Sonar signals processing.
This was an upgrade to the sensor update system (SUDS) sonar submarine tracking systems.
Don't think he'll find many subs in that lake.
He is probably searching for something around the lake area. P-3s like the newer P-8s have a very extensive reconnaissance capability using a wide variety of systems, radar, electronic, infrared etc.
Could be almost anything vehicles, electronic devices phones, camp hidden in the woods.
As a britfag I'm not too familiar with the lake but I assume it is a fairly popular area for recreation so it might be that a boat sank & he is searching for missing persons or a hiker lost near the lake perhaps.
Check local news see if anything comes up. Many US local news outlets are not available to me as I am from across the pond.
No probs
Because of dumb EU censorship rules many US sites block EU searches. Comes up with default 'not available in your region' type messages. Major syndicated outlets it is not a a problem but with some of the smaller local sites they can't be bothered to conform or afford to conform with rules so are blocked.
Can make planefagging a plane crash in the US tricksy at times.
With effort information can be found though.
Seems to be mainly news sites where I come across the problems rather than general digging & searching.
Lost count the number of times I've seen an anon drop a link to an incident only to find it doesn't work for me. Usually gives me clues though where else to search though.
If you are local to the state etc that gives you an advantage knowing the local news stations etc.