An integrated geological-geophysical approach to subsurface interface reconstruction of muon tomography measurements in high alpine regions

Muon tomography is an imaging technique that emerged in the last decades. The principal concept is similar to X-ray tomography, where one determines the spatial distribution of material densities by means of penetrating photons. It differs from this well-known technology only by the type of particle. Muons are continuously produced in the Earth’s atmosphere when primary cosmic rays (mostly protons) interact with the atmosphere’s molecules. Depending on their energies these muons can penetrate materials up to several hundreds of metres (or even kilometres). Consequently, they have been used for the imaging of larger objects, including large geological objects such as volcanoes, caves and fault systems. This research project aimed at applying this technology to an alpine glacier in Central Switzerland to determine its bedrock geometry, and if possible, to gain information on the bedrock erosion mechanism. To this end, two major experimental studies have been conducted with the aim to reconstruct bedrock geometries of two different glaciers. Given this framework, I present in this thesis my contribution to the project in which I worked for 5 years.
Most of the technological know-how of muon tomography still lies within physics institutes who were the key drivers in the development of this method. As the geophysical/geological community is nowadays an important user of this technology, it is important that also non-physicists familiarise themselves with the theory and concepts behind muon tomography. This can be seen as an effective method to bring more geoscientists to utilize this new technology for their own research. The first part of this thesis is designed to tackle this problem with a review article on the principles of muon tomography and a guide to best practice. A second important aspect is the reconstruction of the bedrock topography given muon flux measurements at various locations. Many to-date reconstruction algorithms include supplementary geological information such as density and/or compositional measurements only on the side. A probabilistic framework was successfully set up that allows for such additional data to be included into the inversion. This may be used to better constrain the bedrock geometry. Moreover, this flexible framework allows also for the inclusion of modelling errors in the physical models which may result in a more reliable estimate of the mean and standard deviation of the bedrock position. The third article is concerned with the determination of the effect of rock composition on the muon flux measurements. Researchers in the community use a made-up rock, called “standard-rock” in their calculations. Hitherto, it was unclear in which geological settings this is a valid assumption and in which the induced error becomes too large. Simulations that use this fantasy rock are performed and compared to simulations that use a more realistic rock model. It was found that for felsic rocks the standard-rock approximation is valid over all thickness ranges, while for mafic rocks and limestones this can lead to a serious bias if the rock is thicker than 300m.

2021

dissertation

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