Research On Reverse Design Method Of Antenna Of Logging While Drilling NMR Tool

The antenna is the key component that generates the radio frequency magnetic field and receives the nuclear magnetic resonance signal in the LWD NMR tool.The performance of the antenna will directly affect the detection characteristics of the LWD NMR tool.It is very important to design a probe antenna with high signal-to-noise ratio and low power consumptionThe characteristics of a high-quality LWD antenna include: high transmission efficiency,low power and high signal-to-noise ratio.However,the optimization of these key parameters often restricts each other.Traditional antenna design ideas use manual adjustment of the coil structure to optimize the parameters,making it difficult to obtain optimal results.In this paper,we screen and determine the evaluation parameters of the main properties of the antenna of the LWD NMR probe,and analyze its main influencing factors;then,we select the most static magnetic field area generated by the magnet structure of a new LWD NMR probe designed by the research team,The RF field with good intensity matching is used to derive the antenna model using the target field method to obtain the target function of the reverse design of the RF coil.The RF coil performance evaluation parameters are used as constraints,the minimum value of the target function is obtained.The solution of the model obtains the antenna structure parameters.The result of numerical simulation shows that the antenna obtained by reverse design has significantly improved magnetic field uniformity compared to the initial model.In order to verify the correctness of the results,the initial antenna and the optimized antenna were fabricated and processed,and the experimental verification was carried out.The result shows that the optimized antenna's echo amplitude is increased by 12% compared with the original antenna,and the signal-to-noise ratio is increased by 9%.