The monitoring of seismicity, either natural or induced by anthropogenic activity (altering the stress field in the subsurface, e.g. hydraulic fracturing or geothermal stimulation, CO2 sequestration, wastewater injection, and so on) is of paramount importance for the protection of populations living in areas with high natural seismic activity. Monitoring minimizes the induced seismicity risks and the corresponding impact on operations’ safety and structures’ integrity.
Changes in the subsurface stress regime caused by anthropogenic activity interact with the geological formations and in very specific situations may induce/trigger some seismic activity. Although the resulting earthquakes have generally a low magnitude (micro-seismicity) and thus are not capable to produce damages at the surface, they could trigger moderate seismic events over time. In these conditions, the deployment of appropriate seismic monitoring networks on the surface or in boreholes allows for a real-time detection and identifying the location of (micro) seismic events.
Seismic networks involve the installation of high-quality and high-sensitivity seismic sensors. An example of one of the adopted seismic instruments is shown in Figure 1. The detection of seismic events (both natural and induced) from the continuous recorded waveforms is achieved through automatic processing algorithms (Figure 2) to deliver alerts in real time.
Once an event is detected, its hypocenter is evaluated and the correlation with human activities can be reassessed.
The relationship between seismicity and subsurface operations is generally analyzed considering the long-term catalogue of seismicity and geo-mechanical characteristics of the reservoir.
Figure 1: Example of a seismic sensor adopted for monitoring network deployment.
Figure 2: Example of a coincidence trigger as a result of the STA/LTA algorithm.
Areas of Application: