3D Gravity & Aeromagnetic Inversion, Pillara Region, W.A

Abstract

Constrained 3D gravity and magnetic inversion has been applied to an area of approximately 11 km ´ 4.5 km straddling the Pillara gravity ridge, Lennard Shelf, Western Australia. The main aim was to better define the depth to top limestone. The starting point for inversion was a simplified geological model based on three generalised litho-stratigraphic units: Shale/Siltstone, Limestone, and Basement. A staged inversion procedure was adopted. First the effects of the large property contrasts were accounted for, most notably the density contrast between limestones and clastics. Subsequently the residual gravity and magnetic data were inverted to define more subtle contrasts within the sediments. Gravity inversion involved adjustment of the Limestone contact geometry as well as model densities. The contact was fixed where pierced by drill holes, and a priori upper and lower bounds were imposed on the densities of the geological units. The inferred Limestone contact is a strong determinant of prospectivity, both in terms of depth and in terms of fault displacements. Final stage inversion highlighted coherent intra-sedimentary density trends oriented NE and NNE; these features could be associated with mineralising faults. Aeromagnetic inversion defined a basement susceptibility distribution generally decreasing from the SW to the NE, reflecting the character of the TMI data. More subtle susceptibility trends attributed to the sediments may reflect the underlying structural fabric, though the most pervasive residual gravity features are not strongly developed in the residual magnetic data. 3D gravity inversion is effective the Lennard Shelf as a means for defining the depth to limestone. The reliability of the inversion will be enhanced in areas where the gross geometry of the Limestone contact and basement unconformity are constrained by sparse drilling and/or seismic, and where the densities are well known from drill core determinations or wireline logging. Magnetic inversion can play a supporting role, insofar as it defines the basement topography.