The monitoring of microseismicity during temporary human activities such as fluid injections for hydrofracturing, hydrothermal stimulations or wastewater disposal is a difficult task. The seismic stations often cannot be installed on hard rock, and at quiet places, noise is strongly increased during the operation itself and the installation of sensors in deep wells is costly and often not feasible. The combination of small-aperture seismic arrays with shallow borehole sensors offers a solution. We tested this monitoring approach at two different sites: (1) accompanying a fracking experiment in sedimentary shale at 4 km depth and (2) above a gas field under depletion. The small-aperture arrays were planned according to theoretical wavenumber studies combined with simulations considering the local noise conditions. We compared array recordings with recordings available from shallow borehole sensors and give examples of detection and location performance. Although the high-frequency noise on the 50-m-deep borehole sensors was smaller compared to the surface noise before the injection experiment, the signals were highly contaminated during injection by the pumping activities. Therefore, a set of three small-aperture arrays at different azimuths was more suited to detect small events, since noise recorded on these arrays is uncorrelated with each other. Further, we developed recommendations for the adaptation of the monitoring concept to other sites experiencing induced seismicity.
|Original language||English (US)|
|Number of pages||16|
|State||Published - Dec 13 2018|
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This work is funded by the EU H2020 SHale gas Exploration and Exploitation induced Risks (SHEER) project (www.sheerproject.eugrant agreement no. 640896). We profoundly thank KNMI (Koninklijk Nederlands Meteorologisch Instituut) for support in installing the instruments of the Wittewierum array as well as KNMI and the Groningen field operator NAM (Nederlandse Aardolie Maatschappij) for identifying a suitable location and obtaining the permissions for installation.