Muon geotomography has applications in mineral exploration in existing mines (brownfield) and in undeveloped sites (greenfield) as well as potential applications in block caving monitoring and coal resource estimation. CRM Geotomography Technologies (CRM) has successfully proven the technology by imaging multiple volcanogenic massive sulfide (VMS) deposits in North America, including at Nyrstar’s Myra Falls mine in British Columbia. We are also currently exploring the economic potential of other applications.
Brownfield Mineral Exploration
CRM has developed and built a suite of detectors focused on brownfield exploration. These detectors have a footprint of approximately 1 meter by 2 meters and are approximately 1.5 meters high. They are applicable in contexts where there are suitable access drifts. Our detectors have been developed to handle the harsh and unpredictable environment of a mine, and have been deployed for months at a time without human intervention. The brownfield work is currently focused on
- discovering deposits near existing mine operations, estimating the size, location and shape of dense mineralization;
- developing better targeted drilling programs; and
- helping mine geologists refine their geological models.
By judiciously placing the muon detectors in existing mine drifts, unexplored or poorly known regions of the mine geology can be imaged. We are able to determine whether or not there is an interesting geological structure that warrants further investigation, and we are able to develop a 3D model to guide planning for exploration drilling and to aid in estimating ore quantities. An example of a 3D model developed from field data taken at an existing mine is shown below. First is shown the unconstrained model from muon tomography data only. The model was then constrained with assay data from only fifteen randomly selected drillholes (out of the few hundred available in this field test). This field example clearly demonstrates that in brownfield scenarios
- muon tomography images correctly identify the location of mineralized rock
- the number and placement of detector locations can affect the ability to precisely resolve the full 3D geometry of the density anomaly; however, combining with drill data (or borehole or surface & airborne gravimetry data, if available), yields a compact model that corresponds very well with the known ore shell
Left: “blind” density model from muon tomography using only 4 detector locations; the known ore body is highlighted for comparison. Right: density model from constrained inversion of the same muon tomography data. The density model is much more compact and coincides well with the ore body. The muon detectors were placed in mine drifts below the ore body, indicated by the pink dots.
Greenfield Mineral Exploration
Greenfield applications of muon tomography are technically more challenging because of the aperture limitations for a borehole muon detector. Nevertheless greenfield applications show great promise as a geophysical technique. We have developed a realistic model of a borehole device and have simulated its capabilities, leveraging our expertise and experience from real world brownfield applications. One very important feature of borehole detectors is that they have the ability to image deposits from multiple depths underground, which provides a powerful handle to resolve 3D geometry (in contrast with brownfield where detectors can only be placed at fixed depths within existing mine drifts). In one example simulated scenario, a fence of muon detectors for PQ-size boreholes were situated in spacings of 400 m to image a VMS deposit with two lenses that had a density contrast of 0.7 and 1 gram per cubic centimeter, respectively. Within 3 months, the data from the detectors provided evidence of an ore body at a better than 99% confidence level. Combining the images yielded a 3D model that corresponds very well with the input VMS deposit, as shown in the animation below.
Comparison of model derived from muon tomography measurements to the simulated ore body. The location of the borehole muon detectors is indicated by the row of pink dots.
Caving is a large part of many modern underground mining operations. An important safety concern arises from air voids forming within the collapsing overburden. Such ‘hang ups’ can collapse suddenly and create strong, dangerous air blasts throughout the mine. An example of such a collapse is the event at Northparkes Mine on November 25, 1999 in which a void estimated at 4 million cubic meters suddenly collapsed, resulting in multiple fatalities (discussed further here, for example). Because of the large density contrast and volume of such hang ups, it is possible to discover and image such voids using muon tomography on a compressed timescale compared to mineral exploration applications. This timescale may be appropriate for monitoring for dangerous hang ups in block caving operations.
There are numerous other potential applications for muon tomography. Contact us if you have a case where muon tomography may be able to help.