Optimized slim-line logging NMR tool to measure soil moisture in situ

[Display omitted] •We design a small mobile NMR tool suitable to measure water content in soil.•The tool is optimized to maximize the signal-to-noise ratio.•The optimization takes into account the RF coil-magnet shield coupling.•The tool achieves a penetration depth into the soil of the order of its radius. We report the optimization of a slim-line logging NMR tool carried out by maximizing the signal-to-noise ratio of the NMR measurements. The tool, based on cylindrical permanent magnets of 20cm length and 5cm diameter, has a penetration depth of about 2cm measured from its surface. This is obtained thanks to a large radio frequency coil whose dimensions are comparable to the sensor size. An analytical expression of the SNR as a function of parameters which take into account the interaction between the radio frequency coil and the magnet shielding is developed. In view of the external constrains such as the one imposed by the excavation hole, a proper tool size is determined in the optimization process. Due to its size and properties, the sensor is suitable to measure water content in the vadose zone, which is the zone comprised within the first meters of the Earth surface and whose study is important for improving water management in agriculture and for refining climate models.