UPDATE AT 12:55 ON 1OTH MARCH 2022
The seismic event which occurred at approximately 9.20pm on 9th March was induced by injection testing at Eden Geothermal. These tests have been ongoing since late January and have involved injecting fresh water into the natural fractures near the bottom of the well.
Injection testing induces low level seismic events (microseismicity). Since the start of testing there have been over 300 low level seismic events. However, the event that occurred at 9:20 pm yesterday evening was heard and felt by people in the local community. It was 1.7 Magnitude at a depth of over 3,700 metres.
Magnitude is not the main factor that determines whether an event is felt or heard at surface; this is dependent on the Peak Ground Velocity (PGV) measured in mm/s. PGV is a measure of ground vibration, and the PGV associated with this event was 1.56 mm/s.
Deep Geothermal operations are monitored and controlled in line with the local quarrying and mining industry, using thresholds based on PGV. Eden Geothermal’s operating levels reflect both the thresholds defined by Cornwall Council for quarry blasting and maximum vibration levels recommended within British Standard 6472-2:2008.
- A PGV of 2mm/s is considered by British Standard 6472-2:2008 as the “magnitude of vibration below which the probability of adverse comment is low”, and is also widely quoted in the literature as the approximate threshold below which vibrations are unlikely to be perceived.
- For comparison, a PGV of 8.5mm/s is used by Cornwall Council as the maximum permitted level of vibration from blasting operations during working hours.
- EGL has adopted a cautious approach, using 0.5mm/s as a PGV threshold during pumping operations. As this threshold was exceeded by the event that occurred on 9th March, we have halted operations and are currently reviewing our well testing programme, in order to minimise the likelihood of inducing another felt event.
There is more information about seismicity during well testing, including a video and a downloadable FAQs sheet here . And we will continue to post any additional information on this page.
This event was many times smaller than anything which could cause damage or pose any danger. However, we understand that it has caused alarm and disruption, and are very sorry for this.