DAS System for the Evaluation of Subsea Seismic Data from GEOLAB cable in Madeira Island

DAS technology has emerged as a transformative technology with a vast range of applications, both on land and at sea. These applications span from oil and gas exploration to geophysical data collection, infrastructure monitoring, security, and environmental hazard monitoring, including earthquake and tsunami early warning systems (Landr et al., 2022; Gorshkov et al., 2022). The unique properties of DAS systems can bring high benefits to the demanding field of seismology, as it provides a significant increment in the spatial information that can be obtained from a seismic event. Moreover, the widespread deployment of optical fiber across the Earth's surface, coupled with the relatively low cost per monitoring point for extended distances, has rendered DAS an appealing alternative to traditional seismographs (Li et al., 2023). This is especially true for subsea applications, where the capability of remote sensing is particularly attractive. Remote sensing enables the placement of systems far from harsh environments, often difficult to access, enhancing the feasibility and effectiveness of monitoring efforts. In this work, it was employed a DAS equipment on a dark telecommunication fiber was installed exclusively for research purposes, named GEOLAB, located on the island of Madeira. This fiber spans approximately 50 km, where the initial tests were conducted using a DAS from January 31 to February 14, 2023. The equipment utilized is the HDAS provided by the IO-CSIC. The signal of the fiber was collected with a spatial resolution (or gauge length) of 10 m, resulting in total of 5000 channels, with a temporal acquisition with a frequency of 50 Hz. The DAS system has a chirped pulsed laser as the optical source, generating pulses with a width of 100 ns. These pulses were then amplified using a semiconductor optical amplifier to mitigate intra-band coherent noise. A total of 19 seismic events were detected, and then characterized by performing two-dimensional linear bandpass filtering. We will present the initial findings, particularly the seismic activity resulting from the earthquakes with epicenters near the city of Gaziantep, locatedinTurkey. These events occurred on February 6, 2023, with magnitudes of 7.5 and 7.8 on theRichterscale.