Monitoring temporal opacity fluctuations of large structures with muon tomography : a calibration experiment using a water tower tank

TL;DR

This study evaluates the potential of muon tomography for monitoring temporal density changes in large structures, using a water tower as a controlled experiment to understand atmospheric effects and statistical limitations.

Contribution

It provides a calibration framework and correction methods for muon radiography, clarifying its time resolution limits based on object opacity and atmospheric variability.

Findings

Optimal time resolution for objects around 50 m water equivalent.

Atmospheric fluctuations significantly affect muon flux measurements.

Thicker objects like volcanoes have more stable time resolution.

Abstract

Usage of secondary cosmic muons to image the geological structures density distribution significantly developed during the past ten years. Recent applications demonstrate the method interest to monitor magma ascent and volcanic gas movements inside volcanoes. Muon radiography could be used to monitor density variations in aquifers and the critical zone in the near surface. However, the time resolution achievable by muon radiography monitoring remains poorly studied. It is biased by fluctuation sources exterior to the target, and statistically affected by the limited number of particles detected during the experiment. The present study documents these two issues within a simple and well constrained experimental context: a water tower. We use the data to discuss the influence of atmospheric variability that perturbs the signal, and propose correction formulas to extract the muon flux variations related to the water level changes. Statistical developments establish the feasibility domain of muon radiography monitoring as a function of target thickness (i.e. opacity). Objects with a thickness comprised between $\simeq$ 50 $\pm$ 30m water equivalent correspond to the best time resolution. Thinner objects have a degraded time resolution that strongly depends on the zenith angle, whereas thicker objects (like volcanoes) time resolution does not.