelastic formula,using the least-squares method and error analysis,we obtained the third-order elastic moduli of natural state,dry rocks and saturated rocks.Then,by analogy with Biot theory,we introduced the seepage strain to modify the acoustoelastic theory in the porous media,and established an equivalent model of acoustoelastic theory in porous media.Using the above method,we acquired the third-order elastic moduli of fluid-solid coupling.By comparing the experiment values with theoretical values of the acoustic velocities inrocks,the experiment values and theoretical values show good agreement,and the feasibility of the equivalent model has been proved.For the determination of stress,in this paper,this part is based on the experiments in the first part.Through the transmission method,measured the waveforms in natural condition,dry condition and water saturated condition of rocks with different stress.We found that the amplitude of waveforms along with the increase of stress changing regularly,thus,it is recognized that the amplitude of the waveform carries the stress information.The stress value obtained from the fitting formula is basically consistent with the experimental stress value.This leads us to calculate the stress value by amplitude for the sample with obvious waveform amplitude change under stress.According to acoustic velocity measurement around the hole in rock,this paper measured acoustic velocity around hole of longitudinal wave,shear waves in the rocks under natural state,dry state and water saturated state,the rocks are located in different azimuth angle and different uniaxial stress.We found that in the sample with yellow rust granite stone 1,the variation of velocity with azimuth under load stress is basically consistent with previous researches.That is,the direction of the maximum wave velocity corresponds to the direction of the minimum principal stress,and the direction of the minimum wave velocity corresponds to the direction of the maximum principal stress.Due to the reason of the material of yellow rust granite stone 2,the acoustic velocities of dry and water-saturated samples show abnormal maximum at about 0 °and 330 °.However,the variation of acoustic velocity at other azimuths is similar to that of yellow rust granite stone 1.Thus it can be seen,the variation of azimuth acoustoelastic effect in saturated porous media caused by stress concentration around the hole is basically consistent with the elastic situation.