Getting the best value from gravity gradiometry

Abstract

The critically important steps to get best value from your gravity gradiometry data, assuming your contractor has done his job well in designing and acquiring the data, is the preparation of the representation of the potential field gradients. The ~200m resolving power of existing gradiometer systems approaches what is necessary for minerals applications. In particular, beyond the aircraft, the topographic surface represents the largest and most proximal density contrast encountered in an airborne survey. Hence terrain effects can have significant impact on AGG data. The critical steps are: Terrain correction and determining 'best' terrain density Gridding, using all the measured gradients to constrain the interpolation Smoothing while honouring the 3rd order tensor constraints Anti-alias filtering of the gradient signals so that wave lengths are properly represented in all directories and no distortions exist along line Transformation of the gradients by integration to estimate the gravity or magnetic field. Terrain corrections are a necessary step in the processing of observed AGG data in rugged terrain, in order to highlight subsurface density variations with a minimal overprint from the terrain. We propose a simple and rapid AGG tensor-based method to estimate an optimum bulk terrain density for subsequent terrain-correction. Each of the currently deployed systems for acquiring gradiometry is evolving driven by competition and the users' needs. Mining applications of the technology to directly detect ore-bodies that are anomalies, can now be successful provided the dimensions are of the order of 200m or more.