Reducing risk with geophysical modelling: the advantages of Magnetization Vector Inversion

By Taronish Pithawala and Darren Andrews | December 10, 2013

As the ability to see deeper undercover becomes essential in exploration - sophisticated tools are emerging that allow geoscientists to visualize what’s hidden more quickly and accurately. One of those tools is Geosoft’s Magnetization Vector Inversion (MVI), part of Geosoft’s VOXI geophysical modelling software-as-a-service suite.

MVI allows the magnetization direction to vary within the model and thus take into account the combined effects of remanence, demagnetization, anisotropy and induced magnetization.

The decision to run geophysical inversions, as part of early exploration and target delineation, has become an easier one with the emergence of cloud-powered inversion applications, such as Geosoft VOXI Earth Modelling, that enable geophysicists to generate 3D inversion models faster and more efficiently.

Having overcome the hurdle of computational speed to allow collaboration, improving model accuracy is the next evolutionary step in improving geophysical inversion modelling. MVI is a technique introduced by Geosoft in 2012 which has proven to be helpful in eliminating erroneous assumptions about magnetization. After a year of application in exploration programs, there are a number of project examples, and customer validation, which confirm that MVI results provide a more reliable representation of subsurface geology than traditional susceptibility inversion.

Getting a more accurate picture of subsurface geology

Traditional inversion of magnetic data, or susceptibility inversion, assumes that the magnetization of the local geology runs parallel to the Earth’s magnetic field. It's a risky assumption because there are several geological processes that affect magnetization, such as deformation and alteration. Not taking into account the possible effects of such processes can result in misleading model outcomes.

The MVI technique solves for both the magnitude and direction of the rock's magnetization without requiring any prior knowledge of either parameter, and in the process it accounts for the effects described above. As a result, MVI provides a more accurate picture of subsurface geology than traditional susceptibility inversion and therefore helps the user to better define geological structure and potential drill targets.

Eliminating distortion in low magnetic latitudes

The MVI technique is especially powerful in regions that lie in low magnetic field latitudes, or where there may be non-induced magnetization caused by remanence, demagnetization or magnetic anisotropy. In such areas traditional susceptibility inversions can be severely distorted.

For example, the 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.

Quadrilátero Ferrífero: 3D perspective looking east.

Finding hidden mineralization

Another good example of the value MVI can provide was demonstrated with exploration data from the Osborne mine in Queensland, Australia. Osborne is an ironstone-hosted, replacement-type copper-gold deposit that was discovered in 1989 beneath 30-50 m of deeply weathered cover. Subsequent exploration delineated high-grade primary mineralization dipping steeply east to a vertical depth of about 1100 m.

In 1997 Placer Dome (now Barrick Gold) flew a TMI survey over the area at a 40 m clearance with a 40 m line spacing. To determine if MVI held any advantage over traditional susceptibility inversion in finding new ore at depth, Geosoft ran the historic TMI data through both inversions.

While the susceptibly inversion failed to detect the eastern extension of the mineralization, the MVI showed this extension clearly. The ability to account for demagnetization, magnetic anisotropy, or remanent magnetization made the difference between seeing the mineralized extension or not.

Comparing MVI with susceptibility inversion: Notice that the conventional susceptibility inversion completely fails to support the eastward dipping extension.

For early stage projects, using MVI can help explorers delineate prospective areas and have more confidence in planning drilling programs to test targets, in turn saving time and money. The insight gained by running this quick and intuitive inversion technique is invaluable, especially in the presence of complex magnetic environments.

Interpreting with confidence

In a susceptibility inversion, the value of the outcome depends on the experience of the interpreter in recognizing and accounting for the potential complexities arising from non-induced magnetization. MVI, on the other hand, is much more accessible to a less experienced geophysicist because it accounts for all magnetization, regardless of the type, hence the user can have more confidence in the outcome.

From a geologist’s perspective, MVI is more useful because it takes into account complexity arising from geological processes such as deformation and alteration. As exploration becomes deeper and more complex, having access to an inversion technique that accounts for this complexity is critical to success.

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