The proximity and topography factors in airborne and marine gravimetry

Abstract

The topographic anomaly dominates over contributions from subsurface bodies whose dimensions are comparable to the topography. Its removal from airborne and marine data is made doubly difficult by variations in topographic density over large distances, and the non-proximity of the observations to the anomaly-sources. The smoothing inherent in non-proximity data makes the separation of anomaly components difficult, resulting in a high degree of ambiguity. Various automatic methods of dealing with the topographic anomaly are discussed. None of them can restore gradients already missing in non-proximity data, and none are valid for general application. The blending of anomaly components, which can be easily demonstrated, means that there can be no general form of Nettleton's density profiling concept. There is no general panacea for the topography problem Observing as close to the ground as possible will give the best results and multiple density profiling, based on Nettleton's principle is essential for understanding the ambiguity involved. Regardless of whether topographic density varies laterally or not, a set of multiple-density Bouguer profiles is the only known means of conveying the ambiguity resulting from the interaction between the topographic anomaly and other anomalies. This factor mitigates against contour analysis, where density profiles cannot be used.