Comment by jubjubbird

Apologies for the paywall, but this paper describes imaging cargo containers with gravity gradient sensors (to look for, e.g., plutonium):

https://ieeexplore.ieee.org/document/4215058

Basically you can image anything if there is a density contrast, it's "just" a matter of how good your sensor and inversion algorithm is.

I spend my day-to-day working on measuring gravitational changes to determine groundwater storage change. There's a huge difference between demonstrating something in the lab vs. building an actual field instrument you can use in the real world.