Using offshore seismometers for background seismicity monitoring

The project has also been testing how offshore can be integrated with sesimic networks and arrays for improved background seismicity monitoring. Fortunately, geophones placed on the seafloor for monitoring oilfields can also be used as passive seismic listening devices. By agreement with the Grane production license the project was able to test this option using 10 selected geophones from the Grane permanent reservoir monitoring (PRM) system. Using a dataset for the period between December 2018 and May 2019, we were able to compare detections of seismic events in the region using the existing onshore network (NNSN), the newly installed HNAR array and the selected offshore PRM nodes. 

We could then work out how additional offshore nodes could be used to improve event location accuracy and to evaluate the associated uncertainties. This work is explained in more detail in the paper by Zarifi et al. 2022. Overall, we showed that:

  • Combined use for onshore and offshore stations can significantly improve location accuracy 
  • Use of offshore nodes can significantly lower the detection floor – we expect events of ML>0.5 to be detectable within about 50 km of an offshore array
Figure:Background Seismicity Monitoring to Prepare for Large‐Scale CO2 Storage Offshore Norway. Source: Zarifi et al. 2022. 

However, improving the location accuracy in this region is still challenging and depends on many factors, such as data processing, phase picking, the velocity model, the azimuthal coverage, and the distribution of seismic stations. These challenges are being studied in the current phase of the project HNET3, including testing of a new test deployment of ocean bottom seismometers.