Brener - Magnetometers

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APPLICATIONS MANUAL

FOR

PORTABLE MAGNETOMETERS

by

S. BREINER

Geometrics

2190 Fortune Drive

San Jose, California 95131

U.S.A.

Copyright 1999 by Geometrics. All Rights Reserved . Printed in theUnited States of America. This publication, or parts thereof, may not bereproduced in any form without permission of Geometrics.

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PREFACE

This Manual was written to satisfy most of the needs ofthe average user of a portable total field magnetometer

for both conventional and unconventional applications,including geological exploration, search for lost objects,magnetic measurements of rock or iron specimens andarchaeological prospecting. As the name implies, this isa manual or guide for professional and non-professionalpersons who may not have the time, the requisite back-ground or the ready access to the proper libraries todelve deeply into standard texts, the few that there are,on applied geophysics.

Some of the information that I have included in thisManual may be found in the references cited or drawnfrom obscure sources, or uncovered amongst equationsand confusing terminology in physics or engineering

texts. Many of the facts and instructions in this Manual,however, do not appear anywhere else in print. Forexample, I know of no other readily available referenceon the subjects of magnetic search of buried objects,many of the portable gradiometer applications, opera-tional considerations of proton magnetometers and theeffect of electrical currents on portable total field mag-netometers. Among the less common subjects that arecovered are the magnetic properties and detection ofcommon steel objects, facts concerning the detectionof buried ruins, methods for sketch-it-yourself anomalyconstruction, and some help in interpreting anomaliesat the magnetic equator. I also tried to simplify someaspects of the potentially complex subject of magneticsusing short-cuts wherever possible and deskilling some-what the fine art of magnetic interpretation. For mostportable magnetometer work, I feel this approach isquite adequate. Certainly for the more sophisticatedtechniques required for interpretation of the usually-more-precise aeromagnetic surveys, the reader isadvisedto consult the References or persons knowledgeable inthe subject.

Figures and examples are used liberally in the explana-tions because I feel they assist or confirm ones under-standing of these subjects. Almost all of the profileswere drawn free-hand according to the techniquesdescribed and should not be considered as precisecomputer-derived curves. They do demonstrate that onecan be his own magnetics expert insofar as what isrequired for most of these applications.

The question of units always arises in any technicalpublication. Many magnetic measurements, particularlymagnetic properties of rocks and geophysical research,use cgs, some physics and engineering applications usemks, while geophysical exploration, for most of thereaders of this Manual, still utilizes feet and miles. A mix-ture of units, hopefully not too confusing, was thereforeunavoidable. Subsequent editions of this Manual may be

written specifically in carefully selected metric units.

The various chapters were prepared to be read or utilizedindependent of each other if necessary. For example,someone interested in using the magnetometer forarchaeology but who does not particularly enjoy wadingthrough the mathematics of Chapter V, can proceeddirectly to Chapter VII. He would be aided, however, bysubsequently skimming through Chapter V.

I would appreciate criticism or suggestions should any-one note errors or have suggestions on how I may im-prove later editions. Moreover, if the reader finds thatmy explanations or facts fall just short of what is

required, I am available by telephone or through writtencorrespondence.

SHELDON BREINERGeoMetrics

395 Java DriveSunnyvale, California 94066Telephone: (408) 734-4616

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Contents

I

II

III

IV

INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

MAGNETOMETERS

Instrument Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Proton Magnetometer .........................3Total Field Measurement ......................3Limitations of a Proton Magnetometer.......... 4EARTHS FIELD MAGNETISM

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time Variations ...............................6Magnetic Minerals and Iron .................... 6Induced Magnetization ........................8Remanent or Permanent Magnetization ......... 8FIELD PROCEDURES

AND DATA REDUCTION

Magnetic Cleanliness and Sensor Positions. ... 11Operational Considerations ................... 11

Valid Reading Vs. Noise .................. 11Sensor Orientation .......................12instrument Readings ..................... 12Correction for Time Variations ............ 12High Magnetic Gradients ................. 13

Data Reduction ..............................13Profile Smoothing ........................13Removal of Regional Gradients ........... 14Contour Maps

............................14Construction of a Contour Map ........... 15

V INTERPRETATION

P r e f a c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i i i

Introduction .................................17Asymmetry ..................................17Depth Dependence. ..........................18Other Anomaly Shape Factors ................ 18Geological Models ...........................18Elementary Dipoles and Monopoles ........... 18

Simplified Method for Total Field Signature. ... 19Earths Field Component Behavior ........ 20Dipoles Vs. Monopoles

Vs. Arrays of Poles ..................... 20Configuration of Field Lines .............. 20Dipole and Monopole Fall-Off Factor...... 20Dipole Factor-of-Two ..................... 20Application of Method .................... 20

Contour Presentation of

Dipole and Prism Anomalies ................ 22Anomaly Amplitude .......................... 24

Amplitude Estimates

for Common Sources . . . . . . . . . . . . . . . . . . .24

Dipole and Monopole Signatures

in Vertical and Horizontal Fields ........ 24Maximum Amplitude Given

Magnetization and Generalized Form .... 26Anomaly Depth Characteristics ............... 28

Anomaly Width ................ , .......... 28Anomaly Depth Estimation

................ 28Identification of Anomaly . . . . . . . . . . . . . . . . . 29Fall-Off Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Assumptions on Maximum

Amplitude and Depth Estimates . . . . . . . . . 29

Half Width Rules .......................... 31

Slope Techniques ......................... 31

Other Depth Estimating Methods .......... 31

Interpretation Summary . . . . . . . . . . . . . . . . . . . . . . 3 2

VI MAGNETIC SUSCEPTIBILITY,

MAGNETIZATION AND MAGNETIC

MOMENT MEASUREMENTSIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3

Applications .................................33Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4

Random Sample Rotation

for Magnitude Only ......................... 34

Systematic Rotation

for Magnitude and Direction ................ 35

Dipole in Earths Field ........................ 36

Non-Spherical Object Rotation .............. .36

(cont inued)

V


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Contents (continued)

VII MAGNETIC SEARCH

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Determination of Object Magnetism . . . . . . . . . . . 39

Detectability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Magnetic Anomaly Signatures . . . . . . . . . . . . . . . . 39

Depth/Amplitude Behavior. . . . . . . . . . . . . . . . . . . . 39

Search Procedures.. . . . . . . . . . . . . . . . . . . . . . . . . .4 1

Determination of Magnetic MomentVs. Search Grid Vs. Resolution .......... 41

T r a v e r s e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1

Detailed Mapping

for Pinpointing Location ............. 41

Special Search Topics . . . . . . . . . . . . . . . . . .42

Iron and Steel ............................42Permanent Vs.

Induced Anomaly Sources . . . . . . . . . .. 42Pipelines (horizontal) .............................. 44

Magnetic Markers ........................ 44Archaeological Exploration ................... 45

Introduction.............................45

Magnetic Anomalies

of Archaeological Origin ................ 45Remanent Magnetization ................. 46

Archaeomagnetism ....................... 46Magnetization and

Susceptibility of Soils .................. 46Remanent Magnetization of Soils .......... 47Magnetic Anomaly Complexity ............. 47

VIII GRADIOMETERS AND GRADIENT TECHNIQUES

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

Applications of the Gradiometer . . . . . . . . . . . . . . 49Conditions for Gradient Measurement . . . . . . . . . 49

Gradiometer Sensitivity ....................... 50

Gradiometer Readings in the Field . . . . . . . . . . . . 50

Gradiometer as a Filter .. ........................ 50Calculation of Vertical Gradient ................ 51

Depth Estimates from Vertical Gradients ....... 52

General Expression Involving

Gradients and Coordinates ................ 53

Gradient Vector Diagrams and

Vector Information from Total Field .......... 54I X MAGNETIC MEASUREMENTS OF

ELECTRIC CURRENT DISTRIBUTIONS

Archeological Survey Planning

and Feasibility .................... .47

Archaeological Anomaly Amplitude

and Signatures ................... .47

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 55

Applications ................................. .55Configuration of Magnetic Field of

Electric Current Sources ................... 55

Amplitude of Fields of Current Sources ......... 56

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 58


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I.

