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Zusammenfassung:
In recent years, Distributed Acoustic Sensing (DAS) fiber optic cables installed vertically in wellbores have been used as recording devices in active seismic experiments. Now, for the first time we used both a vertically installed cable in a geothermal injection well and a horizontally installed fiber optic cable to record strain rates and detect earthquakes. The horizontal cable was not deliberately installed for the purpose of this study but has been used for telecommunication for more than a decade. Is measures 15 km and connect two geothermal power plants on the Reykjanes peninsula, an area rich in natural seismicity due to the rifting along the mid-Atlandic spreading ridge and areas of geothermal activity. For 9 days we measured strain rates along the surface cable with a lateral resolution of 4 m and a sampling rate of 1000 Hz. An active hammer seismic experiment was carried out for calibration and and we collected almost 5 TB of data. Recordings of a local ML 1.2 earthquake allowed us to estimated a hypocenter location which lies within several hundred meters from the hypocenter derived from the local seismometer network. The dominant frequency of the ambient noise recorded has a period of 6 s and is caused be ocean waves. Both earthquakes and ambient noise data could be used to identify structural features along the cable. We compared the irregularities and other abnormalities in the record to geological surface maps and found that they coincide with visible surface faults and a volcanic crater row that the cable is crossing over.
Spatial resolution and sensor response are crucial factors both for passive seismic monitoring and active seismic surveys. Using DAS fiber optic cables as sensors offers higher spatial resolution (channel spacing down to 1 meter) than conventional geophone arrays and a comparison to a conventional broadband seismometer located next to the cable shows that the bandwidth of the recordings reaches from 0.1 to 100 Hz. Our results show that existing fibre optic telecommunication cables can be used as passive seismic sensors for surveys and natural hazard assessment. With this study, we demonstrate that this technique offers a cheap and convenient alternative to classical seismic survey
