Drilling-constrained 3D gravity interpretation

Abstract

In interpretation of gravity surveys, it is essential to exploit all the information available from drill holes in order to reduce the ambiguity. Accordingly, a new modelling and inversion methodology has been developed to expedite joint geological/geophysical interpretation of gravity data. The key features of the approach are the enforcement of drill constraints (pierce points) and the imposition of density bounds on geological formations and basement. 3D density models are constructed from close-packed vertical rectangular prisms with internal contacts. Prism tops honour topography, so that terrain effects are modelled, not 'corrected'. Detailed local models can be embedded in regional models to permit fitting of full free-air data, not residual gravity. The geological sense of models is preserved during inversion: the shape and density of homogeneous geological units are adjusted iteratively, subject to the drilling and density constraints. The methodology is illustrated using data from an advanced exploration project in South Australia. Integrated interpretation of a drilled area has been undertaken in four stages. The first stage entailed construction of a 'regional' density model, satisfying gridded gravity data on a coarse mesh over a large area centred on the drill grid. Next, a local density model was created on a fine mesh for the drill grid area, based on drill intercepts and density logs. Thirdly, the detailed density model was inserted into the regional model. Finally, constrained inversion was performed, to adjust the local starting model until a fit to the free-air gravity data was achieved.