INTRODUCTION

This Manual is intended for use as a general guide for a number of very diverse applications of portable

magnetometers, especially the total field proton (nuclear precession) magnetometers. The diversity

of applications and the general complexity of magnetic field measurements limits the depth to which

any one subject can be covered, but further information, if desired, can be obtained through the author

or from any of the references cited.

Among the applications for which this Manual is written are mineral and petroleum exploration, geo-

logical mapping, search for buried or sunken objects, magnetic field mapping, geophysical research,

magnetic observatory use, measurement of magnetic properties of rocks or ferromagnetic objects,paleomagnetism, archaeological prospecting, conductivity mapping, gradiometer surveying, and

magnetic modeling. The terminology, units of measurement, and assumed prerequisite knowledge

are those employed in the field of geology and geophysics.


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II.

MAGNETOMETERS

Instrument Use

The common types of portable magnetometers in usetoday are fluxgate, proton precession, Schmidt fieldbalance, dip needle and other special purpose instru-ments. Field balances and dip needles are mechanicaldevices comprised of pivoted magnets measuring verticalor horizontal intensity or field direction, and are notmuch used today being replaced by the more sensitiveand less cumbersome fluxgate and proton magneto-meters. Portable fluxgate magnetometers employ a satur-able core sensor held in a vertical direction to measurevertical intensity with an effective sensitivity on the orderof several gammas. Fluxgate magnetometers, too, are

slowly being replaced by the proton magnetometerwhich has greater sensitivity (1 gamma or better), abso-lute accuracy, no moving parts, and measures total fieldintensity with freedom from orientation errors. For rea-sons of its increasing utilization and because manyapplications require these features, the proton magne-tometer will be the principal instrument under discussionin the Manual. Much of the Manual from Chapters IIIthrough IX nevertheless applies to vertical componentflux gate magnetometers as well. Anomaly signatures athigh latitudes (magnetic dip 70 o r greater) are practicallyidentical for the two instruments; at other latitudes theydiffer significantly.

Proton MagnetometerThe proton precession magnetometer is so namedbecause it utilizes the precession of spinning protons ornuclei of the hydrogen atom in a sample of hydrocarbonfluid to measure the total magnetic intensity. The spin-ning protons in a sample of water, kerosene, alcohol,etc., behave as small, spinning magnetic dipoles. Thesemagnets are temporarily aligned or polarized by appli-cation of a uniform magnetic field generated by a currentin a coil of wire. When the current is removed, the spinof the protons causes them to precess about the direc-tion of the ambient or earths magnetic field, much as aspinning top precesses about the gravity field. The pre-cessing protons then generate a small signal in the same

coil used to polarize them, a signal whose frequency is

precisely proportional to the total magnetic field intensityand independent of the orientation of the coil, i.e., sensorof the magnetometer. The proportionality constant whichrelates frequency to field intensity is a well known atomicconstant: the gyromagnetic ratio of the proton. The pre-cession frequency, typically 2000 Hz, is measured bymodern digital counters as the absolute value of thetotal magnetic field intensity with an accuracy of 1 gam-ma, and in special cases 0.1 gamma, in the earths fieldof approximately 50,000 gammas.

Total Field Measurement

The total magnetic field intensity, as measured by aproton magnetometer, is a scalar measurement, or simplythe magnitude of the earths field vector independent ofits direction. The measurement can be expressed as inFigure 1a as simply the length of the earths field vector,F, shown here to be 50,000 gammas. A local perturba-

. 5 = 50,000 GAMMAS

TOTAL FIELD

Figure la.

tion, T, of 10 gammas, as might be measured in any ofthe applications discussed herein, is shown in Figure 1bas a vector of arbitrary direction. This disturbance vectoradds to the undisturbed field in the usual manner ofvector addition as shown in Figure lb, paying specialnotice to how the figure would actually appear if boththe 50,000 and 10 gamma vectors were drawn to scale.It is clear from the figure, then, that since the protonmagnetometer measures only the magnitude of the

resultant vector whose direction is almost exactly parallel

F = 50,000 GAMMAS (0(UNDISTURBED TOTAL FIELD) @

I -

- 5 ,006 GAMMAS

R E S U L T A N TRE E L D)Figure lb.

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4 APPLICATIONS MANUAL FOR PORTABLE MAGNETOMETERS

to the undisturbed total field vector, that which is mea-sured is very nearly the component of the disturbancevector in the direction of the original undisturbed totalfield, or where

If + fl= F + compFTwhere IFIG I.Such conditions are almost always valid except in thenear field of large steel objects or in the vicinity of ironore deposits or certain ultrabasic rocks which produceanomalies larger than 10,000 gammas. Thus, the changein total field, A F = compFT, i.e., the component of theanomalous field, T, in the direction of F. (Except wherenoted, CompFT will be referred to simply as the anomalyT.) The proton precession magnetometer, for small per-turbations, can therefore be considered to be an earth's-field-determined component magnetometer.

This property of measuring this scalar magnitude of thefield, otherwise called total field intensity, is very signifi-cant with respect to the asymmetric signatures of anom-alies, interpretation of anomalies, and in various specialapplications. Furthermore, the fact that what is measuredis independent of the orientation of the sensor, allowsthe magnetometer to be operated without attention toorientation or leveling such as would be the case with

a fluxgate magnetometer on the mobile platform of aperson, vehicle, or aircraft. The only limitation of sucha scalar measurement, albeit a minor one, is the factthat the component of the anomalous field which ismeasured is not normally under the control of the ob-server, but rather at the whim of the local direction ofthe earths magnetic field.

Limitations of a Proton Magnetometer

The proton magnetometer has no moving parts, producesan absolute and relatively high resolution measurementof the field and usually displays the measurement in theform of an unambiguous digital lighted readout. Severaloperational restrictions exist, however, which may be ofconcern under special field conditions. First, the protonprecession signal is sharply degraded in the presence ofa large magnetic field gradient greater than 200 gammasper foot (approximately 600 gammas per meter). Also,the signal amplitude from the sensor is on the order ofmicrovolts and must be measured to an accuracy of0.04 Hz of the precession frequency of several thousandHz. This small signal can be rendered immeasurableby the effects of nearby alternating current electricalpowei sources. For these two reasons, a proton mag-netometer cannot usually be operated within the con-fines of a typical building. Developments and proceduresare presented which minimize these effects for the appli-cations to be described in the Manual.


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III.

EARTHS FIELD MAGNETISM

Introduction

The earths magnetic field resembles the field of a largebar magnet near its center or that due to a uniformlymagnetized sphere. The origin of the field is not wellunderstood, but thought to be due to currents in a fluidconductive core. On the surface of the earth the pole ofthis equivalent bar magnet, nearest the north geographi-cal pole, is actually a south magnetic pole. Thisparadoxical situation exists since by convention a north-seeking end of a compass needle is defined as pointingnorth yet must point to a pole of opposite sense or southpole of the earths magnetic field. To avoid possible con-fusion, though, the magnetic pole near the geographical

north pole is, and will be referred to as, a north pole.

