Effects of Anisotropy for P-Wave Velocity on Locating Accuracy of Acoustic Emission Sources in Sandstone

The accurate measurement of wave velocity has a significant effect on the locating accuracy of acoustic emission sources, especially in health assessment and determination for rock structures. There commonly exists some deviation between the measured velocity value and the actual velocity value due to the anisotropy of rock velocity, which causes the large locating error. In this paper, the locating error caused by the anisotropy for P-wave velocity is discussed quantitatively. The locating experiments were carried out on the sandstone sample, whose length, width, and height are 100, 100, and 200 cm respectively. The mineral composition and micro structures for the sample were observed. AE sources were triggered by lead breaks and the arrival times were detected through eight sensors. Then, the sources can be located based on time difference locating method. Results show that the difference of wave velocity is large in different directions, which reaches 2400 m/s. The errors of wave velocity affect the locating accuracy seriously, which is up to 20.5%. A total of 135 groups of velocity values in different directions were measured. It is found that the locating accuracy of repeatedly measuring velocity through diagonal line is higher than accuracy of traditional used axial velocity. In addition, the source locating accuracy can be improved greatly by measuring and solving the average wave velocity in multiple directions. However, the average velocity value corresponds to the locating result is not the best. Tests clarify that the optimal velocity is higher than the average velocity. It is suggested that the wave velocity should be measured in multiple directions for several times when performing the laboratory acoustic emission (AE) sources locating experiments. The conclusions provide important theoretical guidance for the current traditional AE sources locating experiments with only average value of axial velocities.