An improved pseudo-gravity magnetic transform technique for investigation of deep magnetic source rocks

Abstract

The pseudo-gravity transform is one of many possible FFT techniques that can be applied to aeromagnetic data. It enhances the anomalies associated with deep magnetic sources at the expense of the dominating shallow magnetic sources. This transform is an excellent interpretation tool for the detection of deep, magnetic igneous plutons and volcanic piles and the transformed data can be modelled using conventional gravity modelling tools. It is a suitable tool for interpreting deep-seated mineral plumbing systems associated with known, shallow mineral occurrences. The pseudo-gravity transform is derived by an integration of the total magnetic intensity grid data using conventional FFT tools. Padding of the grid around the region covered by the survey introduces long wavelength artefacts into the transformed grids. These long wavelength artefacts can obscure the targets that are the object of investigation. A variety of regional residual separation procedures is applied to the transformed grid to minimise the impact of the long wavelength artifacts. The improved pseudo-gravity transform is applied to the Goulburn 1:250 000 survey in the Lachlan Fold Belt of New South Wales to demonstrate the clear separation of deep sources that are difficult to detect or understand in the context of conventional magnetic image analysis. The results are contrasted with other filter techniques. Interpretive modelling of both the magnetic and the gravity transform data show how to derive more relevant geological information from magnetic surveys. By comparing the pseudo-gravity transform results with lower resolution ground gravity data, it is possible to obtain additional geological information by analysing the correlations.