Applying laterally varying density corrections to ground gravity and airborne gravity gradiometry data: a case study from the Bathurst Mining Camp

Abstract

The influence of topography on gravity and gravity gradiometry measurements is profound and should be minimized prior to geological interpretation. The standard way of minimizing these effects is through the computation of a terrain correction. Terrain corrections require two inputs: topography and density. Often, geology and topography are inextricably intertwined: topography is caused by a change in geology. In geologic environments where there is a structural and (or) stratigraphic control on the near-surface mass distribution, using a single density value in the corrections leads to removal of the topographic effect of rocks having the chosen density. Any remaining gravity signal that correlates with topography is providing geological information. If the objective is to produce a gravity map with minimal topographic signal, then a regionally variable density correction is a means of compensating for this effect. In this paper, we demonstrate how to apply a spatially variable density correction using ground gravity and airborne gravity gradiometry data for the geologically complex Bathurst Mining Camp, northern New Brunswick, Canada. Ground gravity and airborne full tensor gravity gradiometry measurements are subdivided into a series of domains on the basis of the underlying tectonostratigraphic group. Terrain and Bouguer corrections are calculated for each domain using representative density values obtained from drill core and surface sampling throughout the Bathurst Mining Camp. The output from the spatially variable density correction is then compared with previous maps. Overall, the differences are subtle, but the spatially variably density allows for isolated anomalies to be better resolved.

The Henkel Petrophysical Plot: Mineralogy and Lithology From Physical Properties

“…Hinze (2003) recommended that a density value of 2.67 g/cm 3 should be used for Bouguer anomaly correction. More recently, Tschirhart et al (2019) demonstrated that optimal Bouguer correction (minimization of terrain-related signal) is best achieved with a spatially varying density value. Even so the densities they used only varied from 2.72 to 2.83 g/cm 3 .…”

A priori models and inversion of gravity gradient data in hilly terrain

“…More recent examples for producing maps of terrain‐corrected AGG data were given by Tschirhart et al . () using petrophysical data together with geometry of rock units taken from the geological map and Bell Geospace (http://resource.sgu.se/dokument/mineralnaring/Kiruna_AandP_public.pdf) employing a wavelet technique.…”

“…Historically, lateral density variations have been measured directly on rock samples or estimated by correlating measured gravity data with synthetic data from known terrain (Nettleton 1939;Parasnis 1952;Vajk 1956;Rao and Murty 1973;Mankhemthong, Doser, and Baker 2012) and by high pass filtering and 3D inversion (Granser, Meurers and Steinhauser 1989). More recent examples for producing maps of terrain-corrected AGG data were given by Tschirhart et al (2018) using petrophysical data together with geometry of rock units taken from the geological map and Bell Geospace (http://resource.sgu.se/dokument/mineralnaring/Kiruna_Aan dP_public.pdf) employing a wavelet technique.…”

Introduction to the special issue on geophysics applied to mineral exploration

“…Two contributions revisit the gravity method, probably the oldest method in geophysics. Caron et al 2019 use a methodology to remove the effects of overburden in an airborne gravity dataset to enhance the anomalies associated with bedrock sources, whereas Tschirhart et al 2019 show how terrain corrections using regionally variable densities may help to better resolve subtle isolated gravity anomalies. On the methodological side, Morris et al 2019 focus on borehole magnetic techniques that provide a degree of spatial and magnetic resolution that is not achievable with most standard supra-surface magnetic surveys and describe two cases were data corrections were needed to address issues associated with the use of triaxial fluxgate magnetometers sensors in the borehole probes.…”