Multi-span distributed acoustic sensing (DAS) on the Ocean Observatories Initiative Regional Cabled Array 2025-2026: Nokia Bell Labs Multi-span Data

The distributed acoustic sensing (DAS) data collected with the Nokia Bell Labs multi-span system on the OOI RCA from November 2025 to January 2026 is available as binary files that contain both data arrays and metadata. The low sampling rate data (8 Hz) has a total size of 3.9 TB and can be accessed here:

http://piweb.ooirsn.uw.edu/das25/data/MultiDAS/

Python code to read the data, convert it to strain rate, and additional data documentation can be found at the following GitHub page:

https://github.com/uwfiberlab/OOI_DAS_2025

For 3 months beginning in November 2025, a novel multi-span distributed acoustic sensing (DAS) technique from Nokia Bell Labs was tested on the two submarine cables of the OOI Regional Cabled Array extending off Pacific City, Oregon. DAS uses backscattered light to measure strain along a fiber with a spatial resolution of meters. The DAS system tested in this experiment, provided by Nokia Bell Labs, uses the high-loss loopback couplers within optical repeaters to extend DAS measurements beyond repeaters to the entirety of a given seafloor cable. The northern OOI cable has a length of ~480 km and runs roughly perpendicular to shore, across the Juan de Fuca plate out to Axial Seamount. The southern RCA cable has a length of ~350 km and runs southward across the continental shelf, crossing onto the Juan de Fuca plate before looping eastward back towards shore.

On November 5 2025, the Nokia multispan DAS system was deployed on both the north and south cables of the RCA. Each of the RCA cables has two optical fibers, the transmit and receive fibers for the observatory data. The Nokia multispan DAS system sends light down the transmit fiber and receives backscattered light from each optical repeater via the receive fiber. The system was multiplexed such that data could be collected while the RCA continued operating. Data were collected continuously using this system until January 28 2026, with outages from November 12-18, December 8-9, and December 17-23. The spatial and temporal sampling rates were periodically varied, with channel spacing varying between 10-200 m.

Upon initial testing, it became clear that significant nonlinearities within the RCA fibers and an observatory-related synchronization signal on the fibers of both cables was distorting the data collection via strong crossphase modulation. These combined effects mean that significant portions of the cable did not record physically accurate data. Following investigation of these effects, starting on December 2, a spatial mask was applied to the data from both cables to limit the available data to only the portions that were minimally affected by the observatory noise. This spatial masking results in 165 km and 275 km of the northern and southern cable spans being saved, out to 300 km and 350 km maximum distance along the cable, respectively. These unmasked data includes portions beyond three repeaters on each cable. The spatially masked data still contains some non-physical noise, and should be treated with caution. Further investigation into the characteristics of this noise are ongoing and updates will be made to the documentation moving forward. Data post-processing also revealed that, due to latency in the processing chain, the absolute timestamps associated with the data are delayed (behind real-time) by several seconds. Details on this absolute timing delay can be found in the Github documentation.