Abstract
Conventional 3D magnetic inversion methods are based on the assumption that there is no remanent magnetization, and the inversion is run for magnetic susceptibility only. This approach is well-suited to targeting mineralization; however, it ignores the situation where the direction of magnetization of the rock formations is different from the direction of the induced magnetic field. We present a novel method of recovering a spatial distribution of magnetization vector within the rock formation based on joint inversion of airborne gravity gradiometry (AGG) and total magnetic intensity (TMI) data for a shared earth model. Increasing the number of inversion parameters (the scalar components of magnetization vector) results in a higher degree of non-uniqueness of the inverse problem. This increase of non-uniqueness rate can be remedied by joint inversion based on (1) Gramian constraints or (2) joint focusing stabilizers. The Gramian constraints enforce shared earth structure through a correlation of the model gradients. The joint focusing stabilizers also enforce the structural similarity and are implemented using minimum support or minimum gradient support approaches. Both novel approaches are applied to the interpretation of the airborne data collected over the Thunderbird V-Ti-Fe deposit in Ontario, Canada. By combining the complementary AGG and TMI data, we generate jointly inverted shared earth models that provide a congruent image of the rock formations hosting the mineral deposit.
Highlights
In mineral exploration, magnetic data have traditionally been inverted to produce magnetic susceptibility models, which represent magnetization induced by the current magnetic field
Magnetic data contain information about the remanent magnetization of rock formations that are often ignored when inverting for magnetic susceptibility only
We have introduced two novel methods of joint inversion of potential field data in the presence of remanent magnetization: (1) joint inversion subject to the Gramian constraint and (2) joint inversion subject to the joint focusing constraint
Summary
Magnetic data have traditionally been inverted to produce magnetic susceptibility models, which represent magnetization induced by the current magnetic field. The joint focusing stabilizers force the anomalies of different physical properties to either overlap or experience a rapid change in the same areas, enforcing the structural correlation To illustrate both of these approaches, we present a case study of the joint inversion of the AGG and TMI data, which were collected over the Thunderbird V-Ti-Fe deposit in the Ring of Fire area of Ontario, Canada. We compare and contrast the 3D density, magnetic susceptibility, and magnetization vector models of the Thunderbird V-Ti-Fe deposit obtained from standalone, Gramian, and joint-focused inversions Application of both joint inversion methodologies to these data has resulted in 3D models of subsurface formations that have sharper geospatial boundaries and stronger structural correlations than the standalone inverted models. The joint focusing approach provided similar results but was significantly faster and easier to implement
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