Identification of AE multiplets in the time and frequency domains

Previous studies by the authors have revealed that AE multiplets, which are groups of events with closely similar waveforms, can be used effectively to precisely delineate structures inside artificially stimulated geothermal, oil, and gas reservoirs and to determine their response to the stimulation. The similarity of the waveforms among the collected AE events, which are evaluated at some fixed frequency, can be represented by a product of the transfer functions of the source, the earth, and the receiver/recorder. Meanwhile, the low-pass characteristics of the earth transfer function appear more strongly in the coda, where reflected, refracted, mode-converted, and scattered waves arrive randomly at the receiver. This feature of the time and frequency characteristics of the AE signals led to the idea of identifying multiplets in the time and frequency domains. This paper presents results from multiplet identification in the time and frequency domains by using data sets collected at engineered geothermal development sites at Cooper Basin, Australia, and Basel, Switzerland, demonstrating that AE multiplets from different physical phenomena can be clustered by their identification in the time and frequency domains. Keywords: Induced AE, AE multiplet, Coherence, HDR/HFR, Stimulation