Distributed acoustic sensing (DAS) is a recent instrumental approach allowing to turn fiber optic cables into dense arrays of acoustic sensors. This technology based on a simple optical time-domain reflectometer (OTDR) technique is attractive in marine environments where instrumentation is difficult to implement. In this project we use measured optical phase changes in Rayleigh backscattered light of OTDR to quantify strain and strain rate variations on a spatial scale as small as 10 m. The fiber is the sensor that provides the communication media. The validation of DAS technology is currently being tested in different parts of the world from Europe to America. MODAS will contribute to this effort demonstrating its usefulness in the Atlantic Ocean.
Current technology limits the range of DAS to ca. 100 km making them very useful in case, like the Azores, where seismic stations only exist on the Islands with a strong E-W alignment, as demonstrated by Matias et al. (2021) for the domain between Terceira and S. Miguel, linked by a submarine telecommunications submarine cable. MODAS will use the same cable expanding the information provided using the huge amount of spatial data gathered by DAS.
The first challenge that MODAS will address is the development of processing routines do enable real-time use in earthquake monitoring of advanced analysis tools like f-k analysis, apparent phase speed computations, Power Spectral Density, beamforming. The assimilation of these new information to the location routines will be addressed by MODAS.
To our knowledge there is no report yet that tsunami waves were detected by a DAS on a submarine cable. In MODAS we will use infragravity waves as proxy to tsunami waves and we will evaluate if they can be detected and characterized by DAS and information provided in real-time to the central operational room.
Extreme waves are a major cause of flooding in coastal areas. The hazard they represent will increase due to climate change. In MODAS we will use DAS noise data to infer wave height and wave period and to measure the surface currents.
Sound is the sensory cue that travels farthest through the ocean and is used by marine animals, ranging from invertebrates to great whales. Ocean soundscapes are rapidly changing, mostly due to increases in shipping noise. In MODAS we will continuously assess the background level of the noise recorded by DAS and we will try to distinguish between natural sources, like ocean-solid interactions, and anthropogenic sources.
Oceans are experiencing profound changes in physical and chemical properties. Baleen whales’ populations are especially vulnerable to variations at the base of marine ecosystems. Whale vocalizations have been already identified in DAS data and in MODAS we will use one-year long observation period to identify the vocalizing species, their call rate and its variation over time.
One last challenge addressed by MODAS is calibration. It is well demonstrated that strain or strain rate as measured by DAS can be converted to ground motion along the direction of the submarine cable section, if the apparent phase velocity is known. Similarity between DAS converted signals and co-located seismograms is well demonstrated but the absolute value is likely to vary with the cable coupling to the seafloor. MODAS will address this issue by comparing DAS recordings with seismic land stations close to the submarine cable landing point and comparing offshore sections with two Ocean Bottom Seismometers (OBS) to be deployed by MODAS.
The OBS will be equipped with low-frequency hydrophones that will allow for the computation of the scaling function to convert strain to ocean driven pressure generated by large infragravity waves, considered as proxy to tsunami waves.
Surface currents and sea state (wave height and period) computed from DAS data will be validated and calibrated using wave buoy data close to the Terceira landing point and wave and surface current models that are run operationally at IPMA.
The MODAS project will provide, to our knowledge, the first one year monitoring of the ocean using DAS, demonstrating the usefulness of this emerging technology for the real-time monitoring of earthquakes and possibly tsunamis and near real-time monitoring of the ocean environment on some of its critical (and essential) variables, sea state (wave height and period), surface current, ocean sound, shipping noise and whale vocalizations. As in previous DAS experiments, MODAS will expect the unexpected and will be alerted to signals that cannot be immediately interpreted but that the DAS dense array of sensors may help characterize. In the Azores fluid flow from sediments or volcanic activity might be at the origin of the unexpected signals.