Seeing the shades of grey in 3D inversion
By Virginia Heffernan | June 4, 2012
Using 3D models of the subsurface derived from geophysical data is standard practise for guiding exploration in the petroleum industry and has the potential to improve success rates for mineral explorers.
But because geophysical inversion models can provide the same response for different geological scenarios, they can introduce ambiguity into the resulting interpretation. The latest thinking, and technology, has focused on the key factors required to generate 3D models with greater ease and confidence, making their output more reliable and informative as an aid for mineral exploration.
Topping the list of requirements is the ability to integrate as many constraints as possible - including gravity and magnetic data, surface geology and borehole logs. The resulting complete earth model is more likely to pick up nuances in physical rock properties and provide a truer rendering of what lies beneath the surface.
“We’ve got to start thinking about the shades of grey,” says Dr. Bill Morris, a 3D modelling expert and professor at McMaster University’s School of Geography and Earth Sciences . With direct detection of ore bodies becoming increasingly rare, geoscientists must “get past the bull’s eye approach and, instead, put all our little clues together to get the big clue,” he says. “We need to have more physical property databases, and we need to be able to link physical property variations to geological reality.”
Eventually, as the number of constraints in inversion models increases, geoscientists will be able to map regions of alteration in proximity to orebodies,Morris and his co-authors predict in a recent paper entitled Integrating geological constraints in geophysical models.
From the technology side, the ability to rapidly create, modify, iterate and combine data within project timelines is essential for making geophysical modelling a more practical and reliable aid for exploration.
The challenge has been developing lighter workflows for what are currently resource and time intensive modelling algorithms. While 3D inversions using voxel earth models have been available for years, it takes highly trained specialists with powerful computing capacity to produce them. And the associated workflow required to define and introduce constraints is time-consuming.
The newest inversion modelling technology, introduced by Geosoft in April, was developed under the lead of modelling expert Dr. Robert Ellis, a co-founder of the UBC Geophysical Inversion Facility, and relied on input from industry collaborators to address these challenges. Usability and “lighter workflows” were the primary drivers in the development of VOXI , which also harnesses cloud computing to allow geophysicists to work with ever-larger models.
In a recent post in the Exploring with Data blog , Chief Technologist Ian MacLeod tells the development story behind the cloud-based VOXI Earth Modelling service that took over three years and required the work of 29 people to create. VOXI provides tools for making interpretation of 3D inversion models faster,more accurate and accessible to a broader range of explorers, and includes a new Magnetization Vector Inversion (MVI) technique.
The service allows geoscientists to convert magnetic and gravity data directly into 3D models that can be integrated with other project data. Better yet,they can do this in the cloud using Microsoft’s Azure cloud computing platform rather than relying on the limited processing power of their own computers.“Building a multi-core cloud-based algorithm is very different from building a program that runs on a workstation or local cluster,” says MacLeod. “The effort to re-engineer VOXI for Azure took us most of a year to get right.”
Making the software usable is a challenge in its own right, notes MacLeod in his post. “We worked very hard with our collaborators over two years to design and improve the VOXI interface,” he says, “so that everything would work as smoothly as possible and fit within an explorer's natural workflow.”
Speed in generating 3D inversions is one of the essential ingredients that will allow geoscientists to use these tools routinely, enabling them to iteratively improve models as they add constraints and learn more about their projects.
One early adopter says a modeling exercise that took him four hours to complete using his current desktop modelling program, took just two minutes using the VOXI service.
The future of mineral exploration using 3D inversion models looks even brighter under the lens of better and more plentiful data sets that can be used to constrain inversions, especially borehole geophysical data.
“We have a whole suite of new tools that are giving us information that we have never received before,” say Morris. “The other big key is that we are now seeing integrated geological and geophysical model development platforms.”
This will allow geoscientists to move more comfortably between the three main geophysical models (discrete body, lithologic surface and voxel mesh inversion), incorporating elements of each one into a fully constrained inversion.
“The systematic use and development of 3D models for each mining camp will certainly lead to the discovery of new resources in many of the older mining camps,” he predicts.
[Originally published in Earth Explorer Magazine]