The field, or flux, lines of the earth exhibit the usualpattern common to a small magnet as shown in Figure 2.Note that the direction of the field is vertical at the northand south magnetic poles, and horizontal at the magneticequator. An understanding of this geometry is importantwith respect to interpretation of magnetic anomalies. Theintensity of the field, which is a function of the densityof the flux lines shown in Figure 2, again behaves as abar magnet being twice as large in the polar region as inthe equatorial region, or approximately 60,000 gammasand 30,000 gammas respectively. The inclination from Figure 2.

30 60. 90. 120.

60.

120 150 180 150 120 90 60 30 0 30' 60 90 120

Figure 3. The Geomagnetic Inclination in Degrees of Arc from the Horizontal SOURCE: U.S.N.H.O

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Figure 4. The Total Intensity of the Earths Magnetic Field

the horizontal and the total intensity are shown in Figures3 and 4. (NOTE: 1 gamma = 10-9 gauss. Gauss is actuallya unit of magnetic induction and oersted a unit of mag-netic intensity-B and l-l respectively, in physics nomen-clature. By convention in the geophysical community,however, gauss is the unit in cgs of magnetic intensity.In any event, numerically, 1 gamma =lo+ gauss =lo+oersted = 10-9 webers/M=10-9 tesla.)The earths total field intensity is not perfectly symmetricabout the geographical north pole. First, the north mag-netic pole is in northern Canada more than 1000 milesfrom the geographical pole (note again Figure 2). Also,the earth cannot exactly be represented by a single bar

magnet, but has numerous higher order poles and verylarge-scale anomalous features owing to unknown char-acteristics of the generating mechanism in the earthscore. In addition, the solar wind or constant flux of par-ticles and electric currents from the sun distort the fieldlines from what is shown in Figure 2 to a more or lesstear-drop shape with the blunt end towards the sun. Thelast, but for the purposes of this Manual, most relevantdeviation from a symmetric field is the anomalous set offeatures in the earths crust caused by local variationsin the magnetic minerals or other features of interestwhich distort the local earths magnetic field.

Time Variations

The variations described above all refer to the spatialvariations in the earths magnetic field, but there arevariations in time as well. Significant time variationswith periods of seconds, minutes and hours are the direct

or indirect effect of the solar wind referred to above asit distorts the magnetosphere or external magnetic fieldof the earth. Daily or diurnal variations are primarily seenduring the local daylight hours shown for typical daysin Figure 5. Diurnal variations are not predictable, mayexhibit changes as large as 100 gammas or more andare often removed from portable magnetometer data bymethods described in Chapter IV. Superimposed uponthese diurnal variations are more short-period phenomenacalled micropulsations (Figure 6) which are more randomin behavior, generally smaller in amplitude and may occurat any time of the day or night. Micropulsations occurin a broad range of periods between 0.01 seconds upto several tens of minutes with amplitudes from a thou-

sandth of a gamma to several tens of gammas.

Of still greater concern as a possible source of erroneousdata are magnetic storms occurring as often as severaltimes per month with their onset suddenly and simul-taneously throughout the world. Such storms may exhibitvariations of up to several hundred gammas and maylast one day or up to several days. (See figure 7.) Forvery important field measurements, particularly for higherresolution measurements, a recording base station orreference monitor is often used which is examined at thestart of each day for an indication of magnetic stormactivity and also for subsequent removal of the diurnalvariations from field data using time as a correlation.

The earths internal or main field also changes slowlyover years, tens and thousands of years through whatis termed the secular variation. The inclination, intensity


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EARTHS FIELD MAGNETISM 7

MID-NORTHERN AND MID-SOUTHERN LATITUDES

HOURS: 0

DAYS:

1200 2400 1200 2400 1200 2400

DAY 1 DAY 2 DAY 3

EQUATORIAL LATITUDEAFigure 5. Typical Diurnal Variations in Total Field Intensity

10 MINUTES

Figure 6. Typical Micropulsations

50 GAMMAS

Figure 7. Typical Magnetic Storm


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and even the location of the poles varies slowly at a raterelevant, in the context of this Manual, only to observa-ory and archival interests. From time-to-time through

geologic history, the main field has even reversed andhe consequences of these events are extremely impor-ant for a number of portable magnetometer applicationscovered in the Manual.

Magnetic Minerals and Iron

The application of portable magnetometers as treatedherein has, as its primary objective, the identificationand description of spatial changes in the earths field.The time changes described above simply representnoise or interference in the measurements of interest.The spatial variations or anomalies to be mapped forhese applications are those which might occur overseveral feet or several thousands of feet and are usuallycaused by an anomalous distribution of magnetic miner-als or by iron objects or cultural features which may beof interest. The anomalies from naturally occurring rocksand minerals are due chiefly to the presence of the mostcommon magnetic mineral, magnetite (Fe Fe*O,), or itsrelated minerals, ulvospinel, titanomagnetite, maghemite,

etc. which will collectively be referred to as magnetite,a dark, heavy, hard and resistant mineral. The rust-colored very common forms of iron oxide are not usuallymagnetic and are seldom related to the source of magne-ic anomalies. Other magnetic minerals which occur to aar lesser extent are ilmenite, pyrrhotite (with sulphidemineralization), and others of even lesser consequence.

All rocks contain some magnetite from very small frac-ions of a percent up to several percent, and even severalens of percent in the case of magnetic iron ore deposits.The distribution of magnetite or certain characteristicsof its magnetic properties may be utilized in explorationor mapped for other purposes. Iron objects in the earths

magnetic field, whether something buried or intentionallyplanted for subsequent retrieval, would also create adetectable magnetic anomaly. Cultural features associ-ated with mans habitation can frequently be detectedhrough magnetic surveys owing to the contrast in mag-netite associated with numerous artificial features suchas man-made structures, voids, or the enhanced mag-netic effects ofbaked clays and pottery (see Chapter VII).

nduced Magnetization

Magnetic anomalies in the earths magnetic field arecaused by two different kinds of magnetism: inducedand remanent (permanent) magnetization. Induced mag-netization refers to the action of the field on the material

wherein the ambient field is enhanced and the materialtself acts as a magnet. The magnetization of such ma-erial is directly proportional to the intensity of the am-bient field and to the ability of the material to enhancehe local field-a property called magnetic susceptibility.

The induced magnetization is equal to the product ofhe volume magnetic susceptibility, k, and the earths orambient field intensity, F, or

Ii = kFwhere Ii is the induced magnetization per unit volumen cgs electromagnetic units, and F is the field intensityn gauss. (Note: in some texts, the specific magneticsusceptibility or susceptibility per unit weight (gram) isused) For most materials, k is much less than 1 and, inact, is usually ilO- cgs or smaller. If k is this small andpositive, the material is said to be paramagnetic and,

when negative, diamagnetic. For magnetite, k is approxi-mate/y 0.3 cgs and is ferrimagnetic while for iron alloys,k may vary between 1 and 1,000,000 and such materialsare called ferromagnetic. Both ferrimagnetic and ferro-magnetic susceptibility are also a function of the fieldintensity in which they are measured. In all cases, in thisManual, the field intensity is assumed to be the ambientearths field intensity between 0.3 and 0.6 gauss

A parameter similar to k is the magnetic permeabili ty,cc, which is the ratio of the magnetic induction, B, tothe field intensity, F (Magnetic induction is the magne-tization induced in the material). B includes not only themagnetization of the material, but also the effect of thefield itself and is expressed by

B = F+4171where B is in gauss. Therefore as stated above,

and

p=$

/J = 1 +4nkThus when k is very small, as in air, /.J x I and when kis 0.1 or larger I-( is generally one order of magnitudelarger. The susceptibility k can be thought of as theabsolute ability and p the relative ability of a materialto create local magnetization. The measurement of per-meability is most often used for materials where ~1 ismuch greater than 1, typically iron, steel and otherferromagnetic alloys.

