Finally, let's examine the shape of the anomalous magnetic field for a metallic sphere buried somewhere in the northern hemisphere, say near Denver.
As in the previous examples, the Earth's main magnetic field induces an anomalous field in surrounding the sphere. The anomalous field is now oriented at some angle, in this case 45 degrees, from the horizontal. By looking at the direction of the anomalous field, Fa, in comparison with the Earth's main field, Fe, you can see that there will be a small negative anomaly far to the south (to the right in the diagram) of the sphere, a large positive anomaly just south of the sphere, and a small, broad, negative anomaly north of the sphere. Notice that the magnetic anomaly produced is no longer symmetric about the sphere. Unless you are working in one of those special places, like the magnetic poles or equator, this will always be true.
We can consider the situation in the Southern hemisphere, at the same magnetic latitude, by making some mental adjustments to the same diagram. If we take North to be to the right, and reverse the direction of the main-field arrows, we have created the southern-hemisphere setting. The shape of the induced, anomalous field will be the same, but the direction will reverse as a consequence. The outcome is that the profile will have exactly the same shape! A generalisation is that, where there is an isolated anomaly due to magnetisation by induction, the maximum will always be on the equator side of the anomaly, and the minimum will be on the pole side.
What does the anomaly look like if we take the profile EW rather than NS, in any of these cases?
From this simple set of examples, you now see that it is indeed more difficult to visually interpret magnetic anomalies than gravity anomalies. These visual problems, however, present no problem for the computer modeling algorithms used to model magnetic anomalies. You simply need to incorporate the location of your survey into the modeling algorithm to generate an appropriate magnetic model.
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