Portable nuclear magnetic resonance (NMR) instruments are widely used in many fields. However, the large volume, low uniformity and end effect limits the application of portable NMR instruments. In order to improve the uniformity and compensate the end effect, a Halbach structure with 9-layer permanent magnet is proposed, which is optimized by axially adjusting the magnet height based on the Halbach array principle and Quality factor (Q) is introduced to represent the magnetic field uniformity at both ends of the central cylinder region. Each layer consists of 16 permanent magnets with trapezoidal cross section and the total volume is Φ240 × 141.8 mm. Through simulation, it is found that the final magnetic flux density is 1.09 T and the uniformity is 418 ppm in the central region (Φ20 × 20 mm) of the optimized structure. The proposed structure has the advantages of small size, compactness in structure and homogeneity, which is very suitable for portable NMR systems.

The gravitational effects of ocean tide loading, which are one of the main factors affecting gravity measurements, consist of three components: (1) direct attraction from the tidal water masses, (2) radial displacement of the observing station due to the tidal load, and (3) internal redistribution of masses due to crustal deformation. In this study, software for gravitational effects of ocean tide loading was developed by evaluating a convolution integral between the ocean tide model and Green’s functions that describe the response of the Earth to tide loading. The effects of three-dimensional station coordinates, computational grid patterns, ocean tide models, Green’s functions, coastline, and local tide gauge were comprehensively considered in the programming process. Using a larger number of high-precision coastlines, ocean tide models, and Green’s functions, the reliability and applicability of the software were analyzed at coastal stations in the China Earthquake Gravity Network. The software can provide the amplitude and phase for ocean tide loading and produce a predicted gravity time series. The results can effectively reveal the variation characteristics of ocean tide loading in space and time. The computational gravitational effects of ocean tide loading were compared and analyzed for different ocean tide models and Green’s functions. The results show that different ocean tide models and Green’s functions have certain effects on the calculated values of loading gravity effects. Furthermore, a higher-precision local ocean tide model, digital elevation model, and local tidal gauge record can be further imported into our software to improve the accuracy of loading gravity effects in the global and local zones. The software is easy to operate and can provide a comprehensive platform for correcting the gravitational effects of ocean tide loading at stations in the China Earthquake Gravity Network.

Portable nuclear magnetic resonance (NMR) instruments are widely used in many fields. However, the large volume, low uniformity and end effect limits the application of portable NMR instruments. In order to improve the uniformity and compensate the end effect, a Halbach structure with 9-layer permanent magnet is proposed, which is optimized by axially adjusting the magnet height based on the Halbach array principle and Quality factor (Q) is introduced to represent the magnetic field uniformity at both ends of the central cylinder region. Each layer consists of 16 permanent magnets with trapezoidal cross section and the total volume is Φ240 × 141.8 mm. Through simulation, it is found that the final magnetic flux density is 1.09 T and the uniformity is 418 ppm in the central region (Φ20 × 20 mm) of the optimized structure. The proposed structure has the advantages of small size, compactness in structure and homogeneity, which is very suitable for portable NMR systems.