Extending Seabed DAS Beyond Permanent Infrastructure: Redeployable Fiber Nodes with Passive Cable Storage, Robotic Placement, and Multi-Leg Sensing

Temporary seabed deployment of sensing fiber can extend distributed acoustic sensing (DAS) beyond the few locations served by permanent submarine cables. A previous note (Bakulin, 2026a) proposed laying disposable fiber on the seabed from a vessel or from an underwater vehicle that carries the fiber spool. This note moves the cable storage off the vehicle and onto a seabed-resident frame. The frame carries a compact DAS interrogator, batteries, and one or more passive figure-eight cable baskets pre-loaded with sensing cable. The baseline frame is placed on the seabed and later recovered by conventional means; the underwater vehicle, autonomous or remotely operated, performs the core deployment task - acquiring the free end of the cable and pulling it along a planned route while the passive basket pays it out without a powered subsea winch - and, in the baseline configuration, does not transport the node or operate a powered reel. The cable is not respooled subsea; frame and cable are recovered by conventional or ROV-supported means, and reloading, cable exchange and optical reconnection are done on deck. The note then describes how the sensing aperture of a single frame scales. Two or more baskets may be carried on one frame and laid in different directions. Legs may be served by a multi-port interrogator, or by a single interrogator port through a two-fiber cable with a far-end turnaround, so that the fiber runs out one leg, back to the frame, and out the next, placing several physical legs on one serial optical path. Using a conservative illustrative 10 km optical-path envelope - compact interrogators are publicly specified for sensing ranges up to 20 km — two 2 km legs (about 6 km of optical path) fit on one port, and three or four shorter legs would also fit; the optical penalties of the turnaround, the double pass and the far-end loss have not been measured for this configuration and are identified as open questions. The architecture separates conventional frame handling, robotic cable placement, and sensing into three independent pieces, each of which can be developed and de-risked on its own, and offers a low-hardware route to multi-leg seabed arrays that can be deployed, recorded, recovered and redeployed. Aperture then scales at two levels: within a node, through additional baskets, legs, ports or serial paths; and across a survey, through additional nodes - a modular array of distributed-fiber nodes rather than a single long sensing line. Because the frame is designed for intervention, the same architecture supports campaign, serviced and connected operating models. Concept note, September 2026. This note documents a concept-level architecture and possible variants developed prior to any BIRD Energy Center award. It builds on Bakulin (2026a) and includes further system-level contributions by Bakulin and Dupré. Nothing herein is a specification, commitment, or performance claim. No proprietary or confidential third-party information is knowingly disclosed. No external funding supported this work. Component technologies and associated background intellectual property remain with their respective owners. This note does not determine ownership, inventorship, or licensing rights in any underlying or future implementation.