Choosing the right magnetic field inversion method for your geological situation

February 19, 2015 | by Taronish Pithawala

When it comes to creating exploration models with magnetic field observations, the common approach has been to produce a susceptibility model from conventional inversion methods. This approach makes the fundamental assumption that the subsurface is magnetized solely by induction and in the direction of the Earth's inducing field. 

Geoscientists have now realized that this fundamental assumption is invalid more often than not. Remanent magnetization can be caused by several geological processes including: burial, iron oxidation, and even how long a body has been sitting in the present magnetic field. Furthermore, there are anisotropic effects that can alter the magnetization direction; for example highly magnetic iron ore bodies often have their magnetization direction aligned with their long axis – commonly down plunge or down dip. 

In some situations, where geophysicists are confident that magnetization is induced solely in the direction of the Earth's magnetic field, conventional susceptibility modelling places a very helpful constraint on the model – and yields satisfactory results. However, this is a rare case and 100% confidence is not guaranteed.

In other scenarios, making sense of the conventional susceptibility result often requires unavailable information (from drilling) to define the source and extent of remanent magnetization and anisotropy. In the absence of this information, reliable interpretation of the conventional susceptibility model is all but impossible. Geological insight is therefore hard to obtain from the magnetic field data as the spatial extent and structure of the anomaly is obscured by effects not accounted for by the modelling algorithm. Here, conventional susceptibility models are not recommended.

As an example, the Quadrilátero Ferrífero, (Iron Ore Quadrilateral) in Brazil hosts among the largest commercial iron deposits in the world. The region also lies at low latitude (inclination -29º, declination -21º) in the geomagnetic field and the vector of magnetization is rotated to align with the bedding of the iron formation. These factors prevent traditional susceptibility inversion from providing reliable results. However because MVI accounts for non-induced magnetization, the inversion result is more reliable and consistent with the true location and geometry of the iron formation. Shown above Quadrilátero Ferrífero: 3D perspective looking east.

Geosoft introduced Magnetization Vector Inversion (MVI), as part of its cloud-powered VOXI Earth Modelling platform, in 2012 to address this challenge. MVI is a method for solving the magnetization amplitude and direction regardless of the source and extent of remanent magnetization or anisotropy. The result is a model of the magnetic source bodies irrespective of how they were magnetized. The MVI method is widely applicable in all geological situations since it is not negatively affected by the processes that alter magnetization direction. It is the recommended inversion approach for magnetic field measurements.

Several applied examples have since been published demonstrating the use and benefit of MVI in regions of low magnetic latitude, such as South America; regions of known remanent magnetization and when modelling highly magnetic bodies.  MVI has also proven to be beneficial at regional scales or in data-poor environments where little is known about the nature of the magnetization.

MORE: Inversion of magnetic field data from the Arctic to the Andes

Validating or improving geological models with magnetic field data is an important part of creating a unified 3D exploration model to guide drill planning. This can only be obtained when there is consistency in the methods used and in the integration of the data.

The benefit of the Magnetization Vector Inversion technique, now proven in the field, is its ability to produce results that are consistent with more geological environments than conventional susceptibility modelling. 

The end result is an improved understanding of the subsurface from magnetic field data and a more reliable 3D exploration model.

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