3D density imaging of cosmic-ray muography with adjustable angular resolutions

Using cosmic-ray muons as the probe for density structures, muography has advantages in detecting cavities and fracture zones for engineering applications in the mountainous area, because it is not disturbed by vibration and electromagnetic noise and does not require fieldwork over rough terranes. However, the conventional muon detector is bulky and often requires a long observation time to accumulate a statistically significant number of muon events. This study presents a new concept of muon detector with smaller sensor arrays and adjustable angular resolutions for improved portability and efficiency. The detector uses two 7×7-pixel plates to record the arrivals of moun particles; the size of each pixel is 5×5 cm². The angular resolution can be adjusted by changing the plate separation. A smaller separation has a lower angular resolution of ray paths but can accumulate sufficient muon events more quickly. Firstly, the empirical formula for calculating observed muon flux is introduced. The observed flux can be simulated for the direction with a known zenith angle by calculating opacity data along the path. Then we develop a forward algorithm of opacity data simulation at variable plate separations to ensure the accuracy of flux calculation. In our example, a mountain model with a low-density fracture zone is designed. Four muon detectors are placed at the western, eastern, northern, and southern feet of the mountain. The forward flux data at the four detectors is converted into opacity data through the corresponding relationship calculated by the empirical formula for inversion. Finally, our synthetic inversions find that a smaller plate separation and a lower angular resolution can still accurately locate the fault location using mountain-foot observations, but at the cost of lower spatial resolution in the inversion model. Our numerical simulations have proven the feasibility of 3D muon tomography with adjustable angular resolutions for more efficient applications in engineering problems.