SECONDARY ACOUSTIC EMISSIONS IN STRESSED ROCKS

Abstract eng:
Within the past few decades, different studies have been conducted to examine frequency characteristics of Acoustic Emissions (AEs) due to the loading stress on rocks. Ohanka ( [1], [2] ) conducted a few experiments on stressed rocks and observed AEs in different frequency bands ranging from a few hundred hertz to over 1 MHz. While in these observations in some cases the emissions from the same events presented time and phase difference (Figure 2b and 2d of [1]), the cause remained unexplained. In the present study, a theory is given to explain the phase and time difference between the related acoustic emission signals. The theory proposes that the delayed propagated AE wave with lower frequency components is due to the widening of the existing crack. The result is utilized to propose explanations for other related lab and field observations, including simultaneous increase in the event rates in differing frequency bands of the AE signals (Figure 3a and 4a of [1], and 5a, 6a, 7a, and 8a of [2]). The theory is also exploited to describe the frequency range difference between seismology, micro-seismic, and acoustic emission measurements (shown in Figure 1 of [3]).

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