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I wonder if it is of any use for a geophysicist, if we measure the magnetic field strength and direction of the earth. Could we make valid statements about the composition of the the earth, earth crust or core? What if we detect detect variations from the expected field strength, could that help us to draw a conclusion on the underlying local composition the earth?

I only know about one direct use for the magnetic field direction of the earth. Are there more use cases?

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Magnetic surveys are used for prospecting for oil or minerals.
On top of the earth's magnetic field there are small contributions form magnetic materials in the surface rocks, especially granites.
You can use this to either find large bodies of volcanic rock that migth have minerals or diamonds - or alternatively you can find large volumes with no magnetic field which might be oil or gas reservoirs.

The tricky part is that the local fields are only a fraction of a percent of the overall Earth's field and the Earth's field varies by much than this - so there is a complex mapping process to take out the natural background.

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Do you have any directions to such mapping projects? I can imagine there are many private surveys, which are not publicly accessible. Is there any interesting open survey project or even book/article you could recommend? –  elcojon Jan 12 '13 at 20:28
There is probably lots of info on the USGS site –  Martin Beckett Jan 12 '13 at 20:39

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