Figure 3 – Seismoacoustic (AE) monitoring output during specimen loading: stress/load history, AE hits and cumulative activity, event parameters, and example waveform with FFT spectrum 2.3 Methodology for stress determination The analysis was based on data from the seismoacoustic measurements containing axial force in kN and AE hit counts per step (Hits). In the first step it was necessary to determine stress generated during UCS test. The specimen cross-sectional area is determined based on the diameter size: = 24 (1) where is in mm and is in mm2. Then axial stress is computed from force and area. Using in kN and in mm2 using equation: [MPa] = [kN] ⋅1000 [mm2] (2) Then cumulative hits are computed as: cum( )= ( ) =1 (3) Many studies of AE in rocks identify the Kaiser stress from the cumulative AE hits– stress curve, typically by locating an inflection point or by applying bilinear regression and taking the intersection stress as the Kaiser stress. Because interpretation procedures can be operator-dependent and are explicitly noted in the literature as disputable, the proposed algorithm applies an objective threshold-based change detection on an AE activity metric derived from hits versus stress. The following analysis uses the derivative of cumulative hits with respect to stress:
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