[Paper Review] Effects of upward-going cosmic muons on density radiography of volcanoes

K. Jourde, Dominique Gibert+6 more|arXiv (Cornell University)|Jul 25, 2013

Astrophysics and Cosmic Phenomena23 references3 citations

TL;DR

This study identifies upward-going cosmic muons as a significant source of noise in volcano density radiography using muon tomography, demonstrating that such fluxes can mimic downward muons and bias opacity measurements by up to 50%. Using high-resolution time-of-flight measurements, the authors develop and validate a statistical correction method that reduces reconstruction errors, enabling accurate 3D density tomography of volcanoes like La Soufrière and Mount Etna.

ABSTRACT

Muon tomography aims at deriving the density structure of geological bodies from their screening attenuation produced on the natural cosmic muons flux. Because of their open-sky exposure, muons telescopes are subject to noise fluxes with large intensities relative to the tiny flux of interest. A recognized source of noise flux comes from fake tracks caused by particles that fortuitously trigger the telescope detectors at the same time. Such a flux may be reduced by using multiple-detector telescopes so that fortuitous events become very unlikely. In the present study, we report on a different type of noise flux caused by upward-going muons crossing the detectors from the rear side. We describe field experiments on La Soufriére of Guadeloupe and Mount Etna, and give details on the high-resolution clocking system and the statistical procedure necessary to detect upward-going muons. We analyse several data sets acquired either in calibration or in volcano tomography situation. All data sets are shown clearly biased by upward-going noise flux whose intensity may amount to 50% of the measured total flux in given directions. Biases produced on density radiographies by this kind of flux are quantified and correction procedures are detailed. Examples for La Soufriére and Mount Etna are given.

Motivation & Objective

• To identify and quantify the impact of upward-going cosmic muons on muon tomography data in volcano density radiography.
• To address the challenge that upward muons mimic downward muons, introducing significant bias in opacity measurements.
• To develop and validate a correction procedure using high-resolution time-of-flight data to remove upward flux contamination.
• To ensure accurate 3D density reconstruction in volcanic monitoring, especially for detecting small density contrasts related to magma or hydrothermal processes.
• To demonstrate the necessity of correcting for upward muon flux in field deployments where telescopes face open atmospheric volumes below their rear side.

Proposed method

• Utilized muon telescopes with three detection matrices and 1 ns time resolution to distinguish particle trajectories based on time-of-flight differences.
• Applied a time-of-flight criterion: Δt = t_rear - t_front > 0 for forward (downward) muons and Δt < 0 for backward (upward) muons.
• Used statistical analysis of data sets from La Soufrière and Mount Etna to isolate upward-going muon fluxes based on trajectory inclination and time correlation.
• Implemented a correction model based on polynomial fitting of opacity correction ratios as a function of path obstruction and opacity.
• Applied the correction to raw tomographic data using Eq. 14a and the Tang model for atmospheric muon attenuation at sea level.
• Validated corrections by comparing uncorrected and corrected tomography results, assessing density fluctuations and consistency across view angles.

Experimental results

Research questions

• RQ1 To what extent do upward-going cosmic muons contaminate the downward muon flux used in volcano density radiography?
• RQ2 How does the intensity of upward muon flux vary with telescope orientation and surrounding atmospheric volume?
• RQ3 Can high-resolution time-of-flight measurements reliably distinguish upward from downward muons in field conditions?
• RQ4 What is the quantitative impact of upward muon flux on reconstructed opacity and density values in tomographic images?
• RQ5 How effective is the proposed statistical correction method in restoring accurate density distributions in 3D tomography?

Key findings

• Upward-going muons were detected in field experiments at La Soufrière and Mount Etna, with flux intensities reaching up to 50% of the measured total flux in certain directions.
• The upward flux caused significant bias in opacity reconstruction, with correction ratios ranging from 1.0 (no correction) to 1.5 for obstructions of 1000 m (opacity ~1500 hg·cm⁻²).
• For obstructions between 200 m and 1000 m (opacities from 300 to 1500 hg·cm⁻²), the correction factor increased from 1.0 to 1.5, indicating a strong dependence on path length and density.
• Without correction, raw tomography results showed unphysical density drops to 0.5 g·cm⁻³ near the horizon, especially in regions with high backward obstruction.
• After applying the correction, density fluctuations were significantly reduced, and the reconstructed density values became consistent across different view angles, particularly for β between 50° and 85°.
• The correction was most effective in regions with strong backward obstruction and high atmospheric path length, where upward flux was most intense, and less effective for steep upward trajectories (α_F < -10°).