Editing Magnetometer (section)

====Proton precession magnetometer====
{{Main|Proton magnetometer}}
''Proton precession magnetometer''s, also known as ''[[proton magnetometer]]s'', PPMs or simply mags, measure the resonance frequency of [[proton]]s (hydrogen nuclei) in the magnetic field to be measured, due to [[nuclear magnetic resonance]] (NMR). 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 [[part per million|ppm]].<ref>Dr. Ivan Hrvoic, Ph.D., P.Eng. "[https://web.archive.org/web/20131212042401/http://info.orrvweb.com/wp-content/uploads/whitepaper/gemprotonaccuracy.pdf Requirements for obtaining high accuracy with proton magnetometers]". GEM Systems Inc., 11 January 2010.</ref>

A direct current flowing in a [[solenoid]] creates a strong magnetic field around a [[hydrogen]]-rich fluid ([[kerosene]] and [[decane]] are 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 the [[wikt:ambient|ambient]] magnetic field, they [[precession|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, [[amplifier|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.

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 metre increments.

Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000 nT/m, which is adequate for most mineral exploration work. For higher gradient tolerance, such as mapping [[banded iron formation]]s and detecting large ferrous objects, [[#Overhauser effect magnetometer|Overhauser magnetometers]] can handle 10,000 nT/m, and [[#Caesium vapour magnetometer|caesium magnetometers]] can handle 30,000 nT/m.

They are relatively inexpensive (< US$8,000) and were once widely used in mineral exploration. Three manufacturers dominate the market: GEM Systems, Geometrics and Scintrex. Popular models include G-856/857, Smartmag, GSM-18, and GSM-19T.

For mineral exploration, they have been superseded by Overhauser, caesium, and potassium instruments, all of which are fast-cycling, and do not require the operator to pause between readings.