Which we have, not only is there a coarse scale global magnetic field model (two versions, in fact) there are detailed high resolution maps for large areas of the world (eg: all of Mali, all of Australia, most of Canada, much of the central 'stans) where mineral exploration has taken place.
The positioning is complicated by the drift over time (there's a "new" global model every five years and for specific times & locations interpolation is performed) and fluctuates daily (as the Earth turns there's a diurnal pulsing in the induced magnetic field).
This is where "a bit of signal processing" comes into play.
The good news is the maths, the field testing, and the application of it is all a good 50 years old.
The positioning is complicated by the drift over time (there's a "new" global model every five years and for specific times & locations interpolation is performed) and fluctuates daily (as the Earth turns there's a diurnal pulsing in the induced magnetic field).
This is where "a bit of signal processing" comes into play.
The good news is the maths, the field testing, and the application of it is all a good 50 years old.
Typical exploration mapping company:
http://mcpharinternational.com/services/mining-exploration/a...
High resolution gridded geophysics for the Northern Territory, magnetics, gravity, radiometrics, etc:
http://www.nt.gov.au/d/Minerals_Energy/Geoscience/index.cfm?...
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