Magnetization Vector Inversion (MVI) is a modern technique which is gaining acceptance as an effective tool for subsurface exploration in areas where magnetization does not necessarily run parallel to the earth’s magnetic field, a more common scenario than geoscientists have traditionally appreciated.
The global applicability of the technique to target delineation was demonstrated by Michael Webb, principal of Perth-based Blue Sky Geoscience at a recent ASEG Geophysical Inversion workshop, where he, in collaboration with Rob Ellis, Principal Earth Modelling Scientist with Geosoft, presented inversion results from magnetic surveys on three blind greenfield exploration projects operated by Anglo American: a Cu-Ni-PGE project in Finland; IOCG and mafic intrusive Ni-Cu targets near Cooper Pedy in South Australia; and a copper porphyry project in northern Chile.
MVI, a feature of Geosoft’s VOXI Earth Modelling, provided useful insights for target delineation on all three projects. Unlike susceptibility inversion, the technique allows for magnetization effects that are not related to induced magnetism. These include magnetic remanence, self demagnetization and magnetic anisotropy resulting from geological processes such as local tectonics and alteration. As a result, MVI often provides a more reliable picture of subsurface geology.
Magnetic ignimbrite cover in Chile
The copper porphyry project lies under cover along the Domeyco fault system in northern Chile, home to some of the largest copper porphyry deposits in the world, including Escondida and Collahausi. This low magnetic latitude project has a relatively flat magnetic field but the prospective rocks are covered by a weakly magnetic ignimbrite. To complicate matters further, gullies (quebradas) dissect the ignimbrite and correlate with the linear magnetic highs outlined by the draped helimag survey flown over the area.
In fact, the quebrada response dominates the TMI image making interpretation challenging. One approach in this situation is to compute the response from the quebradas using VOXI with its Cartesian Cut Cell method which provides a highly accurate TMI terrain response. Following this approach, Webb found that the short wavelength magnetic anomalies associated with the topography could be explained using a dissected surface layer of constant susceptibility. Webb then subtracted the results of the forward model from the observed data to generate residual data for both MVI and susceptibility inversions. The two methods gave very different outcomes.
“There was very little overlap,” says Webb “If you were to drill a hole into the susceptibility anomaly, you could be drilling a kilometre away from the MVI anomaly.” The company can now decide to either do more modelling or test the inversion results.
Magnetic Remanence in South Australia
Cooper Pedy in southern Australia is best known for its opal deposits, but Anglo American was interested in the area’s potential for both IOCG and mafic intrusive nickel deposits.
The company drilled a large negative magnetic anomaly under 200m of barren cover, hoping to intersect mafic rocks. Instead, what was pulled from 300 metres downhole was a gneissic rock with strong magnetic remanence (testing by CSIRO returned an average Koenigsberger ratio of 600). The main remanence carrier was titaniferous hematite. Because of the strong magnetic remanence, susceptibility inversion provided a completely nonsensical model. “The MVI, on the other hand, provided a realistic result”, says Webb.
Remanence in Northern Finland?
The Cu-Ni-PGE project is located north of the Arctic Circle in Finland. Although the area was devoid of outcrop, the application of a combination of traditional exploration techniques (i.e. base of till sampling and VTEM) and in house geophysical technology (SQUID EM) successfully identified subsurface anomalies. Subsequent drilling outlined Cu-Ni-PGE sulphide mineralization hosted predominantly by a serpentinized ultramafic olivine cumulate rock.
When Webb ran unconstrained models using both magnetic susceptibility inversion and MVI, the two methods produced different interpretations of the plunge and extent of the mineralization. When he took the modelling a bit further by constraining the MVI using magnetic susceptibility measurements from the core, the MVI fitted the exploration model, but the amplitude component implied that the magnetic intrusive was more extensive than currently defined.
Webb concluded that if MVI had been used for modelling in the early stages of exploration, the drilling priorities may have been somewhat different. He has recommended measuring the magnetic remanence of the core to determine if it could account for the difference.
Overall, Webb finds VOXI to be a flexible software service that can be used for both forward modelling as well as unconstrained and constrained inversion.
“The MVI approach in areas with obvious magnetic remanence and at low magnetic latitudes is a great tool that is producing geologically reasonable targets that can be tested by further modelling or drilling” he says. “And the software’s ease of use is a bonus.”