J. Eur. Opt. Society-Rapid Publ. 22, 11( 2026) 111
Fig. 1. Experiment setting in Faial, Azores. The monitored cable by HDAS is shown in thick white. The two OBS are shown in red. The land stations are shown by green triangles. The epicenter location is highlighted with a yellow star.
The specified attenuation is 0.12 dB / km at 1550 nm. The cable is 170 km long, unpowered, and contains no repeaters. It was laid mostly on the ocean floor, with protection levels that vary with seafloor depth; the depth increases rapidly away from the coastline. In Figure 1, the monitored cable by HDAS is shown as a thick white line. The two OBS are shown as red dots. The three land stations are shown as green triangles. The epicentre location is highlighted with a yellow star. DAS data was collected on a dark fiber from an unused telecom fiber pair. At the ALTICE cable-landing station, we connected a HiFi Distributed Acoustic Sensor( HDAS) to the Fibroglobal terminal equipment. The HDAS is a chirped-pulse /-OTDR interrogator prototype developed by UAH and CSIC [ 8 ]. Figure 1 presents the Experimental setting in Faial Azores. The monitored cable by HDAS is shown in thick white. The two OBS are shown in red. The land stations are shown by green triangles.
An FC / APC connector was used to minimize backreflections at the interface with the submarine cable, preventing signal degradation and potential damage to the HDAS photodetector. The interrogator is identical to the CP-DAS system described in [ 9 ], based on /-OTDR using chirped optical pulses, a patented technology. A patented Raman-amplification range-extension module increases the sensing reach from 35 to 40 km to up to 70 km per fiber access. The manufacturer specifies a strain resolution better than 5 ne( nanostrain). In this experiment, the HDAS interrogated 50 km of cable with a 10 m gauge length and 10 m spatial sampling, yielding 4,992 channels, with an acoustic sampling frequency of 1 kHz. The strain time series were later downsampled to 50 Hz, which was sufficient to resolve
the seismic signals of interest while reducing data volume. For calibration and validation of the HDAS recordings, the MODAS project deployed two OBS units developed at IDL, advanced seabed instruments designed for longterm offshore monitoring. Each OBS records ground motion with a broadband seismometer and seismic – acoustic signals in the water column with a hydrophone. The deployment sites( Fig. 1) lie approximately 500 m from the submarine cable, allowing direct comparison between OBS and DAS measurements. Accurate timing for joint analyses with HDAS was ensured by synchronizing the OBS internal clock at the start and end of the experiment. The seismometers recorded leveled ground-velocity data on the three components( N, E, Z) at 250 Hz, while the hydrophone channel was sampled at 100 Hz. Each OBS includes an HTI-04- PCA / ULF hydrophone( 100 s – 8 kHz), a Güralp Certis broadband seismometer with a 120-s corner period, and a Güralp Certimus data logger. Accurate timing for joint analyses with HDAS was ensured by synchronizing the OBS internal clock at the start and end of the experiment.
3 Preliminary analysis of earthquake data
The DAS and OBS recorded several earthquakes over a period of five months. Figure 2 shows one of these earthquakes, recorded at around 12:36 a. m. on September 24, 2024, with a magnitude of 2.7 on the Richter scale and an epicenter located 20 km from the submarine cable, according to information provided by Instituto Português do Mar e Atmosfera( IPMA). However, we can also observe