Muon Tomography for Geological Repositories.

Abstract:

Cosmic-ray muons are subatomic particles produced in the upper atmosphere in collisions of primary cosmic rays with atoms in air. Due to their high penetrating power these muons can be used to image the content (primarily density) of matter they pass through. They have already been used to image the structure of pyramids, volcanoes and other objects. Their applications can be extended to investigating the structure of, and monitoring changes in geological formations and repositories, in particular deep subsurface sites with stored CO₂.

Current methods of monitoring subsurface CO₂, such as repeat seismic surveys, are episodic and require highly skilled personnel to operate. Our simulations based on simplified models have previously shown that muon tomography could be used to continuously monitor CO₂ injection and migration and complement existing technologies.

Here we present a simulation of the monitoring of CO₂ plume evolution in a geological reservoir using muon tomography. The stratigraphy in the vicinity of the reservoir is modelled using geological data, and a numerical fluid flow model is used to describe the time evolution of the CO₂ plume. A planar detection region with a surface area of 1000 m² is considered, at a vertical depth of 776 m below the seabed. We find that one year of constant CO₂ injection leads to changes in the column density of about 1%, and that the CO₂ plume is already resolvable with an exposure time of less than 50 days. The attached figure show a map of CO₂ plume in angular coordinates as reconstructed from observed muons.

In parallel with simulation efforts, a small prototype muon detector has been designed, built and tested in a deep subsurface laboratory. Initial calibrations of the detector have shown that it can reach the required angular resolution for muon detection. Stable operation in a small borehole within a few months has been demonstrated.