Magnetic Resonance (MR) refers to a technique in physics known as “Nuclear Magnetic Resonance” or NMR. This is because we use powerful magnetic fields to affect the nucleus of water molecules for direct characterization. There is no radioactivity involved as “nuclear” merely refers to nucleus of the hydrogen atoms. This is depicted in the image below:
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. 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.
Magnetic resonance is a non-invasive physical method to directly detect the magnetic moment of hydrogen protons
A hydrogen proton is a spinning particle with a positive charge – hence it has a non-zero magnetic moment
When placed in a static magnetic field (Bo):
a) the magnetic moment preferentially aligns in the direction of the static field, and
b) the magnetic moment precesses (wobbles) about the static field at a frequency directly proportional to the static field strength
Fnmr = Bo * (42.58 MHz/T)
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 aquifer porosity and permeability.
Just like in the term used for medical MRI (Magnetic Resonance Imaging), we generally choose to drop the “N” from “NMR” when describing our products except when making technical explanations as “MR” is a more commonly known term.
For more details on how Magnetic Resonance works, watch this short 10-minute video by Dave Walsh, President of Vista Clara.
Read Halliburton’s definitive resource:
NMR Logging Principles and Applications