The high-energy primary cosmic ray-induced muons are one of the most penetrating forms of naturally occurring radiation on Earth. In skilled hands muons can be harnessed to provide information on the subterranean deposits and structures. Let’s take a look how this is done.
Muons and muon radiation
Nearly all radiation is stopped already in a just few tens of centimetres of any soil. As the underlying rock formations are made of even denser materials, common types of radiations do not penetrate them effectively. However, natural muon radiation is an exception.
Muons are highly penetrative charged particles created in the upper layer of the earth’s atmosphere as a result of the interaction of primary cosmic radiation – mainly high-energy protons – and atmospheric nuclei. The muon flux on ground is well known and almost constant. Muons that form one of the major components of secondary cosmic radiation are similar to electrons but 207 times heavier. Thus, the high-energy muons penetrate matter far more deeply than electrons. In fact, some of the highest energy muons can get through even kilometres of rock.
Another consequence of heaviness of muons is that they suffer very little scattering while penetrating through material. Hence the muons that pass through soils and rocks have very straight paths. The high penetration capability and low amount of scattering of muons ultimately means that they are extremely well suited for tracking providing reliable angular information. The muon absorption rate in the rock is utterly independent of the chemical composition of the rock, and directly measures the density length (density x path length).
Muography in geological applications
Like all cosmic rays, secondary muon radiation is also directional, that is, all detected muons come from the sky. Our muon detection system of one or more probes measures both the path lines of muons in soils and rocks and the number of muons that pass through the probes per time unit. In a nutshell, we are able to extract average density images from the rock and soil columns the detected muons have passed through with different angles. This is carried out by combining the angular information of muon paths and the measured muon flux at the given depth.
The muon absorption in the soil and rock overburden can be measured if the muon detector is placed in an underground tunnel or a drill hole. Muons can then be used to search for new ore deposits and unexplored natural caves, forgotten mine tunnels or other geological structures by making observations of density variations in the geological realm.
In most cases, the imaging and interpretation challenges are greater in solid geological bodies than those in rocks containing open spaces. This is due to the fact that the relative density gradients in the former are generally less drastic in character than in the latter. Further complexities are met with the need to squeeze muon-detection technology in probes small enough to fit into the standard drill holes. Muon Solutions is developing their own branch of innovative and novel technologies to get through all such technical obstacles.
Measuring muon flux in a given depth is a powerful tool to get information on the media the muons interact with. In geological applications muography (aka muon radiography and muon radiography) can be used to examine the interior of large-scale geological bodies and structures in the similar way as standard X-ray radiography is used to image the interior of smaller objects. However, while the X-rays are applicable only to objects of up to 1 m in thickness, muography visualises density distributions inside much larger objects (even in kilometre scale).
Muography has the capability to derive an areal density along the measured muon paths. The areal density profile images extracted using muography can be examined as horizontal density maps or, in certain circumstances, 3D muon tomographic images. In the latter case the density data can be imported to third party 3D software, such as Surpac or Leapfrog. If our muon detectors measure continuously, a time-lapse 3D (i.e. 4D) density tomography imaging is possible. This method can be used to detect temporal variations in relative densities in rock realm. Fault structures with underground recharge capacity for ground water could be one possible target for this approach. However, these are just some examples and the number of possible applications depends solely on imagination.
In summary, muography is an entirely non-destructive and passive imaging technique that uses the perpetually present background secondary cosmic ray radiation as the energy source for probing the interior of geological objects and structures in the surface of our planet. The measured muon flux at different depths in tunnels or vertical, inclined or horizontal drill holes determines the attenuation of muon flux and thus the density differences between different stratigraphic units or other geological bodies, or the fluids contained within these units.