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Impact of seismic source processes on ground motion variability of induced earthquakes in geothermal reservoirs, mines and laboratory experiments

Subject Area Geophysics
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 359163592
 
Final Report Year 2022

Final Report Abstract

The scientific advances of the project can be summarized as follows: 1) Within the project the methodological approaches for seismic source parameters assessment have been further developed and tested on data coming from the different scales. The methodological approaches include high-quality source parameters assessment (seismic moment, source size, static stress drop) from seismic waveform data using spectral ratio technique, and adjusting of the hybridMT tensor package to handle data from different magnitude ranges. The developed methodologies have been thoroughly tested during the project course on seismic data from laboratory experiments on rock samples (earthquakes of sub-mmm size), in-situ injection experiments (earthquakes of dm-m size), and induced seismicity (earthquakes of 100m – 1000 m size). The hybridMT tensor package is publictly available. We also developed the full-waveform technique for calculating the detection thresholds and expected ground motions that can be parameterized by complex source properties and path characteristics. 2) The analysis of correlations between technological operations such as injection of fluids into the subsurface, structural inventory of the reservoir, and resulting seismic response shed new light on the potentials for controlling of induced seismicity associated with exploitation of geothermal systems. It was found that most of geothermal reservoirs behave in predictable way considering their seismic response. This is evidenced by the time-invariance of seismic energy release (e.g. seismic moment rate) with respect to the hydraulic energy input rate. We concluded that any deviation from time-invariance of seismic-to-hydraulic energy ratio during the course of injection operation should trigger the re-consideration of injection operations. This procedure have been successfully applied to St1 Helsinki injection in 2018 and 2020, where the near-realtime observations of seismic and hydraulic energy were used as a feedback to guide the injection operations, limiting the possibility for occurrence of LME at this site. It is worth to note that the associated study (C) gained significant coverage in media outlets. This, and other follow-up studies (D) (E) pursued within the project emphasized the importance of injection rate on the maximum magnitude during the course of stable seismic response of the reservoir (by stable response we mean that the maximum magnitude is bounded or can be explained with any existing models of maximum magnitude). The important of injection rate on seismic hazard has been confirmed in a series of laboratory experiments pursed in the project. We also found that the stability of seismic response of particular geothermal system is strongly conditioned on structural inventory of the reservoir (e.g. existence of major faults, active tectonics, stress conditions), which ultimately may lead to rapid changes in the seismic hazard during development of the geo-reservoir. In that respect, our studies highlighted the necessity for high-resolution close-by and near-realtime (an likely AI-aided) monitoring present even before the fluid injection. Having such system in place will enable detection of any deviations from stable seismic response in due time, reducing the seismic hazard.

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