Muon Geotomography In a Nutshell

Muons are a type of elementary particle [1]  that are essentially a heavier version of the electron, and are naturally abundant from cosmic rays [2]. High energy muons, which travel at nearly the speed of light, are virtually unaffected by the earth’s magnetic field or by temperature and pressure variations in the atmosphere. Therefore the muon flux at the surface of the earth is very nearly uniform, both spatially and temporally.

Because muons are so massive, they penetrate matter much more effectively than other charged particles. The rate at which they lose energy depends on the amount of matter traversed by the muons, and to a lesser degree, on the nuclear properties of the medium they pass through.

This means that muons are stopped quicker in denser material. Thus, by measuring the relative rate of muons that penetrate to some depth underground along a particular direction, one can directly measure the average density towards the surface in that direction.

This is important because density is an important geophysical property that can be associated with many high-valued geological structures, such as various types of mineral deposits or coal seams.

Given our knowledge of muon properties, we can develop a highly precise model for the expected flux of muons along each direction at an underground detection location. If we count the muons that are incident on a detector from each direction, and compare to the expected number of muons, we can determine the location of density anomalies within the field of view of the detector.

Dense regions reduce the muon flux from a particular direction for each detector.

In this way, muon tomography works very much like an X-ray of the earth, where instead of using artificially produced radiation sources, we use highly penetrating, naturally abundant muons.

Model Building

In much the same way that a CT scan constructs a 3D model of an object from multiple 2D X-ray images, we can combine the information from multiple detector locations to build a 3D model of geological structures within the surface of the earth. Furthermore, this information can be combined with geological context and other geophysical and geological data such as gravimetry measurements and drill core assays for geological interpretation.

Applications

This technique has obvious applications in brownfield and greenfield mineral exploration. In fact, any geological structure with a sufficient density contrast to the surrounding material may be amenable to this technique. More information on existing and potential applications can be found here.

Because this technology has the potential to detect and image deposits at depth, it will refine the exploration search area which will reduce the amount of expensive drilling required and further efforts to minimize environmental impact.

– Rick Sawyer, Geologist, Nyrstar

Development

CRM Geotomography Technologies is a world leader in exploiting cosmic ray muons for mineral exploration. We have developed and built robust, field-deployable muon detectors, and have tested them successfully in multiple mines. More about the history of this technology, and the progress made by CRM Geotomography Technologies may be found here.