A Short Dead Time Detection Method for Surface Nuclear Magnetic Resonance Based on Decoupling Technology

Surface nuclear magnetic resonance (SNMR) is the only non-invasive geophysical method for direct water detection, and is widely used in groundwater exploration. However, the dead time arising from the coupling effect of the transmitting magnetic field on the receiving coil results in the loss of the high level early free induction decay (FID) signal. To solve this problem, we propose a decoupling method based on symmetrical transmitting coils. A mathematical analysis was conducted to describe the decoupling principle and detection theory based on the novel coil structure. Simulation experiments that performed comparisons with the traditional method showed the potential of the new method for the detection of shallow thin aquifers with short relaxation time signals. Further studies of the detection effects with different coil key structure parameters were conducted and a three-layer coil was developed. Laboratory tests showed that the dead time was shortened to 1 ms, which made it possible to acquire the FID signal earlier.