What is a Proton Precession Magnetometer?
A magnetometer is an instrument used to measure the strength of a magnetic field. Magnetometers are used to measure variations in the earth's magnetic field in order to locate mineral deposits, achaeological sites, buried treasure, or submerged objects such as submarines or shipwrecks. The proton precession magnetometer operates on the principal that the protons in all atoms are spinning on an axis aligned with the magnetic field. Ordinarily, protons tend to line up with the earth's magnetic field. When subjected to an artificially-induced magnetic field, the protons will align themselves with the new field. When this new field is interrupted, the protons return to their original alignment with the earth's magnetic field. As they change their alignment, the spinning protons precess, or wobble, much as a spinning top does as it slows down. The frequency at which the protons precess is directly proportional to the strength of the earth's magnetic field. This is the Proton Gyromagnetic Ratio, equal to .042576 Hertz / nanoTesla. For example, in an area with a field strength of 57,780 nT (such as my home), the frequency of precession would be approximately 2460 Hz.
In a simple proton precession magnetometer, a bottle of fluid rich in hydrogen atoms, usually distilled water or a hydrocarbon such as kerosene or alcohol, is surrounded by a coil of wire which can be energized by a direct current to produce a strong magnetic field. When the current is shut off, the precessing protons induce a very weak signal into the same coil, which is now connected to a suitable output device. This output circuitry may be a frequency counter calibrated to give a direct readout of of magnetic field strength.
Why a Differential Magnetometer?
While a proton precession magnetometer can accurately measure the instantaneous strength of the earth's magnetic field, this field is constantly changing, influenced by such things as manmade interference, diurnal variations, and magnetic storms caused by sunspot activity. When searching for a magnetic target with a mobile magnetometer, such variations could be mistaken for a target's magnetic signature, or could mask a target's signature completely. To minimize the effects of such short-term variations, two sensor coils separated by an appropriate distance can be used and their outputs compared. This is known as a differential magnetometer or gradiometer. In the absence of a magnetic target, both sensors will be exposed to the same strength field, and their outputs will be identical. When one sensor nears a magnetic target, its output frequency will differ slightly from the other, regardless of instantaneous variations in the total magnetic field.
In the differential proton precession magnetometer's simplest form, the audible ~2000 Hz. frequencies from the two sensors are combined and fed to an audio amplifier and speaker or headphones. Unequal magnetic fields will produce unequal frequencies, audible as a beat frequency or waver in the output signal. Listen to a magnetometer signal (courtesy of Phil Barnes).
Barnes & Associates
24631 Via San Fernando
Mission Viejo, CA 92692.
M.L. Dalton Research
6035 Aberdeen
Dallas, TX 75230
(214) 691-4925
My homemade proton precession magnetometer is based on Phil Barnes's update of Nicholas Wadsworth's original 1968 design. I wanted a magnetometer for an additional tool in our local group's shipwreck hunting arsenal, in addition to two sidescan sonars. The electronics package will be carried in the boat while the two sensors are towed behind. Since Barnes's plans are for a land based, hand carried unit, I am making several modifications to his design.
Once the coil was wound and cable connections soldered on, the coil was given an all-over coat of epoxy resin, followed by a layer of fiberglass cloth and two more coats of resin. Brass pipe plugs with brass rings attached provide both a means of filling the sensors with distilled water and attachment points for the tow cables.
The polarize / detect timer and relay are on one small circuit board, and the preamp, active filter and audio amp stages are on another. Once I put all the pieces together and hooked up the controls, I found I had a serious feedback problem. At that point the project got put on the shelf until I can find time to refine my design - it's diving season! I may end up mounting the audio section remote from the rest of the circuit, or try shielding the preamp. I'll post more information as I progress. Bookmark this page & check back - and send me any suggestions or experiences you may have!
Update - October, 1999. Well, the mag has been on the shelf for two diving seasons, so I got it out and regrouped. I'm trying a few changes in the preamp, as well as separating the preamp from the filter / audio section. I'm also considering making a set of toroidal sensors, based on some very persuasive arguments in Jim Koehler's comprehensive research.
When I started out on this project, I had no idea how to design a proton magnetometer sensor - how large, what shape, how much polarization current, etc. Rather than re-invent the sensor, I built upon the example of Nicholas Wadsworth and Phil Barnes, modifying their design to adapt to a boat-towed application. I have recently corresponded with a scientist at a leading magnetics laboratory in Russia, who analyzed my sensor design utilizing the lab's proprietary software. He found that the signal level would be rather weak at about 0.18 microvolt, and the signal-to-noise ratio fairly poor at 35. He made several suggestions for the design of my "next generation" sensors: diameter in the 70-100 mm range (signal-to-noise ratio improves with increased sensor diameter); coil length equal to inside diameter; coil depth 10 to 50 percent of inside diameter; two coils wound in opposite directions spaced about 40 percent of inside diameter apart to cancel induced noise. Wound to Vladimir's specifications, the sensors should yield a signal level of 0.28 microvolt with a S/N ratio of 100 or better, using the same polarization current. As far as polarization current goes, the more the better, short of burning out the coils: more current => stronger field => more protons aligned => stronger signal. The new sensor coils will be mounted inside the sensor tube, maximizing the volume of water subject to the polarizing field.
Since putting this Web page up, I've received several requests for circuit diagrams or construction plans for my mag. Since it doesn't really work yet, I'm obviously not ready to provide detailed information. I hope to be able to offer a packet when I get the instrument "perfected". Meanwhile, thanks for your interest in my project! To get you started, here's a block diagram of my magnetometer:
Thanks to:
Jim Koehler
Vladimir Sapunov
Martin Reany
Martin Lowen
Phil Barnes