Hold on how does that work?
I have had a sort of sci-fi idea that sufficiently sensitive gravitational field measurements coukd detect the passing of submarines (I am not sure on the maths tbh) - which would render a lot of nuclear strategy moot.
Just need to get a grasp on the maths
The Eotvos pendulum (an instrument aka. Eotvos torsion balance) designed in 1888 started this kind of measurement. It was used commonly by the 1920s by geophysicist for mapping underground deposits by measuring the gradient of the gravitational field very precisely.
This instrument was deprecated later by even better tools for surveying.
The instrument was initially constructed for the experiment showing that inertial and gravitational mass are the same (well, linearly correlated) to a great precision: https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_experiment
https://www.nature.com/articles/118406a0 (pretty useless link, but a famed periodical)
Detecting submarines is way harder, practically impossible. as others have already pointed out.
(IIRC) Royal Navy trialed it (officially) for the first time last year.
Gravitational Detection of Submarines, PM Moser 1989
This one's never going to happen.
Geologic mass concentrations are an entirely different story: you get a gravitational monopole, which is a more reasonable inverse square law. (No monopoles for a submarine, because by design they have a mean density equal to water—as the paper explains).
Range is pretty short but still large enough that you can do it from an airplane flying over.
Are you looking for density variation between the parts and airspaces of a submarine?