Proton Precession Magnetometers Principles
Post on 03-Jan-2017
Proton Precession MagnetometersSometimes called the grandfather ofmagnetometers, proton precession unitsserve an important function for earthscience professionals seeking inexpensivesolutions for low-sensitivity fieldwork oracademic. Major applications include:
Locating buried ferrous containers (ex.contaminant waste drums) inenvironmental surveys.
Locating and tracing buried pipelinesin utility-type surveys.
Locating abandoned wells for de-commissioning and sealing.
Teaching of geophysical methods.
Other applications, such as mineralexploration surveys, UXO, archaeologyand other surveys not listed may bebetter performed with Overhauser orOptically Pumped Potassium systems.
Proton precession has limitations, such asorder of magnitude lower sensitivity thanOverhauser and two orders of magnitudelower sensitivity than Potassium. Otherlimitations include low sampling rates,high power consumption, and additionalweight.
However, precession methods are stillviable in many applications. GEMs protonprecession system has particularadvantages as described elsewhere in thisdocument.
PrinciplesThe method is based on the spin ofprotons in a magnetic field according tothe Larmor equation.
Typically, a sensor bottle containing afluid with a large number of protons (suchas kerosene) is subjected to a large directcurrent in a coil wound around the bottle.The current creates a correspondinginduced field in a direction perpendicularto the earths magnetic field.
The current is then shut off and theprotons precess with a frequency that isproportional to the magnetic induction.With GEM, an advanced frequencycounting algorithm is used to determine
the magnetic intensity with an accuracythat is unmatched by other systems.
When choosing a proton precessionsystem, we recommend that you carefullyconsider the system specifications on anequivalent footing to ensure that you areobtaining the maximum value for your
investment. The following table comparesspecifications for different manufacturers.
Note that manufacturers are not namedexplicitly as GEMs practice is to maintainintegrity and ethical practice in itscustomer-focused communications.
Specification GEM GSM-19T Family Other 1 Other 2
Configuration Options 15 10 2
Cycle Time 999 to 0.5 sec 2, 1, 0.5 sec 999 to 1.5 sec
Environmental -40 to 60 Celsius -40 to 60 Celsius -20 to 50 Celsius
Gradient Tolerance 7,000 nT/m ? 1000 nT/m
Magnetic Readings 299,593 140,000 5,700
Operating Range 10, 000 to 120,000 nT 20,000 to 110,000 nT 20,000 to 90,000 nT
Power 12 V @ 0.62 A 12 V @ 0.65 A 9 1.5 D Cell Batteries
Sensitivity 0.1 nT @ 1 sec 0.1 nT @ 2 sec 0.1 nT @ ? sec
Weight (Console/Sensor) 3.2 Kg 3.45 Kg 4.5 Kg
Automatic Grid Setup Yes No No
Integrated GPS Yes No No
Internet Upgrades Yes No No
Simultaneous Grad Yes Yes No
Warranty 2 Years 1 Year 1 Year
Waypoint Navigation Yes No No
Waypoint Programming Yes No No
GEM has 15 configurations that areconstructed from 5 basic building blocks,including mag, grad, vlf, mag/vlf, andgrad/vlf. Each of these building blockscan be augmented with GPS and / orWalking mode.
The Walking mode is a mode thatsimulates the high-density coverageprovided in airborne surveys. Theoperator advances to a station and takesa reading. When the data is dumped fromthe unit, the system automaticallyinterpolates intervening values.
Cycle time is an important parameter asit controls survey efficiency. GEM has abasic mode (GSM-19T) that cycles from999 to 3 seconds this mode requiresthe operator to stop to take ameasurement.
Alternately, an enhanced version (GSM-19TW) enables the operator to acquiredata much faster in Walking mode.
The test of an instrumentation system isits robustness in temperature extremes.The GSM-19T proton precession unit hasbeen used around the world.
It is fully experienced in environmentallychallenging settings, ranging from harshpolar climates to the steaming jungles ofequatorial countries to dry deserts.
Gradient tolerance is key when operatingin areas where magnetic fields varyrapidly (either due to geology or to thepresence of cultural objects or ferrousdebris).
Comparative tolerance is especially highfor GEMs system. Note that where veryhigh gradients are encountered, it may bedesirable to consider Overhauser orOptically Pumped Potassium instruments.
In 2001, the v6.0 system was launched.It includes standard 4 Mbyte memory thatcan be upgraded in 4 Mbyte incrementsto a maximum of 32 Mbytes.
When choosing your magnetometer,gradiometer or base station solution, it isimportant to make sure that you will haveenough memory to make it through adays surveying without interruption. TheGSM-19T is designed with this objectivein mind.
