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 For concepts and details about the gravity inversion process, see Li and Oldenburg (1997).
The 1,900 data points presented as observed data in the pre-processing section were inverted using the algorithm described in Li and Oldenburg (1997). The earth was modeled as 38,000 cuboidal cells, each having a constant density. The cell dimensions were 25 metres in the central region of interest, and increased to 200 metres toward the edges and bottom. A minimum structure (or smoothest) model was specified for the model objective function, and a reference model of zero g/cc was assigned. Positivity was included as a further constraint because in this case, massive sulphides were known to be denser than the host rock. Data were assigned errors of 3% plus 0.01 mGal, and the inversion converged to the desired misfit.
Figures to the right compare the observed gravity and the predicted gravity data that were generated by the final model of this inversion. Move your mouse over the figure to see the predicted data. It is evident that a reasonable match was obtained.
The recovered model shown in the interpretation section provides the density of each cell in terms of the density contrast because the reference model was set to zero, and data were provided as anomaly values. The recovered density contrast had a maximum of 1.4 g/cc which is in accordance with the known densities of 4.5 g/cc for the massive sulphides and 2.8 g/cc for the host rocks.
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January 9, 2007
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