I am a geologist, not a geophysicist, but have a rudimentary knowledge of the use of various geophysical surveys used in mineral exploration. So here is a simple explanation to the queries raised by several posters today.

Airborne surveys are almost always simultaneous EM and Mag surveys. Magnetometers aboard the aircraft measure the magnetic field variations caused by the bedrocks below. The mag results are always released as contoured maps, colour or otherwise. What the maps show is the relative magnetic strengths of the various rock types. Typically ultramafic rocks such as peridotite and pyroxenite, have a strong magnetic signature due to their relatively high concentrations of magnetite in the rock say compared to most felsic volcanics and sediments. The shapes of the contours reflect the shapes of bodies surface.

EM surveys measure the electrical conductivity of bodies. Metal sulphides and graphite are very strong conductors. Also, the concentration of the conductors and their depth of occurrence will be reflected in the strength of their response. The responses for each "channel" detected by the instrument used, supposedly reflects different depths of response. Different instruments, using somewhat different technologies (e.g. frequency or time domain) have different sensitivities or penetration. According to the literature, VTEM surveys have the best depth penetration or sensitivity.

Chromite is an insulator, but so are barren rocks. Hence, I would expect that the former would not mask any underlying sulphide concentrations from the EM surveys. What determines whether or not "conductors" are detected which are then classified as "anomalies" depends on their concentration and depth below surface.

Hope this helps.

Respectfully submitted

geoprof