Inasmuch as magnetite and its distribution is of suchgreat importance for a number of these applications, itis important to understand its relation to common rocktypes. The susceptibility k of magnetite was given as

approximately 0.3 cgs which may actually vary between0.1 and 1.0 depending upon its grain size and otherproperties. The magnetic susceptibility of a rock con-taining magnetite is simply related to the amount ofmagnetite it contains. For example, rock containing 7%magnetite will have a volume susceptibility of 3 x 10-3 cgs,etc. Typical susceptibilities of rocks are given below, butmay vary by an order of magnitude or more in most cases:

altered ultrabasic rocksbasaltgabbrograniteandesiterhyoliteshaleshist and othermetamorphic rocksmost sedimentary rockslimestone and chert

-10-4 to 10-z cgs- 1 0 -4 to 10-3- 1 0 - 4

-10-5 to 10-3- 1 o-4

-10-5 to 10-4-10-3 to 10-4

-10-4 to 10-S-10-6 to 10-S-1 O-6

Typically, dark, more basic igneous rocks possess ahigher susceptibility than the acid igneous rocks andthe latter, in turn, higher than sedimentary rocks.

Remanent or Permanent Magnetization

The remanent or permanent magnetization, I,, (the formerascribed to rocks, the latter to metals) is often the predom-inant magnetization (relative to the induced magnetiza-tion) in many igneous rocks and iron alloys. Permanentmagnetization depends upon the metallurgical propertiesand the thermal, mechanical and magnetic history of


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EARTHS FIELD MAGNETISM 9

the specimen, and is independent of the field in whichit is measured. Magnetite may have a remanent magne-tization, lr, of perhaps 0.1 to 1.0 gauss, ordinary ironmay have a permanent magnetization between 1 and 10,and a permanent magnet may be between 100 and 1,000gauss or larger. Chapter VI will describe simple methodsfor measuring both the remanent ,and induced magneti-zations and the magnetic susceptibility of rocks andmiscellaneous objects. Chapter VII more fully describes

the magnetization of iron objects.

The remanent magnetization is of great importance inmapping and interpretation, and in the fields of paleo-magnetism, archaeological exploration, and archaeomag-netism. The remanent magnetization of magnetite is asstated independent of the present earths field. By andlarge, the high values of remanent magnetization arerelated to the effects of heating, whether naturally heated,as in the case of igneous rocks, or artificially heated, asin the case of baked clay, pottery, and other man-madeobjects found in archaeological sites. Prior to suchheating, small regions, called domains, within each mag-

netite crystal would be more or less randomly-oriented.During heating, particularly at high temperatures, the

domains reorient themselves, which upon cooling, tendto align themselves more or less in the direction of theambient magnetic field and thus parallel to each other,thus creating a net magnetization fixed with respect tothe object. This remanent magnetization may be as muchas 10 or more times greater than the induced magnetiza-tion for many rock types. Thus, the net magnetizationmight be considerably higher than would be indicatedmerely by consideration of the susceptibilities listed

above.

The remanent magnetization of a rock or object may ormay not be in the same directionnas the present earthsfield for the object may have been reoriented and becausethe earths field is known to have changed its orientationin geologic and even historic time. Rocks are frequentlyreversely magnetized so that measurement of this rem-anent magnetization is a useful aid to interpretation ifthe rocks which produce an observed anomaly are,indeed, accessible. The fields of paleomagnetism andarchaeomagnetism in particular depend upon the precisedetermination of the orientation of the frozen paleo-field as it is measured in a given rock or other specimen,

and methods for measuring such will be described inChapter VI.


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IV.

FIELD PROCEDURES AND DATA REDUCTION

Magnetic Cleanliness and Sensor Positions

Most of the applications for portable magnetometersrequire that the operator be relatively free of magneticmaterials on his person. The importance of checkingoneself cannot be over-estimated if measurements onthe order of 1 gamma are desired. In field surveys, theusual magnetic material one may have may include, ofcourse, the obvious such as a rock pick, Bruntoncomp-pass, pocket knife, or instrument console and the not-so-obvious effects of the pivot in eyeglasses, the pants clip atthe top of mens trousers, the light meter in a camera,the magnet in the speaker of a tape recorder, metal in aclipboard, some mechanical pencils, some keychains,

and the steel shank in ones shoes or boots. Of course,some of these items cannot be altered or left behindand some are not significant in any event. The sensoritself should be kept clean to avoid possible contamina-tion by magnetite-bearing dirt on the sensor surface.In order to check the heading effect, i.e., the effect oforientation on the observed field intensity during a fieldsurvey, the operator can take readings at each of thefour cardinal directions while pivoting about the positionof the sensor and note the changes. If the maximumchange is typically less than 10 gammas, the averagereadings on a line will probably not be affected by morethan 5 gammas and individual readings by less than thisinasmuch as readings along the profile are more-or-less

along a given heading f perhaps 30 about one orienta-tion. If a sensitivity of 1 gamma is desired, the headingerror should be less than several, preferably 2 gammasor less and depending upon the desired sensitivity, theoperator should make some effort to face in the samedirection, if possible, for all readings on a given traverse.

The sensor for a proton magnetometer may be carriedon a 8-foot (2.2 meter) staff, on a backpack, on anextended staff to 12 feet (4 meters) or more as necessary,or by a second person as represented in Figure 8. Thesensor on an 8-foot staff is by far the most commonmeans for field measurements removing the sensor suffi-ciently far from the console and from the operator so as

not to be much affected by normal items of clothing, etc.The purpose of mounting a sensor on an extended 12foot or longer staff is to remove the sensor from the

ti

locally disturbing effects of highly magnetic surfacematerials, such as surface laterite, glacial till, or highlymagnetic outcropping rocks. The sensor may also beraised in the case of very high magnetic gradientswhich would otherwise ruin the magnetometer signaland prevent any reading whatsoever (see following sec-tions). An additional reason for an extended staff will bedescribed in Chapter VIII in reference to vertical gradientmeasurements. There are also occasional reasons for asecond person carrying the sensor while the first personcarries the console together, perhaps, with magneticor other materials that must necessarily be on his personsuch as pick, tape recorder, another instrument or rocksamples.

The sensor may be carried in a backpack pouch formore convenient field operation where 5 or 10 gammasensitivity is all that is desired, but care should be takento check the effects of the batteries and console (par-ticularly the very magnetic alkaline batteries). The back-pack pouch frees the hands for taking notes, pushingaside the underbrush and, in general, balances the loadof the console and decreases fatigue.

