A note to just be a bit careful passively monitoring ocean acoustics, it’s easy to fall foul of military / security forces, they don’t like anything that can fingerprint a vessel.
I worked on DAS acoustic monitoring for subsea power cables (to monitor cable health!), turns out they are basically a submarine detection system.
Reminds me of how the Navy heard the OceanGate submarine implode immediately when it lost contact en route to the Titanic, but waited several days before they admitted that because at the time noone even knew they had such a system of hydrophones in place. I wonder what else they have that we don't know about. The oceans are not just unexplored as a habitat, but also as an intelligence theater.
Pretty sure a fair number of people knew the US Navy and others had hydrophones in place, they've always been coy about it though.
For interest:
* it's one reason we know so much about ocean tempretures and tangentially have great data on climate change being real, and
* they had some cool R&D vessels:
FLIP was originally built to support research into the fine-scale phase and amplitude fluctuations in undersea sound waves caused by thermal gradients and sloping ocean bottoms. This acoustic research was conducted as a portion of the Navy's SUBROC program.
That was not the first time such data was used to from and a wreck. They have released locations for things like downed airliners for years, decades. Everyone knows about SOSUS. The classified bits are its locations and exact capabilities.
I remember at the time it felt a little bit suspicious to me. Only after everyone already knew it had imploded, the navy came out to say their hyper advanced detection system for enemy submarines had of course also detected it.
I live near a sub listening station. Schools tour the base. Their hydrophones are built on site in long oil filled tubes. They can hang these from the listening barges or lay them on the bottom. Hydrophone arrays can be paired with anchors attached to a sound triggered buoy. The buoy sinks with the anchor and will be released when it gets the right signal. With this setup the Navy can lay down semi-permanent arrays without a surface buoy.
the systems are pretty public, for instance the UK tender for Atlantic Net is easy to read. And the russians have Bastion which we known well about as well.
Good advice but there's a bit of a difference between a device (or even several) you can knock together yourself and throw out of the side of a (surface) boat vs access to a whole undersea cable which (I have just learned) is what you need for DAS. Plus, if you can do it yourself with virtually no resources, it's a safe bet that any potential adversaries are already doing something many orders of magnitude greater.
Supposedly new submarines are so quiet that they can't be detected anyway. I'm sure there's a large element of exaggerating abilities here, but there's definitely an element of truth: in 2009, two submarines carrying nuclear weapons (not just nuclear powered) collided, presumably because they couldn't detect each other. If a nuclear submarine cannot detect another nuclear submarine right next to it then it's unlikely your $5 hydrophone will detect one at a distance.
Of course, none of this means that the military will be rational enough not to be annoyed with you.
Very cool and very powerful technology, it'll be interesting to see how fiber sensing progresses, especially with how much undersea fiber already exists. For subsea power cables, is there a parallel fiber dedicated just for DAS monitoring? Do these get bundled in with data fiber runs as well? I've been curious how well DAS can work over actively lit / in-service fiber.
On the cables I worked on they would use a separate fibre, but power cables tended to overspec the number of fibers massively so was never an issue. Some even have two bundles of fibers.
A supplier played whale song they recorded from cables, and said they repackage and sell the same product to defense contractors.
Even biulding the equipment. There are rules about hydrophones at certain certain frequencies. Just putting the plans online might runafoul of export rules. Beware of stringing multiple hydrophones as this article suggests. Put too many on a system and you are into possible beamforming territory ... the tech used for geolocating noises underwater. The USN gets kinda twitchy about such things.
I used to be a submariner and now work in an unrelated acoustic space (acoustic analysis of the electric grid), but I'd love to learn more about the DAS world — my email is in my profile.
I worked on DAS acoustic monitoring for subsea power cables (to monitor cable health!), turns out they are basically a submarine detection system.