Rock’s absorption of neutrons doesn’t matter. Neutron thermalize. Since rock has mostly low atomic weight isotopes, it thermalizes them very well [1]. Once they are thermal they scatter easily.

Gauss’ law is for the electric field. The magnetic field is very difficult to block and we have incredibly sensitive magnetometers (probably ones that can detect magnetic anomalies of a submarine from space)

In fact, thats pretty much how deep submarine communication works very low frequency fields. Thats why I proposed a magnet: a basically static field that cant be blocked.

Is my proposal practical? Well, typically not. But if the alternative is neutron communication, I dont see why not.

[1] to a first approx think of thermalizing as momentum exchange upon collision. If you’re similar in mass to the colliding mass you’ll exchange more momentum. Assuming rock is mostly silicon, w/ a mass 28 times larger than a neutron. And there’s at least a km of rock above miners.

[2] throttle a reactor to modulate neutron? Oh boy!

EDIT: actually the crust is mostly oxygen, atomic mass 16. That only strengthens my argument.