Dear Mr. Akpan:

For samples which the susceptibilities have negative values you may expect to contain mostly diamagnetic minerals. The magnetic susceptibility depends on minerals in which the iron is contained. Since no correlation is observed it means that the iron is mostly concentrated in paramagnetic minerals. The majority of minerals which do not accommodate iron in their lattice are diamagnetic yielding a negative moderate susceptibility, e.g., siliceous minerals, which are the most common constituents in rocks. By contrast, Fe-bearing paramagnetic minerals give a positive read-out which can be higher by several orders of magnitude than those obtained for diamagnetic minerals. Siderite, ankerite and some Fe-bearing phyllosilicates pertain to these minerals. Only a fraction of these minerals are ferrimagnetic, the most well-known of which are magnetite [Fe3O4], maghemite [γ-Fe2O3], ferroxyhyte [δ-FeOOH] and  pyrrhotite [Fe7S8] which can also appear in sedimentary rocks. Another group of Fe-bearing minerals also quite common differ from the afore-mentioned oxidic Fe minerals by their  antiferromagnetic order such as hematite [α-Fe2O3], ilmenite [FeTiO3] and goethite [α-FeOOH].

I hope it will direct your thoughts in the right direction.

Best regards

H.G.Dill

It is most likely the result of remnant magnetization having opposite direction with respect to the present day field.

I agree with Antonio. By "sag", I think you mean you have a negative magnetic intensity anomaly over the intrusions. If the intrusions are mafic or intermediate, they most likely contain magnetite, maghemite or pyrrhotite, which are ferrimagnetic. These minerals can preserve the magnetic anomaly from the time they cooled below their Curie temperature, which could have been in a different direction or with a different intensity than today's field. Ferrimagnetic minerals would have a high positive susceptibility. It will be difficult to quantitatively model these intrusions purely on the airborne magnetic intensity survey and the susceptibility measurements. You would need to measure natural magnetic remanence (and other petrophysical parameters) in addition to susceptibility on accurately oriented samples of the intrusions, and on a sufficient number of samples to be representative. This is my interpretation anyway, based on your info - hope it helps.

Thank you Dill and Schettino for your help. Schettino's suggestion, in particular, has redirected my thought.

A ground magnetic survey with detailed measurement of the susceptibilities of several samples may help.

By sag I think you're probably referring to negative susceptibility which of course will mean the signals are probably over diamagnetic or ferrimagnetic minerals unless except as pointed out by Anthonio Schettino a realignment of the magnetic moments opposite to the present field direction had occurred