Spatially Varying Corrections?
When reducing gravity observations, there were a host of spatially varying corrections that were applied to the data. These included latitude corrections, elevation corrections, slab corrections, and topography corrections. In principle, all of these corrections could be applied to magnetic observations also. In practice, the only corrections routinely made for are spatial variations in the Earth's main magnetic field, which would be equivalent to latitude corrections applied to gravity observations. Why aren't the other corrections applied?
Variations in total field strength as a function of elevation are less than 0.015 nT per meter. This variation is generally considered small enough to ignore. Variations in total field strength caused by excess magnetic material (i.e., a slab correction) and topography could, on the other hand, be quite significant. The problem is the large variation in susceptibilities associated with earth materials even when those materials are of the same rock type.
Recall that in applying the slab and elevation corrections to our gravitational observations, we had to assume an average density for the rocks making up the corrections. Rock densities do not vary much from rock type to rock type. Density variations of 0.5 gm/cm^3 are large. Variations among different samples of the same rock type vary by even less. Therefore, we can assume an average density for the correction and feel fairly confident that our assumption is reasonable.
Magnetic susceptibilities vary be orders of magnitude even among samples of the same rock type. So, how can we choose an average susceptibility on which to base our correction? The answer is we can't. Therefore, instead of applying a set of corrections that we know will be wrong, we apply no correction at all to attempt to account for excess material and topography.