2139 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: :=•,2Y (1) where o 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]=à [kNá ]⋅!<<< [mm2] (2) Then cumulative hits are computed as: ~cum(O)=Ä ~(Å) ) */! (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: ;(O)=ßècum ß®() ( ) )) =ècum())@ècum()@!) ®())@®()@!) (4) This metric expresses the intensity of AE hit accumulation per unit stress increment. At the same time to reduce step-to-step fluctuations, ;(O)is smoothed using a moving average window (default window in pilot analysis was set as length of r=5): ;Ç (O)=$ !Ä ;(k), ) + ? / - )@- r=2A+1 (5)
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