Minimum-structure borehole gravity inversion

The borehole gravity technique has been well established in hydrocarbon exploration geophysics since the 1970's. The concept behind borehole gravity is simply to measure the variation in the Earth's gravitational field while traveling along a borehole. Densities both close to and far from the borehole can be derived from such measurements. However, the borehole gravity technique has not yet been routinely used for mineral exploration because gravimeters that fit in the narrower diameter holes used in mineral exploration have not existed. Such gravimeters are now being developed. Complementary investigation and development of interpretation procedures for borehole gravity data in a mineral exploration context are required. Here, results are presented of a study inverting synthetic borehole gravity data for three-dimensional, mineral exploration relevant Earth models. The forward-modelling on which the inversion is based is a finite difference solution of Poisson's equation. The inversion is performed using a standard minimum-structure algorithm for multiple scenarios of varying borehole locations, amount of borehole data and varying model parameters. The intention is to demonstrate what we can expect to determine about the density variation around and between boreholes given varying amounts and locations of down-hole and surface data. It is observed that the benefits of borehole gravity data depend on the locations of the boreholes relative to the anomalous mass. Inversions which produce images of complex subsurface density distributions are attainable with the most successful models resulting from combined surface and borehole data. Minimum-structure borehole gravity inversion is shown to be a beneficial interpretation option which can provide accurate information of an anomaly's shape with proper depth resolution and density distribution.