We present a case study of applying 3D inversion of gravity gradiometry data to iron ore exploration in Minas Gerais, Brazil. The ore bodies have a distinctly high-density contrast and produce well-defined anomalies in airborne gravity gradiometry data. We have carried out a study to apply 3D inversion to a 20  km2 subarea of data from a larger survey to demonstrate the utility of such data and associated inversion algorithm in characterizing the deposit. We examine multiple density contrast models obtained by first inverting Tzz; then Txz, Tyz, and Tzz jointly; and finally all five independent components to understand the information content in different data components. The commonly discussed Tzz component is sufficient to produce geologically reasonable and interpretable results, while including additional components involving horizontal derivatives increases the resolution of the recovered density model and improves the ore delineation. We show that gravity gradiometry data can be used to deli...

3D inversion of airborne gravity gradiometry data in mineral exploration: A case study in the Quadrilátero Ferrífero, Brazil

Inversion of gravity and/or magnetic data attempts to recover the density and/or magnetic susceptibility distribution in a 3D earth model for subsequent geological interpretation. This is a challenging problem for a number of reasons. First, airborne gravity and magnetic surveys are characterized by very large data volumes. Second, the 3D modelling of data from large-scale surveys is a computationally challenging problem. Third, gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions. These difficulties and how they can be addressed in terms of large-scale 3D potential field inversion are discussed in this paper. Since potential fields are linear, they lend themselves to full parallelization with near-linear scaling on modern parallel computers. Moreover, we exploit the fact that an instrument’s sensitivity (or footprint) is considerably smaller than the survey area. As multiple footprints superimpose themselves over the same 3D earth model, the sensitivity matrix for the entire earth model is constructed. We use the re-weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options. We demonstrate our approach with the 3D inversion of 1743 line km of FALCON gravity gradiometry and magnetic data acquired over the Timmins district in Ontario, Canada. Our results are shown to be in good agreement with independent interpretations of the same data.

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Large-scale 3D inversion of potential field data

Airborne gravity gradiometry (AGG) is becoming a widely accepted tool in exploration. It provides rapid acquisition of accurate gravity data at high spatial resolution with complete coverage over large areas. The data are of significant value to explorers for a wide range of commodities throughout the world. With the advent of airborne gravity gradiometer technology, it is now possible to collect high-resolution gravity with all the attendant advantages.

Performance of airborne gravity gradiometers

We have identified a gap in the literature on error propagation in the gravimetric terrain correction. Therefore, we have derived a mathematical framework to model the propagation of spatially correlated digital elevation model errors into gravimetric terrain corrections. As an example, we have determined how such an error model can be formulated for the planar terrain correction and then be evaluated efficiently using the 2D Fourier transform. We have computed 18.3 billion linear terrain corrections and corresponding error estimates for a 1 arc-second (∼30  m) digital elevation model covering the whole of the Australian continent.

/pdf/fast-fourier-based-error-propagation-for-the-gravimetric-c9m87929wl.pdf

Fast-Fourier-based error propagation for the gravimetric terrain correction

We developed a new method for interpretation of airborne gravity gradiometry data, based on cokriging inversion. The cokriging method that we evaluated minimized the theoretical estimation error variance by using auto- and crosscorrelations of several variables. It does not require iterations and can easily include complex a priori information. Moreover, the smoothing effects in the inverted density structure model can be reduced to a certain extent due to the anisotropy constrain in the covariance model. We compared the recovered models obtained by inverting the different combinations of gravity-gradient components to understand how different component combinations contributed to the resolution of the recovered model. The results indicated that including multiple components for inversion increased the resolution of the recovered density model and improved the structure delineation. Moreover, in the case in which the parameters of the variogram model are not well chosen, cokriging with multicompon...

3D inversion of airborne gravity-gradiometry data using cokriging

Terrain corrections for airborne gravity gradiometry data are calculated from a digital elevation model (DEM) grid. The relative proximity of the terrain to the gravity gradiometer and the relative magnitude of the density contrast often result in a terrain correction that is larger than the geologic signal of interest in resource exploration. Residual errors in the terrain correction can lead to errors in data interpretation. Such errors may emerge from a DEM that is too coarsely sampled, errors in the density assumed in the calculations, elevation errors in the DEM, or navigation errors in the aircraft position. Simple mathematical terrains lead to the heuristic proposition that terrain-correction errors from elevation errors in the DEM are linear in the elevation error but follow an inverse power law in the ground clearance of the aircraft. Simulations of the effect of elevation error on terrain-correction error over four measured DEMs support this proposition. This power-law relation may be used in selecting an optimum survey flying height over a known terrain, given a desired terrain-correction error.

Airborne gravity gradiometry: Terrain corrections and elevation error

The Bass Strait FALCON airborne gravity gradiometer (AGG) survey was flown over an area of the Gippsland Basin in Bass Strait in July 2002. The survey, centred on the gas-producing Marlin and Snapper fields, coincided with a contemporary, detailed marine gravity survey. These marine gravity data, as well as pre-existing data from satellite gravity and sparse marine gravity surveys, were used in comparisons with the FALCON data. The vertical gravity gradient data outlined a major Eocene channel. Interpretation, including modelling of representative seismic lines, revealed the particular usefulness of these data for mapping shallow faults. Conventional vertical gravity from the FALCON survey was better at the longer wavelength features, typically the major tectonic elements such as basin bounding faults, and relative sediment thickness. Comparisons with the marine and satellite gravity data showed that the FALCON gravity reproduced all the information available in the other surveys at wavelengths up to the survey size. At shorter wavelengths, the FALCON data had higher sensitivity than all other datasets, including the detailed marine gravity. Incorporation of longer wavelengths from the satellite or other gravity into the FALCON gravity proved successful in improving the longer wavelength information and regional context. As a result of this study, the standard FALCON processing has been improved to allow such incorporation on all surveys where separately acquired sparse gravity data are available.

Yi Zeng

Papers

Airborne gravity gradiometry: Terrain corrections and elevation error

Applying FALCON® gravity gradiometry to hydrocarbon exploration in the Gippsland Basin, Victoria