Study On Rock Damage Characterization Based On Waveform Correlation

The damage of rock will continue to occur over time under the influence of external factors in underground engineering.Detecting and accurately characterizing the damage can prevent the occurrence of disasters and make the damage mechanism clearly.Among the existing detection methods,the method based on acoustic wave tests can characterize the global damage and defect location by imaging.Studies have shown that the method based on the onset time of the first arrival wave is not sensitive enough to the low-level damage.The method based on coda wave interference is not suitable for characterizing the high-level damage.This paper analyzed the causes of these problems and proposed a rock damage characterization method based on waveform correlation.Firstly,the calculation process of the method is theoretically deduced,then the code of the method is written in the Python programming language,and the numerical simulation is further used to quantitatively verify the algorithm and analyze the influencing factors.Finally,this method is applied in the laboratory experiment.The applicability of this proposed method is compared with other methods,and the influence of the acoustic wave parameters used in the test on the results is discussed.The research contents are concluded as follow:(1)A global wave velocity calculation method based on waveform correlation is proposed to characterize the global damage level of rock.Combined with the wave velocity calculation method based on the onset time of the first arrival wave and the wave velocity change rate calculation based on the coda wave interference,the correlation of each time window of the waveform received by the acoustic wave test before and after the medium change is calculated.Then,the time shift calculated based on waveform correlation for different medium damage levels is deduced.Finally,the new relationship between wave velocity change and time shift is established,making the new solution method of the time-shift and the relationship between time-shift and wave velocity change more reasonable.The accuracy of the proposed method is quantitatively verified in different damage level media.(2)A wave velocity field calculation method based on waveform correlation is proposed to characterize rock damage’s location,range and damage level.Based on the improved solution method of the time-shift,this method integrates the first arrival wave and coda wave as the observation data.Firstly,the back-projection technique is used to obtain the initial wave velocity field directly.Then the Bellman-Ford algorithm is used to solve the shortest path of the global medium.The corrected initial wave velocity field is obtained by inversion according to the onset time of the first arrival wave.The relationship between the time-shift matrix and wave velocity change matrix is obtained by using the probability of wave propagation intensity.The results are used to modify the initial wave velocity field.Finally,a more accurate calculation method of the wave velocity field is obtained.The accuracy of the proposed method in calculating wave velocity field is verified quantitatively with different damage levels and layered media with defects.(3)The influencing factors of the algorithm are discussed.According to the iterative matrix,the number of iterations,the initial value of iterations,the measurement error of travel time and the monitoring layout scheme are selected for analysis.The wave velocity field,the overall error rate,the error rate at the defect,the correlation coefficient with the actual wave velocity field,and the relative error rate of each imaging unit are calculated,respectively.The results show that the proposed method is not affected by the initial value of iteration,and the convergence speed is fast.The survey line layout scheme and travel time measurement error significantly impact the results.The survey line layout should tend to the complete mapping state as far as possible,and reducing the measurement error is conducive to more accurate results.(4)The feasibility of the proposed method is verified by characterizing the damage evolution of granite samples during uniaxial compression.The global damage level,damage defect location and damage level at the defect in the damage evolution process are characterized with the proposed method.The relationship between the number of new adding acoustic emission(AE)events in the specimen during uniaxial compression and the global wave velocity change rate obtained by different methods is compared.And the relationship between the damage characterization of the specimen strain rate image obtained by digital speckle technique and the characterization of wave velocity field obtained by different methods.The results show the feasibility of the proposed method in practical applications.(5)The influence of waveform parameter selection on the results in the rock acoustic test is analyzed.A rock specimen with a crack is used in the experiment.The influence of waveform frequency,waveform type,wavelength,defect size,and relative position on the received waveforms are analyzed by emitting waveforms with different parameters.This provides the guidance for the practical application of the proposed method.