Originally Posted by
Ninthace
If the s/m displaces water equal to its mass - would it even have a gravity signature?
It'll have a different mass distribution, so it will definitely have some sort of signature (whether or not it's measurable is another question). Presumably, if you were designing a new submarine from scratch, you could then try to make the mass distribution more even.
If the sensor struggles in an airborne environment, what about deploying it like a sonobuoy? Either to float, or maybe to sink slowly thorugh the water reeling out a wire to a floating antenna?