MODULATION EFFECTS IN NON-DRILLING NMR IN THE EARTHS FIELD

The paper describes the variant of nuclear magnetic resonance (NMR) in the Earth's field using no prepolarization. This method can be employed to record the NMR signal of underground proton-containing liquids, such as water, located at the depth of more than 100 m without drilling. The non-drilling NMR in the Earth's field is recorded by locating on the surface the circular wire with a diameter of about 100 m. This wire serves as an antenna for the exciting field source and the NMR signal receiver. The radiofrequency pulse with the carrier frequency equal to that of proton resonance in the Earth's field is passed through the wire. When the exciting pulse is switched off, the induction e.m.f. signal caused by the free Larmor nuclear precession in the geomagnetic field is observed. An important case when in the non-drilling NMR in the Earth's field the signal is excited and observed at different frequencies is studied both from theoretical and practical viewpoints. The resulting modulation effects are considered. Such situations arise e.g. either in magnetic rocks or upon magnetic storms.