Muon Tomography in Geophysics
IDPASC Portugal - PHD Programme 2017
2017 / 2018
Experimental Particle Physics | Astrophysics
Universidade de Évora
Laboratório de Instrumentação e Física Experimental de Partículas
Muons are deeply penetrating particles which are naturally present at the Earth's surface, as a consequence of the interaction of cosmic rays with the atmosphere. By measuring the attenuation of the open air flux through different directions, they be used to radiograph, or better said, muograph the interior of large volumes of material, like geological structures like volcanoes or mines to archaeological sites. The existing techniques so far, include gravimetric, magnetic, seismic, electric and electromagnetic methods, or simply drilling, however each technique has limited sensitivity or spatial resolution. In general, muons will allow mapping at deeper levels. Currently, gravimetry is the general-purpose method for density mapping and provides also information of density contrast from measurements of the vertical component of the local gravity field. Similarly to muography (transmission muon tomography), it is linearly linked to the density of the material, but their spatial resolution and sensitivity is different. Muon telescopes can be placed in existing tunnels to observe the muon flux for brownfield mining applications, and the muon tomography images correctly identify the location of mineralized rock. The enhanced 3D density algorithm combines the two or more sensors. or different points of measurements, into a 3D image by optimizing inversion process. The greatest advantage of muography is its high spatial resolution compared with other geophysical methods - in particular with the gravimetry. As gravimetry alike, Inversion of muon data is also affected by non-uniqueness. In fact, the number of muon trajectories may not be enough to the resolution of small scale geological density models. Since both muon tomography and gravimetry are geophysical methods that provide information on the density structure of the Earth's subsurface, our approach to imaging a density distribution is to invert gravity and muon data jointly. Additionally, the resolution in deeper regions not sampled by muon tomography (not possible due to geometric constraints or excessive depth of the targets) will be significantly improved by joining the two techniques. Therefore, there are 3 strategies that will be pursued 1) imaging with muons; 2) muons as input a priori data for conventional inversion of gravity data; 3) imaging with gravity and muon data jointly. The student will develop and implement the theoretical and computational methodologies for inverting the results of muon surveys and for the joint inversion of muon and gravity surveys. These will be tested by simulation of the muon propagation through synthetic 3D models, before applying it to real muon data. The final performance of the methodology will be assessed by comparing the reconstructed density distributions with the preexisting information.