Magnetic Anomaly: Magnetized Sphere in the Northern Hemisphere
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 of the sphere, a large postive 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 at the magnetic poles or equator, this will always be true.
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 alogrithms 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.