The gravity terrain correction - practical considerations

Abstract

The revival in gravity methods stimulated by the availability of reliable positioning and, more importantly, elevation estimation, using relatively low cost GPS methods has transferred the focus on survey accuracy and precision from elevation errors to those implicit in the complete topographic correction. The terrain correction has normally been ignored in low relief terrain since the error in doing so has usually been much less than that related to errors in elevation accuracy. It is now possible to specify and observe gravity surveys that can yield a precision of 0.01 to 0.03 mGal in the Bouguer anomaly in production practice. Unfortunately even quite minor aberrations in local topography near the gravity meter may generate errors in excess of 0.1 mGal and no tight specification is justified unless it also demands detailed description of the local terrain, outlines how to avoid problems, and requires calculation of such corrections as are necessary. This will require a change in observer culture and some suggestions of the scale of the problem and means for avoiding it are given in this paper. There is no doubt that the terrain correction is now the major source of deficiency in gravity survey data bases. The methods used for calculation of the terrain correction also need review and care since digital terrain models are not necessarily of adequate precision or reliability for calculation of the effects close to the observation point.