OOI

RAPID: Multiplexed Distributed Acoustic Sensing (DAS) at the Ocean Observatory Initiative (OOI) Regional Cabled Array (RCA)

Abstract

Distributed acoustic sensing (DAS) on submarine fiber‐optic cables is providing new observational insights into solid Earth processes and ocean dynamics. However, the availability of offshore dark fibers for long‐term deployment remains limited. Simultaneous telecommunication and DAS operating at different wavelengths in the same fiber, termed optical multiplexing, offers one solution. In May 2024, we collected a four‐day DAS dataset utilizing an L‐band DAS interrogator and multiplexing on the submarine cables of the Ocean Observatory Initiative’s Regional Cabled Array offshore central Oregon. Our findings show that multiplexed DAS has no impact on communications and is unaffected by network traffic. Moreover, the quality of DAS data collected via multiplexing matches that of data obtained from dark fiber. With a machine‐learning event detection workflow, we detect 31 T waves and the S wave of one regional earthquake, demonstrating the feasibility of continuous earthquake monitoring using the multiplexed offshore DAS. We also examine ocean waves and ocean‐generated seismic noise. We note high‐frequency seismic noise modulated by low‐frequency ocean swell and hypothesize about its origins. The complete dataset is freely available.

OOI Regional Cabled Array (RCA), Offshore Central Oregon (45.21° N, -123.98° W)

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Dataset Description

The OOI DAS 2024 is to validate the feasibility of optical multiplexing using the submarine cables of the Ocean Observatory Initiative’s (OOI) Regional Cabled Array (RCA), in the context of regional seismicity detection and ocean gravity wave analysis.

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In May 2024, a team of scientists led by Dr. Brad Lipovsky at the University of Washington conducted a test of multiplexed distributed acoustic sensing (DAS) along the more southerly of two submarine cables of the OOI Regional Cabled Array (RCA) extending off Pacific City, Oregon.  DAS uses backscattered light to measure fiber strain with spatial resolution of meters and at frequencies typically recorded by seismometers. Whereas previous DAS experiments on the RCA sought to demonstrate the use of DAS on OOI RCA infrastructure during a time period when the RCA cables were not being used for communication with the RCA instruments, the May 2024 study sought to demonstrate the use of DAS during a time when active communications were taking place on the same optical fiber as the DAS observations.

The 4-day test was conducted from May 6-10 during a scheduled maintenance shutdown of the OOI RCA.  The OOI RCA has two cables extending off Pacific City Oregon, the north cable which extends to Axial Seamount and the south cable that loops back onto the continental margin off Newport to the south of Pacific City.  Each cable has two optical fibers, the transmit and receive fibers for observatory data. The multiplexed DAS observations extended to the first optical repeater on the South cable which is located at about 95 km offshore.

The experiment used an Alcatel Subsea Networks (ASN) OptoDAS interrogator.  From 2024-05-06 15:57:34 to 2024-05-10 14:50:35 UTC, we acquired continuous DAS data on the South cable of the OOI RCA. At 15:00 UTC on May 6 the network was disconnected and we connected the interrogator to the fiber at the outermost module of the submarine terminal system using an in-line C/L filter. Our team tested DAS acquisition parameters for approximately one hour before continuous recording commenced.  Around 19:00 UTC on May 7, the network was reconnected to the RCA South cable, through the C/L filter, and the OOI team resumed data telemetry while DAS data acquisition continued. Shortly before 15:00 UTC on May 10, DAS data acquisition was stopped. The C/L filter was then removed, which required briefly disconnecting and reconnecting the RCA South fiber.

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The data from the experiment are available by FTP.  A description of the dataset is provided via Github repository, including a jupyter notebook that shows how to use the data.

DOI: https://doi.org/10.58046/4WEF-A282

Citation:

Lipovsky, B., Williams, E., & Ocean Observatories Initiative. (2024). RAPID: Multiplexed Distributed Acoustic Sensing (DAS) at the Ocean Observatory Initiative (OOI) Regional Cabled Array (RCA) [Data set]. Ocean Observatories Initiative.

License: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International

NSF Award #2415521: RAPID: Multiplexed Distributed Acoustic Sensing (DAS) at the Ocean Observatory Initiative (OOI) Regional Cabled Array (RCA)