I’ll do a more precise calculation in a day or two, but magnetic materials are noticeable when buried. The intuitive reason is the spins in ferromagnetic materials align themselves with the external field, increasing the field inside the magnet so strongly, that it changes earths field noticeably even at somewhat large distances, and even when buried.
The thing to optimize for is likely something like field change when buried of an object large enough to be visible on some realistic grid size, per dollar.
As a quick guess, something like a spiral or other shape 1km wide made from ferite magnet 1x1m cross-section would have been noticed by a survey using WW2 instruments when buried 10m deep, in an area where some survey was done.
I’ll do a more precise calculation in a day or two, but magnetic materials are noticeable when buried. The intuitive reason is the spins in ferromagnetic materials align themselves with the external field, increasing the field inside the magnet so strongly, that it changes earths field noticeably even at somewhat large distances, and even when buried.
The thing to optimize for is likely something like field change when buried of an object large enough to be visible on some realistic grid size, per dollar.
As a quick guess, something like a spiral or other shape 1km wide made from ferite magnet 1x1m cross-section would have been noticed by a survey using WW2 instruments when buried 10m deep, in an area where some survey was done.