How to detect (open to friendly speculations) minute magnetic field changes ?
In application such as; * Detecting a crashed small aircraft in a very dense forest from another small aircraft fliying at a hight of 1000-4000 feet AGL. * Detecting metal pipe 10-20m below in ground. * Detecting a small metal boat sinked to depth of 00-200m from the surface of the sea. * Detecting earth magnetic field variations from a low orbit atellite or from a high altitude flying baloon.