Constrained gravity 3D litho-inversion applied to Broken Hill

Abstract

We present an example of a constrained inversion method that uses a categorical property, lithology, as the primary model parameter. A 3D geological model is supplied as the starting model. The topology of this model is used as prior information, together with parameters that define the probability density function of a secondary property, density, and observations of the gravity field. The 3D geological model is a prediction of the geometry of geological interfaces given points where these interfaces have been observed, structural observations of the orientation of these interfaces and a table describing the relationships between the various geological units. The interfaces are modelled as equipotential surfaces, and cokriging is used to interpolate between the supplied points. The inversion method returns a number of models which are consistent with the supplied prior information. This ensemble of acceptable models can be analysed statistically to derive conclusions. A 3D geological model of Broken Hill was constructed from a range of geological inputs. Constrained inversion of ground gravity was then carried out. Prompted by the results of initial inversions, a number of adjustments were made to both the geological model and estimates of the density for each of the units. The final inversion was used to demonstrate a high degree of internal consistency amongst these amended forms of the prior information. The use of airborne gravity gradiometry (AGG) data was deferred pending modification of the forward modelling algorithm to incorporate the band-limited character of these observations.