An Improved Air-Core Coil Sensor With a Fast Switch and Differential Structure for Prepolarization Surface Nuclear Magnetic Resonance

Abstract

Surface nuclear magnetic resonance (SNMR) technology based on a prepolarization field is a new geophysical method that offers a higher resolution for detecting shallow groundwater than traditional SNMR. However, when a prepolarization SNMR system works in noisy environments (such as cities), it is difficult to obtain high-quality data because of the low signal-to-noise ratio (SNR). Moreover, a traditional air-core coil sensor system has a relatively large dead time, which can cause early signal loss and further reduce the SNR. To solve these problems, we proposed an innovative air-core coil sensor design method to improve the detection capability of prepolarization SNMR in complex noise space. To suppress environmental noise and increase the signal amplitude, a passive matching network based on a differential structure was designed, which has a gain of 9.69 dB and a bandwidth of 700 Hz. In addition, a fast switch based on back-to-back technology was also used to replace the mechanical high-voltage relay, and the switching time was reduced to less than $10~\mu \text{s}$ . The effectiveness of the new sensor was tested via experiments in the laboratory and field. The test results showed that the switching speed between transmitting and receiving is thousands of times faster than that of the traditional sensor, and the new sensor can still detect effective free induction decay (FID) signals in a noisy environment where the traditional sensor completely fails. Therefore, this research can effectively improve the SNR and provide strong support for the application of prepolarization field magnetic resonance technology in areas with complex noise environments.