Lithological mapping by correlating magnetic and gravity gradient airborne measurements

Abstract

We report here two new applications of the Poisson relation useful when a joint magnetic and gravity gradient survey has been made. In the first, an apparent susceptibility contrast to density contrast ratio map (which we call the "pseudo-lithology" map) can be estimated. In the second, selected values of the above ratio are used in a "pseudo-magnetic" filter to subtract from the magnetic field measurements the effect of a particular rock type. This would be especially useful as a filter of "geological noise" due perhaps to weathered magnetite products, particularly if the latter had an effect over a large area. As a test of these techniques we have applied them to a simulated joint airborne survey over a model which we designate "Mini-Elura", in which the magnetic magnitude and vertical gravity gradient anomalies are not visible above the noise due to surficial variations. The computed pseudo-lithology map clearly distinguishes the regions of the map dominated by the alluvium source from those due to the "Mini-Elura" orebody, while the pseudo-magnetic filter does very effectively remove the "noise" due to the alluvium from the magnetic map.