Operating range should be sufficient toenable you to take readings wherever youare operating. GEMs proton precession isthe highest rated system.
Please note that precession systems maystill have problems in equatorial regionsdue to the inherent physics. To learnabout advantages of Overhauser, access:
High power consumption is one of thelimitations of proton precession systems.In fact, about 99% of the DC currentapplied for polarization is dissipated asheat energy. This high power need is oneof the reasons that systems are heavierthan Overhauser instruments (i.e. morebatteries are required).
However, proton precession systems willstill perform well for many applications;GEM offers the lowest power consumptionavailable; meaning that field time isprolonged as long as possible.
Sensitivity is a statistical value indicatingrelative uncertainty of repetitive readingsof the same magnetic field intensity. It isdefined as r.m.s. (root - mean - square)value per square root of a unit ofbandwidth (Hz1/2). For example, asensitivity of 1 pT / Hz1/2 means 1 pTr.m.s. (about 3 - 4 pT peak-to-peakdepending on the character of the noise)will be a scatter of readings about any"etalon" (fixed value) of the appliedmagnetic field per 1 Hz of measurementbandwidth.
The sensitivity of quantum units isdetermined by the signal-to-noise ratioobtainable from its sensor, the spectralline width on which it operates and on thegyromagnetic constant, in accordancewith the following equation:
B = k / n Sn
where k is a constant of proportionality, is the spectral line width, n is thegyromagnetic constant and Sn is thesignal-to-noise ratio.
Any well-designed magnetometersreadings will eventually be limited by anoise level that cannot practically besuppressed any further: Sensor thermalnoise is typically the limiting factor in thecase of Proton and Overhausermagnetometers.
The following diagram shows thesensitivity of GEMs proton (GSM-19T),Overhauser (GSM-19) and potassium(GSMP-40) systems. Each system has itsown characteristic sensitivity envelopedefined by sample interval.
G SM -19TG SM -19G SM P-40
Note that the proton sensitivity at 0.5seconds is 0.5 nT, and is cut-off at 0.25nT for illustrative purposes.
Proton precession weight is primarily afunction of battery weight a reflection ofthe high power consumption for DCpolarization.
Automatic Grid Setup
The automatic grid setup capabilitydelivers efficiency in defining your survey.To use this option, you access thecorresponding menu and enter thestarting and finishing points of your firstline, and a direction (clockwise or counterclockwise).
The system then automatically generatesa survey grid of 500 lines. You can use asmany or as few lines as required tocomplete your survey.
Integrated / External GPS
With precise GPS positioning comes theability to significantly speed up surveysand to acquire reliable information fordecision-making.
Integrated GPS is an attractive option inGEMs proton precession systems minimizing the potential for damage toGPS components and minimizing thephysical size of units for reduced shippingcosts. Where sub-metre resolution isrequired, GEM also provides an external
GPS with on-board corrections for real-time DGPS using WAAS or subscriptionservices.
Imagine that your manufacturer has theability to deliver the latest technology toyour magnetometer via periodic webupdates. Now envision the money you willsave no shipping costs to have thelatest modifications or to add functionalityto your unit. GEM provides these benefitsas a standard feature in all its protonprecession magnetometers.
Gradiometer configurations deliver anumber of advantages for near surfaceand deeper work, including resolution ofshallower targets as well as the ability toacquire data without performing diurnalsurveys (other benefits are described inwww.gemsys.ca/Quantum/Technology/Gradiometer%20Advantages.htm).
Its important, however, to have sensorsrecording at exactly the same time particularly if the system is to be used fordiurnal corrections or in areas of high
magnetic gradients. This capability isprovided in all GEM systems, includingproton precession, Overhauser andpotassium.
Waypoint navigation delivers severalfunctions. These include the ability toenter a set of waypoints for use in GPSsurveying mode, and lane guidance. Thesystem guides you to each waypoint -keeping you within a user-defined lane.
The main benefits lie in survey efficiencyand tight survey control for as accuratepositioning as possible. You also have theability to perform surveys along lines thatare non-parallel a useful capability forenvironmental-type or non-standard,roving surveys.
This option enables you to enterwaypoints from either keyboard ordownload from your personal computerfor maximum efficiency.
No system is complete without softwarefor downloading and correcting your data,and GEM provides you with the provenGEMLinkW software for this purpose.
GEM-VIS is an optional packagedeveloped by a major commercialsoftware house for GEMs users. Itenables rapid quality control and analysiswith easy visualization of profiles andstacked plots as well as 2D and 3D grids.