Study on Shortening the Dead Time of Surface Nuclear Magnetic Resonance Instrument Using Bipolar Phase Pulses

Surface nuclear magnetic resonance (NMR) is a geophysical technique for direct and quantitative detection of underground water sources. In recent years, with the application of lightweight instruments in tunnels and mines, coils with small dimensions and multiturn have been used for surface NMR. Increasing the coil turns increases the inductance and causes problems such as severe ringing, long dead time, and significant loss of early stage surface NMR signal. To address these problems, the cause of ringing in surface NMR instruments and the effect of ringing on the dead time are analyzed in this paper. The bipolar phase pulse (BPP) method is introduced to reduce the ringing based on the positive and negative superposition principle and therefore shorten the dead time. A surface NMR signal simulation test and a free-radical solution test are performed to verify the effectiveness of the BPP method. A comparison with technologies such as Q-SWITCH shows that the dead time of the surface NMR instrument decreases from 35 to 5 ms and that the signal-to-noise ratio (SNR) of the collected signal improves by 5 dB.