Topology Optimization-Based Design of Magnet System in Unilateral NMR Logging Sensor Considering Coil Efficiency

Nuclear magnetic resonance (NMR) well logging plays an important role in oil measurement. The magnet system is a crucial component of a unilateral NMR logging sensor as it generates a high-strength static magnetic field with a strong gradient in the remote region of interest (ROI). However, current works on magnet system design are based on experiential structures, and lack a theoretical exploration of the optimal magnet system structure. What is more, during magnet system design, current works often ignore the impact of ferrite core on the radio frequency (RF) coil efficiency, which is crucial for NMR signal strength. In this study, we propose a topology optimization (TO)-based design approach for the magnet system in a unilateral NMR logging sensor, taking into account the RF coil efficiency. First, we conduct sizing optimization using the genetic algorithm (GA) to obtain the regular magnet system structure under a given RF coil structure, both with and without considering coil efficiency. Subsequently, we further optimize the ferrite core structure using TO and finite element method (FEM). We compare and analyze the optimization results for different coil structures and ROI ranges. We manufacture a prototype of the magnet system based on optimization results. The results demonstrate that a desired magnet field distribution is obtained, and the measured results align with the calculated results. Furthermore, we test the signal quality of the designed unilateral NMR logging sensor after tuning and matching the RF coil, and the analysis is presented at last.