Operational Considerations

Valid Readings Vs. Naise

It is important to establish that, in fact, the magnetometer

is providing valid readings. The simplest means of con-firming that what is being observed is a magnetic fieldreading and not random, meaningless instrument read-ings (i.e., noise) is to take several readings in successionin one location without moving anything, and note therepeatability. Successive readings should be withinf 1 gamma, + 0.25 gamma or f 1 count for whatever thesensitivity setting. Valid readings should not,, underany naturally-occurring circumstances including mag-netic storms, vary by as much as* 10 orf 100 gammasin a few seconds; if such is observed, the readingsrepresent either noise or a degradation of the signal-to-noise ratio with the observed corresponding loss in termsof sensitivity. Under certain circumstances even succes-

sive readings repeating to within several gammas maystill represent noise. To confirm that these readings areindeed magnetic field, simply kill the signal by placing,

Figure 8. Sensor Carrying Positions

11


14/58


momentarily during the reading, something magneticadjacent to the sensor such as ones shoe, watch, certainrocks, etc. Random readings varying by 10 or 100 gam-mas or more would then be observed in addition to theirdeviating considerably from the readings without theobject present. Another but less certain method is totake readings at intervals of increasing distance from anobject or location known to produce a magnetic anomaly.

Typical reasons for a proton magnetometer not producingvalid readings may be: electrical noise from AC powerlines, transformers or other radiating sources; high mag-netic gradients from underlying rocks, nearby visible orhidden iron objects, fence lines or improvised iron hard-ware improperly used nea the sensor; improper orienta-tion of the sensor (even when omni-directional) ;expiredbatteries, incorrect range setting or instrumen tfailuresbroken or nearly broken sensor cable, and other mal-functions usually described in the instrument operatingmanual.

Valid but distorted readings may result from severalother conditions including the above effects of highmagnetic gradients, magnetic dirt or other magneticcontamination on the sensor and any magnetic bias onthe operator. Time variations (Chapter III and following)and the effects of direct current in distant power linesand trains (Chapter IX) can also distort magneticobseryations.

Sensor Orientation

According to the theory of operation of the proton mag-netometer, the total intensity, measured as the frequencyof precession, is independent of the orientation of thesensor. Th e amplitude of the signal, however, doesvary (as sir+@ with the angle between the directionof the applied field within the sensor and the earths

field direction. Variation of signal amplitude does notnormally affect the readings unless there is simply insuf-ficient signal to be measured accurately, i.e., a minimumsignal amplitude is required above which a variation inamplitude does not affect the readings.

Ideally, the applied field in the sensor should be atright angles to the earths field direction. The directionof the applied field is governed by the configuration ofthe polarizing coils in the sensor which are commonlyeither solenoids (cylindrical) or toroids (ring or dough-nut-shaped). The solenoid produces an applied fieldparallel to its axis, whereas the toroid produces a fieldwhich is ring-shaped about the axis of the toroid (con-sult the instrument operations manual to determine the

direction of these axes with respect to the sensor hous-ing). Solenoids are used because they produce some-what higher signal than a toroid and are less perturbedby electrical noise, whereas a toroid is inherently omni-directional. In the ideal case, a solenoid should be heldhorizontal and in any direction in a vertical field, andshould be held vertical in a horizontal (equatorial) fieldfor maximum signal amplitude. A toroidal sensor shouldbe held with its axis vertical in a vertical field, and point-ing north in an equatorial field to obtain maximum sig-nal. A field which dips greater or less than 45, shouldbe treated as though it were a vertical or horizontalfield respectively.

Instrument ReadingsMeasurements are normally made at regular intervalsalong a grid or otherwise selected path whose locations

are noted for subsequent plotting. Simple pacing isusually adequate with readings every 6, 10, 50, 100,500,or even 1,000 feet (2 to 300 meters), as anomalies, field,and either geological or search requirements dictate.Traverses may be selected along pathways or otheraccessible routes and occasional locations noted on anaerial photograph or map using paced distances inbetween. The density of readings along the traverseshould be related to the wavelength of anomalies of

interest such that several readings are obtained for anysuch anomaly. A single trial line with relatively densestations is usually attempted first to determine the re-quired station density. It is important never to hold themagnetometer sensor within one or two feet of theground, if possible, in order to avoid effects of minorplacer magnetite which usually collects on the surfaceof the ground, and also to avoid the effects of micro-topography or outcropping rock surfaces.

Readings may be noted in a field notebook or, if desired,on a miniature tape recorder, but care must be takento magnetically compensate the speaker magnet andmotor following the theory given in Chapter VI if one is

to use a recorder. The convenience of the recorder isthat only one hand is needed and the data may beplayed back for fast, convenient plotting.

Correction for Time Variations

Some ground magnetic surveys require correction fordiurnal and micropulsation time variations. Correctionis required if the anomalies of interest are broad (thou-sands of feet) and typically less than 20 to 50 gammas,or if the profile lines are very long, or if the objectiveof the survey is a good magnetic contour map expres-sive of deep-seated anomaly sources. Also, if the surveyis performed in the high magnetic latitudes in the auroralzone where typical micropulsations are 10 to 100 gam-mas, correction for such variations would be necessary.On the other hand, if one is merely interested in profileinformation of anomalies of several hundred gammas orif the anomalies are only 20 gammas but can be traversedcompletely in less than 5 minutes, no time variationcorrection is needed. Perhaps most surveys fit the lattercriteria an ddo not actually require any such correctionfor time variations.

The simplest method of correcting for time variationsinvolves repeated readings in the same orientation atthe same station at different times during the survey.If a smooth curve is drawn through the readings plottedas a function of time (every hour or so), these valuescan be subtracted from all other readings provided thateach reading also includes the time at which it wasobserved. To avoid an extremely long and repeatedwalk to a single reference station, it is also possible todouble-back to take a second or third reading on eachgiven traverse to determine at least the time variationsfor that traverse. Still another technique is to emulatewhat is done on aeromagnetic surveys, namely, obtainrapidly acquired measurements on tie lines or lineswhich cross the principal traverse lines at each endand perhaps in the center. The stations common to eachtraverse and tie lines should be known and occupiedwhile facing the same direction to avoid heading errors.The simplest method for using these tie lines is to make

each intersection agree by linearly distributing the erroron each traverse line and holding the tie line valuesfixed-provided the ti eine data were acquired rapidly.


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FIELD PROCEDURES AND DATA REDUCTION 13

A local recording base station, i.e., diurnal station moni-tor, is the most ideal method and certainly the mostaccurate for removing time variations. The time varia-tions can readily be removed from each reading, againassuming that the time is noted for each reading on thetraverse to within a minute or so of the base station.The base station should not be further away than 100miles from the area of the survey for agreement withina few gammas and should be positioned more than 200

feet away from local traffic and other disturbances (seeChapter VII). The diurnal base station, if left to continuerecording during each evening, can indicate magneticstorms in progress and may be examined at the startof a survey day to determine if any useful measurementscan actually be obtained during such conditions. Duringa magnetic storm, it is best not to obtain field data withthe objective of removing the storm variations as thesurvey magnetometer and base station may not agreebetter than 5 or 10 gammas.

High Magnetic Gradients

In the case where an extremely high magnetic gradientdestroys the signal as evidenced by successive non-

repeating measurements, it may be necessary to raisethe sensor up to 10 or 12, sometimes 15 feet in orderto move the sensor to a region of lower gradient. Thiswill only happen over outcropping or nearly outcroppinglarge masses of perhaps altered ultrabasic rocks, mag-netic iron ore deposits or ore bodies containing a largepercent of pyrrhotite and in the near vicinity of buriediron objects in the applications for search. Such anevent would only occur if the gradient exceeds severalhundred gammas per foot. If the span of high gradientis not too wide, it may not actually be necessary toobtain measurements precisely at the highest gradient.Measurements on either side of the anomaly can beextrapolated or be used to at least indicate the contacts

of such a highly magnetized formation. Furthermore,as the signal disappears and the readings diverge con-siderably from f 1 or 2 counts, it may be worthwhile tonote approximate indications of magnetic field gradientwhich on some instruments is displayed on the frontpanel as signal amplitude (which is a function of gradient).In areas of highly magnetic surface conditions, as notedin a previous section but where a signal is still obtained,another alternative in acquiring meaningful data otherthan that of using an extended staff would be to make2 to 5 measurements per station, for example, at thepoints of a cross centered at the actual primary stationlocation. The average of these readings would later beused to draw a profile. In this way, some of the surface

noise is averaged out.

