How geophysical inversion boosts confidence at every exploration stage

By Virginia Heffernan | December 10, 2013

From validating geology on grassroots projects to finding new ore around existing mines, geophysical inversion is taking some of the risk out of high stakes mineral exploration.

Mineral explorers have traditionally ruled out running inversions in all but the latest stages of exploration because they are complex and time consuming.  Recent technology developments, however,  have turned inversion into a fast and responsive tool that can be used at every stage to delineate targets with greater speed and accuracy. 

Stage 1: Project Generation

Until the 1960s most deposits were discovered by prospecting, or looking for clues on the surface with the help of outcropping geology. But geophysical techniques are increasingly taking the role of the prospector as exploration moves deeper into the subsurface. 

In most cases, especially in countries with robust government databases, airborne geophysical information is available to help locate a prospective area. Until recently, however, many explorationists wouldn’t have considered running an inversion at such an early stage because of the complexities in setting up the inversion parameters, accessing the appropriate computing power and the inability to iterate inversion results to support time-critical exploration decision making.

Cloud-powered applications such as Geosoft’s VOXI Earth Modelling have resolved these technical challenges, making large, multi-parameter geophysical inversion modelling faster, more responsive and effective as a tool to assist with project generation.  A challenge that remains for many explorers is understanding how inversion benefits exploration at the different stages and knowing what to expect from results.

For project generation, 3D inversion of gravity or magnetic data can provide clues to what is happening in the subsurface even if there is no outcrop to be found.

As exploration moves ever deeper into the subsurface, the next generation of prospect selection will ultimately be driven by just this kind of predictive model, according to Rio Tinto’s head of exploration Stephen McIntosh who presented on technology’s potential to improve mineral discovery success at the 2013 ASEG-PESA International Geophysical Conference in Melbourne.

Stage 2: Prospect Targeting

The more information gathered and integrated into the conceptual model, the more detailed and reliable that model will be. Explorers can manipulate and interrogate 3D inversion models from any perspective to pick up on subtleties in the data they might have missed had they been confined to a 2D picture.

“By converting plan maps of field data to physical property solids, you make them much easier to integrate with geology,” says Bob Ellis of Ellis Geophysical Consulting Inc., who is routinely running 3D inversions for his clients using Geosoft VOXI.  “The explorationist is more inclined to use the information in that form than as an image of the data.”

If drill data are available, 3D inversion can be especially helpful in delineating new targets. For example, Aben Resources’ goal is to find more gold zones on their Justin project in the Yukon that are similar in nature to "POW", a mineralized zone delineated by airborne magnetics and subsequently confirmed by drilling.

Using VOXI’s iterative reweighting inversion focusing (IRIF), the company created a 3D model of the magnetic data and compared it to the wireframe model derived from drilling results from POW. The two matched, giving Aben confidence that drilling similar targets on the property would yield new mineralization.

Iterative Reweighting Inversion (IRI) focused susceptibility model in blue; isosurface value = 0.04 SI. Geophysical model correlates to known geology, with larger susceptibility contrasts (>0.04 SI) contained within the hornfels and skarn units.

“I was really impressed with the VOXI IRIF technique and how well it fit with the geology we actually see,” says Mike McCuaig, a geologist for Aben’s consultant, Terralogic Exploration. “It just gives you way more confidence that your modelling is realistic.”

Stage 3: Resource Definition and Expansion

As a project advances and even more information becomes available, inversions provide an opportunity to detect subtleties in the data that might have been overlooked otherwise.

At Gold Resource’s El Aguila gold-silver mine in southeastern Mexico, for example, where magnetic resonance can distort traditional magnetic inversions, a VOXI Magnetization Vector Inversion (MVI) of airborne magnetic data showed good correlation with the underlying geology and identified new targets  with similar properties to the mineralization at La Arista, a producing mine just southeast of the El Aguila mine.

At La Arista, structurally controlled mineralization consists of quartz veins and stockwork hosted in Tertiary rhyolite intrusive, breccia, andesite, tuff, and agglomerate. In addition to the epithermal veins, Cretaceous sedimentary units below or adjacent to the mineralized host are targets for base metal skarn mineralization.

“In this case, we were looking for lithologies that occur near known deposits and low amplitude magnetic sources that would be consistent with skarn mineralization in the carbonate part of the sedimentary sections,” says Ellis, the geophysical consultant who identified susceptibility contrasts associated with skarn mineralization when he ran an MVI on the airborne data from the project. “The inversion modeling gives you the source geometry and depth to these targets so you can further evaluate whether they fit your targeting model.”

Another example of the value MVI can provide comes from the Osborne mine in Queensland, Australia. Osborne is an ironstone-hosted, replacement-type copper-gold deposit discovered in 1989 beneath 30-50 m of deeply weathered cover. Subsequent exploration delineated high-grade primary mineralization dipping steeply east.

To determine if MVI held any advantage over traditional susceptibility inversion in detecting the eastern extension, Geosoft ran historic aeromagnetic survey data for Osborne through both inversions.

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

A comparison of magnetization vector inversion  with conventional susceptibility inversion, in the left and right panels respectively. The conventional susceptibility inversion completely fails to support for the eastward dipping extension.

Whether generating new prospects or finding new ore around existing operations, inversion can save time and money by providing a picture of the subsurface and more reliable targets for follow up and drilling.

“Inversion helps to prove a geological concept you can then take with you to another area that appears to have the same geological setting and do so with confidence,” says Geosoft’s Darren Andrews. “It’s a valuable tool for the geologist to have at every stage of exploration from grassroots to resource definition.”

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