Principles of Operation
A direct current flowing in a solenoid creates a strong magnetic field around a hydrogen-rich fluid (kerosine, and decane is popular, and even water can be used), causing some of the protons to align themselves with that field. The current is then interrupted, and as protons realign themselves with ambient magnetic field, they precess at a frequency that is directly proportional to the magnetic field. This produces a weak rotating magnetic field that is picked up by a (sometimes separate) inductor, amplified electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength or output as digital data.
The relationship between the frequency of the induced current and the strength of the magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hz nT−1. Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer can reach 1 ppm.
The frequency of Earth's field NMR (EFNMR) for protons varies between approximately 900 Hz near the equator to 4.2 kHz near the geomagnetic poles. These magnetometers can be moderately sensitive if several tens of watts are available to power the aligning process. If measurements are taken once per second, standard deviations in the readings is in the 0.01 nT to 0.1 nT range, and variations of about 0.1 nT can be detected.
For hand/backpack carried units, PPM sample rates are typically limited to less than one sample per second. Measurements are typically taken with the sensor held at fixed locations at approximately 10 meter increments.
The two main sources of measurement errors are magnetic impurities in the sensor, errors in the measurement of the frequency and ferrous material on the operator and the instruments, as well as rotation of the sensor as a measurement is taken.
Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000 nT m−1 which is adequate from most mineral exploration work. For higher gradient tolerance such as mapping banded iron formations and detecting large ferrous objects Overhauser magnetometers can handle 10,000 nT m−1 and Caesium magnetometers can handle 30,000 nT m−1.
Read more about this topic: Proton Magnetometer
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