In the absence of anomalous surface conditions and forreasons more fully described in Chapter VIII, it may beuseful for both geological and search applications tomeasure the vertical gradient of the total field. Thevertical gradient is obtained by making 2 total fieldmeasurements, one over another, taking the differencein the readings and dividing by the distance betweenthem.

Data Reduction

The profiles when plotted should be smoothly varyingand expressive of the anomalies of interest. (NOTE:The nature of the disturbances or anomalies of interest,theirwidth character, signature, and amplitude are dis-cussed in Chapter V, following.) Should there be anexcessive amount of such geologic/magnetic noise, at a

wavelength much shorter or much longer than is ofinterest, it is possible to apply simple filtering or smooth-ing techniques to facilitate interpretation of the profile.As a rule of thumb, never remove or filter out anomalieswhose wavelength is on the order of the depth to sourcesof interest. A number of advanced techniques for dataenhancement or filtering as employed in airborne sur-veys or well-gridded ground surveys will not be dis-cussed within the scope of the Manual but are listed

to acknowledge their existence: vertical derivatives,upward and downward continuation, reduction-to-the-pole, bandpass filtering, trend surface filtering, spectralanalysis, trend enhancement, magnetization filtering,and others most of which are applied to two-dimensionaldata.

Profile Smoothing

Anomalies of very short wavelength (much shorter thanthe probable depths to sources of interest) may bepresent and caused by the magnetic effects of the mag-netometer operator, or simply by surface magnetizationcontrasts in the surface or near-surface materials asmentioned earlier. In removing such effects, the eye

itself tends to enhance what one is seeking. Anothersimple and obvious method is of course to pencil ortrace through the noise. A more objective technique isto apply a running average or weighted running averageto the data (see Figures 9 and 10). For a 3-point weighted

Figure 9. Profile Smoothing

MAGNETOMETER READINGS

. :( AX1 d + sx2 + cx )

ABOVE. 3 POINT WEIGHTED RUNNING AVERAGE

FOR SIMPLE RUNNING AVERAGE

AtStC- = a

3

FOR 5 PT. WEIGHTED RUNNING AVERAGE

A+ZS+4C+2D+E10 --c

Figure 10. Running Averages


16/58


running average, for example, one would multiply thevalue at a given station by 2, add the values of the twoadjacent stations, divide the sum by 4. This value is thenset aside for that station for later recompilation of a newprofile while advancing to the next station to performthe same procedure (see figure 10). A 5-point runningaverage might utilize a weighting factor of 4 for the cen-tral point, 2 for each adjacent point, and 1 for the out-side points while dividing by 10 to obtain the averaged

value. More sophisticated techniques are also possiblesuch as polynomial curve fitting, least squares, digitalbandpass filtering, etc. The number of points or intervalover which the averaging or filtering is to be performedfor removal of such noise should be much shorter(perhaps 11~ to I/& than the anomalies of interest.Removal of Regional Gradients

In most cases, the anomalies of interest usually appearsuperimposed on a much broader anomaly which is notof interest. This broader anomaly, or regional gradient,due to the main earths field or very deep or distantsources, may appear simply as a component of slope intne curve and although it is subjectively determined, is

often removed from the data in order to better examinethe anomaly. This gradient is removed from a singleprofile as shown by Figure 11 by drawing a straight lineor broadly-curved line through the non-anomalous por-tions o f the curve. The values are then subtracted ateach station and replotted to present the residualvalues, hopefully expressing only the anomalies ofinterest which in this case would be the anomaliesoccurring at the shallower depths. The vertical gradient,measured according to the methods prescribed inChapter VIII, also serves to remove or largely reducethe regional gradient.

Contour Maps

Most survey traverses are not sufficiently close nor well-arranged in plan to allow the compilation of a contourmap. Such is usually the case when only mineral explor-

ation data are desired, in broad reconnaissance surveys,or when surveying in extremely rugged terrain wherelarge areas are otherwise inaccessible. Profiles are, infact, usually adequate particularly for anomaly sourceswith very long, horizontal dimensions. Contour maps arenevertheless useful in cases where little is known of thegeology or magnetic sources, where the anomalysources are either small or extremely large, or for ascer-taining, on a more objective basis, the distribution of the

anomalous masses or very subtle longer wavelengthfeatures. Many surveys for search also require broadcoverage and perhaps contour map presentation. Analternative to the construction of contour maps forbroad two-dimensional coverage is the presentation ofoffset profiles or profiles plotted on abscissae whichalso serve to show the location of the traverse linesdrawn on a map.

Constructing a contour map requires that large effectsfrom diurnal variations or the heading effects, if any, beremoved; that is, that there be a single datum level forall traverses or readings. The process of removing theregional gradients, as described above, frequentlyserves to remove these other sources of errors as welland is satisfactory as long as one is not interested inthe longer wavelength anomalies removed as part ofthe gradient.

The following guide should be useful to one not familiarwith the techniques of constructing a magnetic contourmap, or plan view of the anomalous total intensity. Acontour or iso-intensity map is analogous to a topo-graphic map and is a map on which are drawn lines(contours) of equal intensity, at convenient and regularintensity intervals, as would be observed were a mag-netometer used to occupy every point on the surfaceof the ground. The contoured values are at best extrapo-

lations and interpolations across areas where measure-ments are not actually taken. Such a map is drawnwith the knowledge that the magnetic field is smoothly

LINEAR REGIONAL

NON-LINEAR REGIONAL

Figure 11. Removal of Regional Gradient

RESIDUAL


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FIELD PROCEDURES AND DATA REDUCTION 15

varying and on the assumption that one is interestedonly in broad features expressed by such a map. Fea-tures much smaller than the spacing between adjacenttraverses should be examined on a profile basis only andshould not be sought nor included on a map presentation.

Construction of a Contour Map

Given a set of readings obtained on a traverse, the time

variations, if significant, should be removed, perhaps theregional gradient removed and the profiles smoothed.Values are then selected from these smoothed profilesat widely-spaced intervals not less than, say, I/* or l/d thespacing between adjacent traverses or at similarly spacedbut significant points on the profile, namely, maxima,minima, inflection points, etc. In other words, the valuesto be contoured should be more-or-less equally distrib-uted in plan view. Anomalous features which obviouslyextend across several traverses might be included also.The total intensity values thus selected and representa-tive of the principal features are posted at their properlocations on a base map made of material which willsupport numerous erasing of penciled lines and includingreferences to location.

Examine the dynamic range of the values and select 5or 10 intensity levels through this range at convenientvalues such as every 20, 100, or 1000 gammas. Drawthese contours according to the instructions below andthen fill in the intermediate contour lines, i.e., every 10,50, or 500 gamma contours, depending upon whichcoarse valuse above were originally selected, until con-

/

tours appear in all segments of the map. Magneticintensity values and contours should, in theory, besmoothly varying and should thus be smoothed at thelater stages of contouring by removing sharp bends orcorners. After such smoothing, other contour lines asneeded to cover the map adequately are carefully drawnbetween the fair-drawn contours and appropriate labelsapplied. In areas of steep gradients, only a few coarsecontour lines are drawn to avoid numerous and insigni-

ficant fine details. Since closed contours (closures)appear the same for maxima and minima, they are dif-ferentiated by applying hashure marks or other indica-tions on the inside of the minima.

