Potential-field geophysical data inversion for 3D modelling and reserve estimation (Example of the Hajjar mine, Guemassa massif, Morocco): magnetic and gravity data case

Abstract

Geophysical data inversion is a tool, which can be used to recover the subsurface distribution of physical properties from field data. Each type of geophysical data can be inverted using one or more inversion algorithms. In this paper, a set of geophysical magnetic and gravity data of the Hajjar area in Morocco, covering an extent of $3.2\times 1.6~\mathrm{km}^{2}$, were used to make a 3D model of an orebody and to estimate the mineral reserve by potential-field geophysical data inversion and excess mass estimation. We thus promote the development and application of potential-field geophysical data inversion using the softwares Geosoft Oasis Montaj and Voxi Earth Modelling™ and the evaluation of its power compared to the excess mass estimation method. The process of inversion begins with data processing, then moves to analysis and interpretation, and ends with unconstrained Cartesian cut cell inversion. The results show a variation of $-$0.22 mGal to 1.59 mGal for the gravity residual anomaly map, leading to have density variations from $2.45~\mathrm{g}/\mathrm{cm}^{3}$ to $4.22~\mathrm{g}/\mathrm{cm}^{3}$, and a variation of $-$232 nT to 1018 nT for the reduced magnetic anomaly map.Moreover, data inversion allowed us to create a 3D model of the orebody and of the adjacent geological formation, and to estimate the different parameters that characterize the orebody derived from the inversion results, which have been confirmed from survey data: (depth $\approx 160$ m; maximum density $\approx 4.22~\mathrm{g}/\mathrm{cm}^{3}$; minimum density $\approx 3~\mathrm{g}/\mathrm{cm}^{3}$; mean density $\approx 3.61~\mathrm{g}/\mathrm{cm}^{3}$; thickness of the overburden $\approx $120 m; dip $\approx 45^{\circ }$; morphology ${\approx }$ lens; volume $\approx 4.8 \times 10^{6}~\mathrm{m}^{3}$).It was therefore possible to evaluate the reserve, and to validate the reliability of the inversion by having a root mean square error between the exploited reserve and the calculated reserve of 13.5%, i.e. an insignificant difference between the real and calculated magnetic and gravity orebody responses, which support the validity of the results.

Keywords

• Geophysical data inversion
• Cartesian cut cell inversion
• Density
• Susceptibility
• Hajjar mine