"Abstract
Gravity and magnetic exploration, also referred to “potential fields” exploration is used to give geoscientists an indirect way to “see” beneath the Earth’s surface by sensing physical properties of rocks (density and magnetization, respectively). Gravity and magnetic exploration can help locate minerals, faults, geothermal or petroleum resources, and ground-water reservoirs. Potential field surveys are relatively inexpensive and can quickly c over large areas of ground. The primary goal of studying potential fields is to provide a better understanding of the subsurface geology. The methods are relatively cheap, non-invasive and non-destructive environmentally speaking. They are also passive – that is, no energy needs to be put into the ground in or der to acquire data. The small portable instruments (gravimeter and magnetometer) also permit walking traverses. "
http://www.os.is/gogn/unu-gtp-sc/UNU-GTP-SC-09-13a.pdf
And to along with hunting subs in a P3 Orion that use a "Magnetic Anomaly Detector" - MAD.
Or people using a common 'metal detector' to find coins or whatever.