The Significance of Station Spacing in Subsequent Modelling of Basin Structure from Enhancement of Gravity Data – an Example from the Ngalia Basin, Australia

Abstract:

Enhancements such as the vertical derivative provide advantage in imaging gravity data. Those same advantages extend to modelling and inversion, but place greater demands on sample density and resolution of the data. The Proterozoic Ngalia Basin in the Northern Territory, Australia has proven uranium deposits. The structure of the basin is poorly constrained by vintage seismic and sparse drilling, with only two basement intersections in the deeper parts of the basin. A model of the basin as a container was required for an integrated study of the introduction, distribution, deposition and enrichment of uranium as a guide for regional exploration. At the start of the project the existing gravity data consisted of stations at 11 kilometre spacing, with closely spaced stations along the sparse network of seismic lines. This coverage did not provide clear gravity mapping of the basin, which is superimposed on a strong regional gradient and is complicated by gravity anomalies due to variations in density of the basement that surrounds and underlies the basin. During the project new gravity data was acquired to cover large parts of the basin at 4, 2 and 1 kilometre spacing. The resulting new gravity image does not differ in major form from the previous image but includes much greater detail. Imaging of that detail with the vertical derivative transform supports an interpretive separation of the gravity expression of the basin itself. That advantage was extended to modelling and inversion by directly matching the vertical derivative enhancement with vertical derivative model computations. This focus on matching the data enhancement was of clear advantage in developing a model optimised to the most relevant content of the data. The anomaly separation remains interpretive rather than definitive, but has provided a simplistic but geologically acceptable 3-D model of the basin for input to other components of the prospectivity study. An identical advantage would have been obtained by modelling the vertical derivative of gravity directly measured to the same resolution by an AGG survey.