The position of the various contours is selected bymanually (eye and mental calculation or by using pro-portional dividers, although not really necessary) inter-polating linearly between all the neighboring values asshown in Figure 72. In this case, it was decided to drawcontours at 10 gamma intervals. The contour line neardata point value 91 would subsequently be smoothedto pass through this data point following the guidelinesgiven above.

Contour lines should never cross nor pass between pairsof data points which are both higher or both lower thanthe value of the contour. Also in some regions of zeroor near zero gradient such as at a saddle point (regionbetween two adjacent maxima or minima), there existsan ambiguity in the direction of the lines. However, itdoes not matter under such conditions which of the twopossible sets of contours are drawn.

0

/,081/*SO 9or 89

---*!/

l 75//

\ /kc0 80 l 84 /0

80+c 0/ - g o -/ ) I 0

l 96

90: 0 0

l 85-,/ -@ 910 /0\9c / I/100,

I. *\

90, 0100 / I/ 1 / * - l 101

l 97 //l 92

096 D A T A* 90 MANUALLY INTERPOLATED POINT- - - CONTOURS DRAWN THROUGH

INTERPOLATED POINTS

Figure 12. Interpolation and Contouring


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v.INTERPRETATION

Introduction

Total magnetic intensity disturbances or anomalies arehighly variable in shape and amplitude; they are almostalways asymmetrical, sometimes appear complex evenfrom simple sources, and usually portray the combinedmagnetic effects of several sources. Furthermore, thereare an infinite number of possible sources which canproduce a given anomaly. The apparent complexity ofsuch anomalies is a consequence of the net effect ofseveral independent but relatively simple functions ofmagnetic dipole behavior. With an understanding ofthese individually simple functions however, and givensome reasonable assumptions regarding the geology,

buried object or whatever other source one is seekingto understand, a qualitative but satisfactory interpreta-tion can usually be obtained for most anomaly sources.

The interpretation, explanation and guide presentedhere is directed primarily towards a qualitative interpre-tation for both geological reasons as well as search

applications, i.e., an understanding of what causes theanomaly, its approximate depth, configuration, perhapsmagnetite content or mass, and other related factors.But even if qualitative information is derived from thedata, it is important to have applied a reasonable amountof care in obtaining precise measurements. Quantitativeinterpretations are possible, but are applied more to air-

borne data, entail relatively complex methods for depthdetermination, and are the basis for a relatively largebody of literature on the subject, references to whichare given in the Manual.

An anomaly represents a local disturbance in the earthsmagnetic field which arises from a local change in mag-

DIPOLE

netization, or magnetization contrast as it is termed. Aprofile, for example over a very broad uniformly mag-netic surface, although magnetic itself, will not exhibita magnetic anomaly as there is no local change inmagnetization. A local increase or even decrease on theother hand would constitute such a change and producea locally positive or negative anomaly.

The observed anomaly expresses only the net effect ofthe induced and remanent magnetizations which usuallyhave different directions and intensities of magnetization.Since the remanent magnetization is so variable andmeasurements of its properties seldom made, anomalies

are all interpreted in practice as though induced magnet-ization were the total source of the anomalous effects.

Asymmetry

The asymmetrical nature of total field anomalies is pri-marily a consequence of the directions of the field linesof the locally created magnet or source and the earths-field-component nature of a total field magnetometer inthe usually-inclined direction of the earths magneticfield. Recall that a total field magnetometer measuresonly the component of any local perturbation which isin the direction of the earths magnetic field at that point.

Anomalies in the earths field, whether created by inducedor permanent magnetization, exist as arrangements of

magnetic dipoles, monopoles (effectively), lines of dipolesand monopoles and sheet-like distributions of such poles.It is important therefore to understand the nature of thedipole or monopole field as it will be shown that a sum-mation of such elementary forms will explain the mostcomplex characteristics of anomalies and facilitate theirinterpretation. Notice, for example in Figure 73, the con-

MONOPOLE


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Figure 14. Effect of Depth on Anomaly Width

figuration for such fields as they would appear if onewere to measure the direction of the anomalous field.

Depth Dependence

Another significant characteristic of a magnetic anomalyis its variation with the depth between the magnetometerand source, the deeper the source, the broader theanomaly as expressed in Figure 74. It is this propertywhich enables one to determine the approximate depthto the source independent of any other information con-cerning the source. If one familiarizes himself with only

one subject in this discussion on interpretation, it shouldbe the general characteristics of anomaly wavelength,or width, as a function of depth. A knowledge of thissubject allows rapid and easy interpretation of anomaliesof interest when numerous anomalies arising from vari-ous depths appear in the observed total intensity profile.

Other Anomaly Shape Factors

Other factors which affect the anomaly shape and ampli-tude are the relative amounts of permanent and inducedmagnetization, the direction of the former, and theamount of magnetite present in the source comparedto the adjacent rocks. The actual configuration of thesource, that is, whether it is narrow, broad or long in

one dimension and its direction in the earths field, alsocontrol the anomaly signature.

Geological Models

Geological anomalies are interpreted in terms of muchsimplified geological models which very much facilitateinterpretation procedures. The first simplification is theassumption that magnetization is uniform within someelementary prismatic form and that the magnetization isdifferent outside this form, i.e., there is a magnetizationcontrast. Typical of the kinds of geologic sources thatare assumed to cause anomalies are those which areshown in Figure 15.

DIKE DIKE FAULT

As was stated, in any potential field method the givenmagnetic signature can be produced by an infinite com-bination of sources so that there is no unique interpreta-tion. For example, the same anomaly could be producedby the peculiar distribution of magnetite (unrealisticgeologically), and a uniform distribution of magnetitewithin the prismatic form (realistic), both of which areshown in Figure 76. It must be emphasized that not onlyare simplifications required, but a reasonable geologicframework must be used as a guide when consideringthe various possible sources. A typical set of anomaly sig-

natures of various sources might appear as in Figure 77.Elementary Dipoles and Monopoles

Since anomalies are explained herein in term of variousarrays of dipoles and monopoles, it is important to exam-ine their geometry and intensity characteristics. A mag-netic dipole produces a field with imaginary lines of fluxas shown in Figure 78. The intensity of the field, whichis proportional to the density of the flux lines is drawnas lines of equal intensity to express this relationship.From Figure 78, notice that 1) the intensity of the dipoleis twice as large off the ends of the dipole as it is at thesame distance off the side of the dipole. This explains,for example, why the earths magnetic field is approxi-mately 30,000 gammas at the magnetic equator and

60,000 gammas at its poles; 2) the direction of the fieldoff the side of the dipole is parallel to the dipole itself,but opposite in sense; 3) the direction of the tangent ofthe field lines of a dipole are parallel along any radialline from the dipole.

A monopole has field lines which point radially in or outfrom the positive or negative monopole respectively. Theintensity is constant at a given distance and any directionfrom a monopole. In actual fact, there are no magneticmonopoles, but only dipoles whose ends are far apart.For all practical purposes, however, monopoles exist interms of the distance to the source and such geologicalconfiguration as shown in Figure 73.

