Muon borehole detector design for use in 4-D density overburden monitoring

The increase in atmospheric CO₂ concentrations has initiated research into carbon sequestration methods. One possibility is to store CO₂ in subsurface porous reservoirs. Monitoring the injected CO₂ plume is vital because escaping CO₂ poses health and environmental risks. Typically, seismic reflection methods are used to determine the change in density due to the replacement of brine by CO₂ in the reservoir but this is expensive and not continuous. A potential alternative is to use cosmic muon tomography to measure density changes in the reservoir as a function of time. This paper describes the development of a muon detector that will be capable of being deployed in boreholes. The detector will be designed to have the required dimensions, an angular resolution of approximately 2°, and be mechanically robust. The prototype design is based on alternating layers of scintillating rods, which can provide 4-D reconstruction of the overburden to detect small changes in density at depths up to approximately 2 km. Geant4, a Monte Carlo simulation code, is being used to develop models to guide the design of the physical configuration. Preliminary testing and measurements have been performed to validate the simulation predictions and optimize physical performance parameters. The simulated and preliminary experimental results are presented here.