ALBERT

Description: Imaging the internal structure of faults remains challenging using conventional seismometers. Here, the authors use deployed fibre-optic cables to obtain strain data and identify faults and volcanic dykes in Iceland. Such fibre-optic networks are pervasive for telecommu-nication and could be used for hazard assessment. Natural hazard prediction and efficient crustal exploration requires dense seismic observa-tions both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. In the manuscript Jousset et al. (2018), we demonstrate that strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW Iceland.

Description: We present updated tomographic results obtained using seismic data recorded around geothermal reservoirs located both on-land Reykjanes, SW-Iceland and offshore along Reykjanes Ridge. We gathered records from a network of 234 seismic stations (including 24 Ocean Bottom Seismometers) deployed between April 2014 and August 2015. In order to determine the orientation of the OBS stations, we used Rayleigh waves planar particle motions from large magnitude earthquakes. This method proved suitable using the on-land stations: orientations determined us- ing this method with the orientations measured using a giro-compass agreed. We focus on the 3D velocity images using local earthquakes to perform travel time tomography. The processing in- cludes first arrival picking of P- and S- phases using an automatic detection and picking technique based on Akaike Information Criteria. We locate earthquakes by using a non-linear localization technique, as a priori information for deriving a 1D velocity model. We then computed 3D velocity model by joint inversion of each earthquake’s lo- cation and velocity lateral anomalies with respect to the 1D model. Our models confirms previous models obtained in the area, with enhanced details. In a second step, we performed inversion of the Vp/Vs ratio. Results indicate a low Vp/Vs ratio anomaly at depth suggesting the absence of large magmatic body under Reykjanes, unlike results obtained at other geothermal field, sucha as Krafla and Hengill. We discuss implications of those results in the light of recent IDDP drilling in Reykjanes.

Description: Natural hazard prediction and efficient crust exploration require dense seismic observations in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor capacity and distribu- tion. More and denser networks are being deployed on volcanoes, sedimentary areas in order improve of capability to image and monitor the crust. It has been suggested for some months that fibre optic cable technology is able to record strain and is useful in industry related exploration and may be useful for seismology. We review the distributed Sensing technologies for such applications. In this study, we demonstrate that direct strain determination is now possible with conventional fibre-optic cables deployed for telecommunication and is a new tool for earthquake location, for crustal exploration using unexpected sources and provides key records for understanding earthquake and fault structure and behaviour. Extending recently distributed acoustic sensing (DAS) studies, we provide spatially un-aliased broadband nano- strain data. We record seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW Iceland. We identify with unprecedented resolution structural features like normal faults and dykes in the Reykjanes Oblique Rift, allowing to infer new fault dynamic processes. Comparison with conventional seismometer recordings corroborates dynamic and stable spectral amplitudes between 0.1-100 Hz bandwidth. The networks of fibre-optic telecommunication lines worldwide may be used as seismometers opening a new window for Earth hazard monitoring and exploration.

Description: Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration.

Description: 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