Some aspects of regional‐residual separation of gravity anomalies in a Precambrian terrain

Abstract

The Bouguer gravity field measured over two Archean greenstone belts of northwestern Ontario is analyzed using three different regional‐residual separation techniques. The purpose of the analysis is to obtain a residual map suitable for gravity modeling studies to help define the subsurface characteristics of the greenstone belts and associated granitic areas. The methods used to derive the regional and residual maps are spectral factorization, upward continuation, and graphical smoothing. The substantial differences in the three sets of maps emphasize the ambiguity and subjectivity of the separation process. Each method may yield nonunique results. For example, in the spectral factorization technique, the filter design is dictated by the clarity of the slope change between the short‐ and long‐wavelength features and, in the case of the upward continuation technique, by the choice of the continuation height. The graphical method is empirical clearly nonunique. The regional map obtained through graphical smoothing is the most satisfactory for the purpose stated since it bas been designed to have minimal contributions from the shallow and broad greenstone masses outcropping at the surface. These features are clearly visible in the spectrum‐based regional map and to a lesser extent in the upward‐continued regional map. All three types of residual maps follow the general outline of the geologic units and thus are probably equally useful for a qualitative study of the anomaly shapes. However, for quantitative modeling purposes, the graphically produced residual is most suitable, since it can be successfully fitted by models that are consistent with the known surface geology and measured density values. The location, spatial extent, and the amplitudes of the analytically produced residual anomalies, in many areas, show poor correlation with the surface measurements, rendering these maps less satisfactory.