Detecting minute magnetic field changes

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.
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<D.N. Scalbile> wrote in message

lookup SQUID magnetometer
Dave
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A digital compass has a ferite core and 2 windings ( actually 3) but the 2 are switched off then on as they turn on they are effected by the earths mag fld . and its easy to do for todays op amps are very good
BTW Im writting a good OpSys for small tings like robots . Later i'll expand it for large PC's i call it NewForth . Fits in 8KB . also im hobbiing on ATMEL8515 and have a way to increase the 4KB code space ! I use lists of addresses in SRAM to mimic and increase the code space .
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The common name for that is a flux gate. I just googled, here's an interesting article with a useful table of what devices cover what range. As someone else posted, it looks like SQUID wins for the most sensitive.
http://www.sensorsmag.com/articles/1298/mag1298/main.shtml

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magic
D.N. Scalbile wrote:

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