Unveiling Subsurface Secrets: Advances in Neutron Imaging for Soil Research

Abstract

As an opaque medium, soil conceals processes beneath its surface, posing a challenge for direct observation. To overcome this limitation, scientists employ imaging techniques based on radiation types capable of penetrating the soil, offering insights into its internal structures. While X-ray imaging is the most commonly used method due to its widespread availability, our focus lies on neutron imaging, an alternative and complementary modality. The distinctive advantage of neutron imaging lies in its heightened sensitivity to light elements, particularly hydrogen, and its greater penetration depth in many metals compared to X-rays. Hydrogen's significance emerges in studying the distribution of water and organic matter within soils. While the spatial and temporal resolution of neutron imaging falls slightly short of laboratory-based X-ray imaging, it operates within the same order of magnitude. Neutron imaging, often flux-limited, has prompted methodological advancements to enable time-series experiments in two and three dimensions on a scale pertinent to soil studies. Our efforts have concentrated on refining methods for conducting time-sensitive experiments, and we have pioneered the concurrent use of neutron and X-ray imaging. This dual modality approach enhances the reliability of quantitative analysis by leveraging the advantages of images from the two modalities. Quantitative analysis has been a primary focus, leading to the development of correction methods that substantially enhance the accuracy of gravimetric water content quantification based on gray levels in acquired images. The exceptional sensitivity to hydrogen enables the quantification of water content even in unresolved pores, showcasing the primary advantage of neutron imaging. Incorporating advanced denoising techniques further diminishes uncertainties in the results. Beyond imaging-related innovations, our current endeavors extend to the provision of dedicated sample environments for porous media experiments. This new equipment encompasses balances, pumps, and signal-logging devices integrated into the instrument control system. This integration improves experiment control and facilitates the logging of metadata associated with each image. In this presentation, we offer a comprehensive overview of applications benefiting from these developments, showcasing the state-of-the-art performance of neutron imaging techniques in porous media research. Our commitment to refining methodologies, advancing quantitative analysis, and providing specialized sample environments underscores our dedication to pushing the boundaries of neutron imaging for a deeper understanding of soil processes.