This might seem a little strange because presumably a void would not have a magnetic field. However rocks do, so the situation I am considering is a body of rock which has permanent magnetization in one constant direction spread across the whole rock. Without any voids the magnetic field intensity will be equal everywhere at the surface of the rock. But what happens if you remove some rock and create a void? Presumably the field intensity will drop off accordingly, you would get an anomaly peaking at the surface at the point connecting the magnetic field lines to the void. Is my view simplistic, is it possible the void would act as a dipole?