Surface nuclear magnetic resonance (SNMR) is a method to detect groundwater information, which is also called nuclear magnetic resonance sounding (MRS) by some scholars. Considering the application space of this method in studying geoscience problems and the difference between it and medical magnetic resonance imaging, the former is adopted. In this method, magnetic resonance imaging system (NUMIS) is used to detect groundwater information.
The method of SNMR detecting groundwater information is to use the nuclear magnetic resonance effect produced by the difference of nuclear relaxation properties of different substances. That is to say, using the relaxation characteristics of hydrogen nuclei (protons) in water and using nuclear magnetic resonance water finder on the ground, the variation law of nuclear magnetic resonance signals generated by water protons in the formation is observed and studied, and then the existence and temporal and spatial occurrence characteristics of groundwater are detected.
Nuclear magnetic vibration is a physical phenomenon based on nuclear characteristics, which means that substances with nuclear paramagnetism selectively absorb electromagnetic energy. Theoretically, the only condition for applying NMR technology is that the nuclear magnetic moment of the studied substance is not zero. The hydrogen nucleus in water has nuclear paramagnetism and its magnetic moment is not zero. Hydrogen nucleus is the nucleus with the highest abundance and the largest magnetic rotation ratio in paramagnetic material with nuclei in the stratum. Under the action of a stable geomagnetic field, hydrogen nuclei precess around the direction of the geomagnetic field like a gyro, and its precession frequency (larmor frequency) is related to the intensity of the geomagnetic field and the magnetic rotation ratio of the nucleus.
Hydrogen nuclei are at a certain energy level under the action of geomagnetic field. If the protons in the groundwater are excited by an alternating magnetic field with a pulling frequency, there will be a transition between nuclear energy levels, that is, nuclear magnetic resonance.
Usually, the coil (transmitting coil) laid on the ground is electrified with alternating current with the frequency of Lamol, and the hydrogen nuclei in the underground water are excited by the alternating magnetic field formed by underground alternating current to form a macroscopic magnetic moment. This macroscopic magnetic moment produces precession motion in the geomagnetic field, and its precession frequency is unique to hydrogen nuclei. After the current pulse is cut off, the same coil (receiving coil) picks up the nuclear magnetic resonance signals generated at different excitation pulse times, and the signals decay exponentially. The signal intensity or decay rate is directly related to the number of protons in water, that is, the amplitude of NMR signal is directly proportional to the free water content in the detection space. Thus, the technology of directly detecting groundwater information is formed, and the method of detecting groundwater information by ground nuclear magnetic resonance is formed.