The people, science and technology behind discovery

3D EM Surveys promise to reduce drilling costs

by Virginia Heffernan on MARCH 8, 2013 technology

Field technician setting up one DC/IP and MT survey station.

"The mining industry has traditionally focused on drilling, but high-density omni-directional 3D surveys can reduce drilling costs by focusing drill targets."

The growing ease with which geophysical data are collected, inverted and visualized in three dimensions stands to change the way mining companies conduct their exploration programs.

By densely sampling the subsurface in all directions, rather than interpolating 3-D images from 2-dimensional information, explorers are able to target their drill-holes  more accurately, cutting down on the cost and time required to investigate areas with limited data.

“The mining industry has traditionally focused on drilling,” says Kevin Killin, vice-president of interpretation for Quantec Geoscience, “but high-density omni-directional 3D surveys can reduce drilling costs by focusing drill targets.”

More accurate mapping of the subsurface has become an even stronger imperative as explorers look for smaller and ever more complex ore bodies to replace larger and shallower deposits that have already been discovered and mined.

While sophisticated 3D survey technology such as Quantec’s Orion 3D can cost 50-70% more than traditional 2D surveys, the greater chance of drilling success justifies the cost, Killin says.

The high-performance computer resources, skills and talent necessary to interpret the resulting data are not insignificant. Quantec has amassed a team of 14 geophysicists with post-graduate credentials to help both junior and large companies fully utilize the data collected on their properties.

In late 2011, for instance, Nevada Sunrise Gold ran an Orion 3D geophysical survey over an eight square kilometres section of its Golden Arrow gold project in Nevada in hopes of finding new pockets of gold mineralization to bolster existing resources.

Golden Arrow lies along the northeastern margin of the Walker Lane structural zone, a past producer with current resources exceeding 30 million ounces of gold and 400 million ounces of silver. Intrusive-hosted gold and silver deposits at Golden Arrow represent both low-sulphidation and high-sulphidation type mineralization.  Historic production on the property came from discrete high-grade ore shoots within quartz-adularia-gold veins, but more recent exploration has focused on defining disseminated gold mineralization in two zones known as Gold Coin and Hidden Hill.

[Click to enlarge]

3D Voxel models of resistivity and chargeability in the Hidden Hill region created in Geosoft Oasis montaj using the Orion 3D dataset. Since data is highly sampled in all directions, the models were created using 3D gridding techniques instead of interpolating between 2D inline sections.

Quantec acquired and inverted the 3D data from the Orion 3D survey at Golden Arrow and combined the results with historical lithology, assay data, and ongoing exploratory data. By delineating zones of high resistivity (the host intrusive) and high chargeability (both mineral and host associated), the survey identified new zones of mineralization and confirmed the previously defined Hidden Hill deposit.

Of the 21 reverse-circulation holes drilled in the new target areas, 18 intersected widespread mineralization assaying above 0.2 gram gold per tonne. The survey also changed the geological understanding of the deposit. The “basement” rock was previously assumed to be andesite, and all the historical holes were stopped in this lithology.   The Orion 3D system proved that the potential host rock is under the perceived basement.

Traditional 2D survey layouts use discrete lines, generally spaced an equal distance apart, to measure geophysical signals originating in the subsurface. In contrast, 3D surveys such as Orion use an array layout that collects uniform and omnidirectional information, resulting in a detailed volumetric representation of the subsurface that can optimize drilling success.

“If the deposit is 50 to 100 metres wide and your survey line spacing is 200 to 300 metres, you may not intercept that body on any line,” says Killin, noting the potential inaccuracy and serendipitous nature of the traditional 2D layout. “And if you extrapolate the information you have, it may not show the accurate location and geometry of the body unless you happen to run right over it. Moreover, if you have a body that is close enough to the line to give a response, it will spuriously appear as if it is lying directly underneath that line while it may in fact be so far offset that you will miss it with your drill.”

[Click to enlarge]

Clockwise from top-left; Aerial view of 3D survey layout (bounded by purple rectangle); Oblique view showing survey stations at the surface (yellow and red) and subsurface samples collected by Orion 3D loggers (green). The grey surface indicates depth of investigation for this particular survey; Cross-sectional view of subsurface sampling density.

On the other hand, a typical layout using Orion 3D would collect a couple of hundred thousand samples, resulting in a high-quality dataset that can then be run through a powerful imaging platform to generate and visualize complex subsurface structure with greater certainty.

Quantec uses Geosoft’s Target and Oasis montaj to manage, visualize and share the Orion 3D dataset, incorporating drill hole geology and assay data to increase confidence in the inversion. Data subdivision, which can be a resource intensive exercise, is rendered unnecessary because the internal memory management solution in Oasis montaj can handle very large volumes of data.

Killin says this level of mass computation allows voxel models to be run directly from the dataset, elucidating structurally complex targets and deposits in a 3D software environment that is intuitive and easy to use.

Although Orion 3D is only two years old, it is already generating a lot of interest from the exploration community because it can be customized for each project. “We can modify the layout of the survey for deeper depths, shallow depths, odd-shaped properties, and fine structures such as gold mineralization, or adapt it to a larger area to look for porphyry-type signatures,” says Killin.

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