David, I've posted previously on this regarding [magnetic] gradiometers (and also ionospheric propagation on that day, but that's another matter).
Having - admittedly rather briefly - reviewed this thread I see that you mention using a 'portable magnetometer' in a previous search. Are you able to elaborate on that? While a gradiometer may use magnetometers it is a somewhat different beast to a single magnetometer, and IMV would ordinarily be the weapon of choice when looking for this sort of thing. If you were in fact using a single magnetometer then I would be less surprised at a nil or somewhat confusing result.
FP.