Computer-assisted regional-residual gravity anomaly separation with regularized first and second-order derivatives.

The regional-residual separation of gravity anomalies in crustal and mineral exploration was a graphical-based procedure before the advent of fast digital computers and the need for more efficient algorithms to process large data sets. However, since requiring the supervision of an experienced interpreter, the results once obtained with graphical procedures are often accepted as second to none in producing anomalies with geological significance. Numerical methods based on spectral filtering and robust polynomial fitting have worked in many scenarios but seem not fully effective in replicating in algorithms the kind of results once obtained with interpreter-assisted graphical methods. We develop a procedure (CARRS- Computed Assisted Regional Residual Separation) implemented by a set of short MATLAB scripts which in many aspects simulates the operations of former graphical methods but requires few decisions and minor hand-work from the interpreter. CARRS applies robust polynomial fitting to points with low horizontal gradient and vertical second-order derivative, thus selecting fitting points as a real interpreter would do with graphical approaches in outlining the regional field. A regularized procedure is used to calculate stable first and second-order derivatives. MATLAB codes in companion allow results replication and further exploration with different threshold levels to identify flat domain regions. CARRS is illustrated with airborne gravity data CPRM-1123 freely available to download. A data window of the residual field is used to analyze the distribution of cassiterite deposits in greisen zones of the Paleoproterozoic Velho Guilherme granites in the Amazon Craton, as their distribution appears in the observed gravity anomaly and its corresponding residual field.