High depth resolution design of small diameter nuclear magnetic resonance logging probe based on compensating magnets for soil moisture measurement in situ

• Published in: Journal of hydrology (Amsterdam) Vol. 603; p. 127031
• Main Authors: Zhu, Jinbao, Wang, Hualiang, Zhao, Hanqing, Lin, Tingting
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2021
• Subjects: Compensating magnets; High depth resolution; NMR logging probe; Soil moisture; Uniform design; NMR logging probe; Soil moisture; Uniform design; Compensating magnets; High depth resolution
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2021.127031

•High depth resolution Nuclear Magnetic Resonance logging sensor for soil moisture measurement.•Detailed Nuclear Magnetic Resonance probe optimization process and design scheme.•Accurate measurement results of soil moisture including the proportion of free water and bound water. The soil moisture in the surface layer (1 or 2 m) interacts with the atmosphere through evapotranspiration and influences land–atmosphere interactions, impacts rainfall-runoff processes, regulates net ecosystem exchange, and constrains food security. Thus, the accurate measurement with high depth resolution of surface soil moisture is of great significance to hydrology, meteorology, and agriculture. The nuclear magnetic resonance (NMR) logging technology has the ability to study hydrogen-containing fluid dynamics in porous media and is expected to achieve in situ detection for soil moisture. In this paper, we propose a new design of a small diameter NMR logging probe based on compensating magnets, which increases the signal-to-noise ratio (SNR) and have high depth resolution (1 cm). The uniform design method is adopted to obtain the optimal compensation magnet parameters. Compared with the probe without compensation magnets, the SNR of the new probe can be increased by 5.2 times in theory. Then a soil moisture experiment of three group samples is conducted to demonstrate the effectiveness of the new probe. The experiment results show that the new probe can obtain accurate results of soil moisture, and can also give the proportion of free water and bound water, which has been verified with the results of the Lab-NMR experiments. Finally, the factors that have an important influence on the measurement effect, such as the uniformity of the excitation magnetic field, the radial depth of investigation, and the echo time have been fully discussed.