Magnetic resonance, also called nuclear magnetic resonance (NMR), is the same physics that is used in medical MRI (magnetic resonance imaging). Despite having “nuclear” in the name, NMR does not require a radioactive source. The technique is completely non-radiogenic, and no special permits are required for its use.
In geophysical applications of magnetic resonance, a high-voltage electrical current is pulsed through a coil of shielded wire at a specific frequency—the resonant frequency of hydrogen nuclei (hence “nuclear”). This ‘excites’ the nuclei, causing them to generate an electromagnetic response signal that can be measured on the same coil that was used to excite them.
Because the recorded NMR signal is emitted directly by hydrogen nuclei in water, it is only present if groundwater is present. Therefore, measuring the NMR signal enables direct quantification of the volume and spatial distribution of groundwater. The behavior of the NMR signal also provides information about the pore-scale hydrogeologic environment. It can be used to estimate critical properties governing the flow and storage of groundwater, such as porosity and permeability.
Read more about Applications of magnetic resonance.
Read Halliburton’s definitive resource:
NMR Logging Principles and Applications