A Low-cost, Portable, Ruggedized Cosmic Muon Detector Prototype for Geological Applications

Muons, neutrons and protons observed at the Earth's surface are generated by cosmic ray primaries causing cascades in the atmosphere. Cosmic muon tomography is a cost effective real time monitoring technique that can be applied to determine large scale displacement of reservoir fluids induced by injection of liquid or gas. Such technique would need a detector array with an overall sensitivity tailored to the monitored volume and the expected density change in the target geological formation over the projected injection time. A scalable detector system, able to withstand the harsh conditions of underground deployment is a must for the evaluation of this promising technique. This paper presents the design and construction of a portable muon flux monitor, known as the μ-Witness. The detector is based on coincidence counts between two scintillator panels to be used as an indicator of density-dependent attenuation of cosmic muon flux. The Muon Witness detector (μ-Witness) has been designed to be able to measure cosmic muon flux for periods of time of up to 40 days, using battery power. The prototype has been mounted in a ruggedized case to enable measurements in underground environments. The purpose of this prototype is to evaluate the feasibility of using 3D density tomography in geological applications. The efficiency of the detector has been experimentally determined to be 57±3%. This measurement was performed by comparing the detector response to the response of a larger and more efficient muon counter in the same location. Using Monte Carlo simulations of the cosmic muon flux, and the measured efficiency, the projected sensitivities for density changes in large underground monitored volumes are presented as well as the results of a test run in a shallow underground facility. Along with a detector prototype, a model of the muon attenuation inversion must be developed in order to take into account the different energy and angular distribution of the cosmic muons, and other contributing factors such as altitude, magnetic field rigidity and time of the year. There has been extensive work on characterizing the cosmic ray showers and this work uses one of such parameterizations to model the cosmic muon flux. Monte Carlo simulations can model the passage of particles through matter, among them high energy muons going through Earth's subsurface. The first underground measurements carried out with the prototype are also presented. The μ-Witness detector collected measurements at Pacific Northwest National Laboratory, inside, outside, and in the shallow underground lab, which has a depth of ~30 meters water equivalent (mwe). The μ-Witness count rate was 2.51 ± 0.04 muons/s, 2.61 ± 0.05 muons/s, and 0.40 ± 0.01 muons/s inside, outside and in the underground lab, respectively. Indoor measurements were expected to be lower than outdoors, as the laboratory overhead serves as overburden, and was estimated to be about 2 mwe. From these measurements, assuming an inverse linear attenuation, we can infer the μ-Witness density sensitivity to be -0.0752 count/s*mwe. This figure will aid in the design of a large detector system for field-scale deployment at the ground surface and in boreholes.