Mitigation of induced seismicity
Earthquakes can pose threats to engineering works and to human life. In order to protect people, infrastructure and the environment, a number of common practices can be employed to reduce the risk.
Temporary local seismometer networks deployed before a project can improve earthquake-detection thresholds and provide more complete assessments of natural seismicity to inform safety strategies.
Borehole measurements can be taken to analyse in situ stress. This can help scientists understand local and regional stress states, and identify which faults are more likely to be critically stressed.
Computer models can make estimates of the likely effects of a project on, for example, the stress field, subsurface faults and fractures, and fluid flow. Such models are simplified scenarios of reality and require input parameter values that are usually poorly known, however their results may still be useful.
Pre-fracture injection tests
For fluid-injection projects it is common to inject a small volume of fluid prior to the commencement of full-scale injection. This provides information on how the subsurface may respond to larger-volume injection.
Seismic reflection surveys
Using artificial acoustic signals (e.g. explosions or vibration trucks) transmitted into the ground and recorded at the surface, geophysicists can calculate the travel times of reflected signals in order to construct an image of the subsurface. These images may be two-, three- or even four-dimensional (the fourth dimension being time), and may reveal potentially unstable geological features such as faults.
As well as baseline seismic monitoring prior to the start of a project, seismic monitoring can also be undertaken throughout a project’s lifetime. A user-friendly format is the traffic-light system, where predefined seismic magnitudes correspond to the traffic-light colours. If these magnitudes are exceeded this may result in the modification or termination of a project. Green usually means the project continues as planned, orange may mean the project continues at reduced rates (modification), and red may mean an immediate halt in the project. An example is the UK traffic-light system for shale gas hydrofracturing.