gradient filter/formula

[Ken Hickey]

G'day all   I have a friend who has some aeromagnetic data of an area she is studying. We want to look for edges between granite bodies with slight differences in magnetic intensity. What filters/formula in ERMapper are best to look for edges between granite bodies (ie., zones of max gradient) in all horizontal directions in the plane of the earth's crust? and would I be better doing the filtering in Fourier space? (if so what filter should I use there?).   Thanks in advance Ken Hickey