Application of wavelet transform in evaluating the Kaiser effect of rocks in acoustic emission test

Abstract

The Kaiser Effect is amongst the phenomena that can be detected through the so-called acoustic emission method. This effect can be used to evaluate in-situ stress of the rock. Identification of the point at which the Kaiser Effect is applied is problematic in particular cases where the acoustic parameters increase gradually, and this is where signal analysis methods come into play. In this research, Brazilian test was performed on Phyllite samples and acoustic data was recorded simultaneously. Following an energy-based and count-based parametric approach, the occurrence time and the Kaiser Effect stress were obtained for the test specimens. Then, wavelet transform method was used to estimate the occurrence time of the Kaiser Effect. Continuous wavelet transform analysis was undertaken to analyze the signals. The signal analysis was performed by considering the peak frequency as desired parameter and db5 wavelet as the mother wavelet. In continuous wavelet analysis method, the results were classified into five cluster using the K-means clustering algorithm. Considering the mechanism of the Kaiser Effect, the fifth category was considered for further investigations. The results showed that the Phyllite specimens exhibited good capabilities for recovering the stress memory and evaluating the Kaiser Effect-driven stress, the results further showed a good agreement between the occurrence times obtained from the parametric method and those resulted from continuous wavelet transform technique, so that the corresponding differences in the Kaiser Effect-driven stress are negligible. The level of Kaiser Effect-driven stress obtained from the proposed method were acceptably in agreement with the preloading stress levels.