Remanent Magnetization
So, as we've seen, if we have a magnetic material and place it in an external magnetic field (one that we've called the inducing field), we can make the magnetic material produce its own magnetic field. If we were to measure the total magnetic field near the material, that field would be the sum of the external, or inducing field, and the induced field produced in the material. By measuring spatial variations in the total magnetic field and by knowing what the inducing field looks like, we can, in principle, map spatial variations in the induced field and from this determine spatial variations in the magnetic susceptibility of the subsurface.
Although this situation is a bit more complex than the gravitational situation, it's still manageable. There is, however, one more complication in nature concerning material magnetism that we need to consider. In the scenerio we've been discussing, the induced magnetic field is a direct consequence of a magnetic material being surrounded by an inducing magnetic field. If you turn off the inducing magnetic field, the induced magnetization disappears. Or does it?
If the magnetic material has relatively large susceptibilities, or if the inducing field is strong, the magnetic material will retain a portion of its induced magnetization even after the induced field disappears. This remaining magnetization is called Remanent Magnetization.
Remanent Magnetization is the component of the material's magnetization that solid-earth geophysicists use to map the motion of continents and ocean basins resulting from plate tectonics. Rocks can acquire a remanent magnetization through a variety of processes that we don't need to discuss in detail. A simple example, however, will illustrate the concept. As a volcanic rock cools, its temperature decreases past the Curie Temperature. At the Curie Temperature, the rock, being magnetic, begins to produce an induced magnetic field. In this case, the inducing field is the Earth's magnetic field. As the Earth's magnetic field changes with time, a portion of the induced field in the rock does not change but remains fixed in a direction and strength reflective of the Earth's magnetic field at the time the rock cooled through its Curie Temperature. This is the remanent magnetization of the rock--the recorded magnetic field of the Earth at the time the rock cooled past its Curie Temperature.
The only way you can measure the remanent magnetic component of a rock is to take a sample of the rock back to the laboratory for analysis. This is time consuming and expensive. As a result, in exploration geophysics, we typically assume there is no remanent magnetic component in the observed magnetic field. Clearly, however, this assumption is wrong and could possibly bias our interpretations.