Inversion of borehole magnetic data for prospecting deep-buried minerals in areas with near-surface magnetic distortions: a case study from the Daye iron-ore deposit in Hubei, central China

Significant near-surface magnetic distortions and weakly magnetic responses complicate the interpretation of surface magnetic data and compound difficulties associated with the prospecting of deepburied orebodies in old mines, such as the Daye iron-ore deposit. To address this problem, we used three-component borehole magnetic measurements from multiple drillholes to evaluate a magnetisation inversion approach that incorporates these data to obtain magnetisation intensity distributions. The magnetisation direction was estimated from rock and ore sample magnetic property measurements. Linear optimisation problem equations were solved using the pre-conditioned conjugate gradient algorithm. Tests on two-dimensional synthetic data returned favourable results. In the case study of the Daye iron-ore deposit, several concealed magnetite orebodies were discovered using three-component borehole magnetic measurements. Two-dimensional magnetisation inversion for borehole magnetic data indicated that magnetic sources were located at the contact zones between the diorite and marble rocks, as verified by other drillings. A three-component borehole magnetic survey shows advantages of moving away from shallow interferences and containing important additional information compared with a surface total field and provides a feasible approach to explore deep-buried and concealed minerals in areas with strong near-surface distortions.