Jijjijs f+I 4

ANTICLINE\J) ( M O D E L ) ORE BODY GRABEN (VOID)Figure 15. Geological Model Representations of Common Magnetic Anomaly Sources


20/58

INTERPRETATION 1999Figure 16. Possible Geologic Sources Producing Same Anomaly n

Having outlined the qualitative geometry of the intensityT from a dipole, the quantitative aspects can be con-sidered as follows:

The intensity, T, from a dipole can be expressed as

T2!r3 along the axis, i.e., off the end of the dipole,and T =E along a line at right angles to the dipole,i.e.,

r3 off the side of the dipole,and for a monopole

T =$- in any direction from a monopole, whereM = magnetic moment and r is the distance to the pole.A more detailed mathematical formulation for the inten-sity due to a dipole is given subsequently in this Chapter.

LINES OF FLUX (-4ANDLINES OF EQUAL INTENSITY I- -_)FOR A DIPOLE

Figure 18.

Figure7 7. Typical Anomalies for Simple Geologic Models

Simplified Method for Total Field Signature

From the above description of a dipole and monopoleand with the knowledge of the earths-field-component-nature of the total field magnetometer, it is possible tosketch the signature of an anomaly for any given orien-tation of the dipole (orientation caused by field direction,the direction of remanent magnetization, or by the con-figuration of the geology). It is helpful to draw suchsignatures at various inclinations of the magnetic fieldto understand where the source swould be located withrespect to the signature, the dip of the magnetizationproducing the anomaly, and even for information relatedto the depth of the source. Remember that all anomaliescan be considered as caused by various distributions ofdipolar and monopolar sources and it is possible toproduce any anomaly simply by the super-position of

such dipole ormonopole signatures derived here.

LL L&

L

L

I - _ I1 .\ \

\ - .I_I

I

L

LLL -


21/58


DIPOLES

DIPOLES

Figure 19.

Earths Field Component Behavior

This method of predicting or drawing the anomaly sig-nature depends upon one property of the field, namely,nclination, and three properties peculiar to the dipole

or monopole source, whichever is assumed. The dip ofhe earths field is first considered because this is the

direction, the only direction, of the components of anyocal magnetic anomalies which are measured by a totalield magnetometer. (If one is using a vertical componentmagnetometer, this guide still applies except that insteadof using the earths field as the direction of measurement,simply use the vertical.) In other words, the magneto-meter will only measure the component of a local per-urbation in this direction, i.e., as projected into this

direction. See Figure 20.

Dipoles vs. Monopoles vs. Arrays of Poles

The decision to use dipoles, monopoles, or other con-gurations as the model is based upon the manner in

which the earths field induces a local field and this inurn depends upon the configuration of the geologicbody which exhibits the magnetization contrast and theirection of the field. For example, a long body which

nearly parallels the earths field will tend to be magne-zed along its long dimension. Furthermore, if the bodys sufficiently long with one end near the magnetometer,he anomaly will appear as a monopole seeing only thepper pole with the lower pole removed effectively to

nfinity. If the same long, thin body were normal to theeld, it would then be magnetized through its thinestimension producing the sheet-like array of dipoles ashown in Figure 19.

One may wish to draw on the typical models depictedn Figure 15, the array of poles from a uniform earthseld at various inclinations and orientations of the source.

Whether the monopoles or the dipoles (and its equiva-ent line or sheet distributions) are close or far apart,etermines if the model is to be considered a dipole or

monopole, respectively (see, for example, Figure 34).

Configuration of Field Lines

The first property of the dipole or monopole which is tobe considered is the configuration of the field lines (see

Figure 13). When superimposed upon the componentwhich is measured by the total field magnetometer, itcan be seen that the relative lengths of the disturbancevectors that are measured are those shown in Figure 21for an induced dipole and monopole source. It is therelative length of these disturbance vectors drawn alongthe total field direction that is measured, each disturb-ance vector, in turn, weighted by the intensity functionsdescribed below.

Dipole and Monopole Fall-Off Factor

The next factor to be considered is the variation ofintensity with distance, i.e., l/r3 and l/r* factors for thedipole or monopole fields respectively and as expressedin the preceding equations. The relative intensity fordipoles or monopoles as a function of distance to theircenters as would be observed along a traverse is pre-sented in Figure 22 and described mathematically underAnomaly Amplitude below. This factor multiplies thelength of net vectors in Figure 21.

Dipole Factor-of-Two

The last consideration really only applies to the dipoleand that is a factor of 2 when one is off the end of thedipole compared to a position off the side. In otherwords, at a given distance, the intensity varies by a fac-tor of 2 as a function of the angle between the radialline to the dipole and the dipole axis. This function isshown approximately in Figure 23 for the dipole used inthe example. The monopole possesses radial symmetryand therefore requires no such consideration.

Application of Method

A dipole and monopole signature is thus constructed inFigure 24. The amplitude is dimensionless, but can becompared to a real anomaly by multiplying by a singlefactor derived below from considerations of volume,susceptibility, etc. However, applying these factors evenqualitatively should allow one to draw the dipole andmonopole signatures for variously inclined fields andgeometries. Figure 25, for example, is drawn free-handfor anomalies in vertical field (90 inclination), magneticequator and mid-southern latitudes. By simply sketchingin the earths field direction and the dipoles field lines


22/58

INTERPRETATION 21

Figure20.

Direction of Components Measured by Total Field Magnetometer

Figure 27. Total Field Components of Tangent to Field Lines of Dipole and MonopolerrDIPOLE + MONOPOLE +

Figure 22. Fall-off Rate(Relative intensity or length of vectors in Figure 21)+ Figure 23. Aspect Factor(Relative Intensity of Dipole of Figure 21 with Respectto Angle from Axis at Various Points Along Profile)A DIPOLE

Figure 24. Dipole and Monopole Signatures (Constructed from Figures 20-23 according to methods described in text.)


23/58


without consideration of the other last two factors, it ispossible to appreciate the basis for:

a negative anomaly over sources at the mag-netic equator,

absence of anomalies in the central portion ofelongate N-S anomalies at the equator,

both positive and negative fields for almost anyanomaly,

changes in anomaly character for different direc-tions of the dipole,

asymmetry of anomalies,

monopole which has only positive sense yet formost inclinations still produces a total intensityanomaly with both posit ive and negativeportions.

The simple exercise of drawing such anomalies may also

elucidate other characteristics of signatures, which tomany not familiar with magnetics or such behavior asshown here, appear to be complex and difficult tocomprehend.

Based upon the above procedures, applied qualitatively,and upon the manner in which lines of flux are inducedin various configurations of geologic bodies and ambientfield directions and inclinations, it is possible to derivethe various signatures shown in Figure 26 (drawn free-hand). By varying the effect of depth as it produces ananomaly of longer wavelength, and by building com-posite anomalies such as summing the effect of 2 faultsto create a single wide, shallow dike, it is also possible

to generate a composite curve demonstrating the effectof different sources and different depths which is thetypical observation.

Contour Presentation of Dipole and Prism Anomalies

Profiles of total intensity are usually the only form ofpresentation from ground measurements even when dataare taken on a 2-dimensional array. If measurements are

taken properly, however, it is possible to construct acontour map by the methods described in Chapter IV.It is therefore useful to examine a few special cases ofcontour maps that would beexpected oversimple sourcessuch as a dipole and a wide, vertical prism in various

latitudes. Such a contour map also allows one to extract,even by simple inspection, how a given profile wouldappear at various positions over such simple-shapedforms which is useful information both in search and in

0 MONOPOLE

G DIPOLE

-7 7F aFigure 25. Free Hand Sketch of Dipole and Monopole for Various Inclinations


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I N T E R P R E T A T I O N 23

4

5

SEEGEND

ANTICLINE

OR

RIDGE

(HORIZONTAL CYLINDER)

SILL OR

VOLCANIC FLOW

\DIPPING DIKE

/DIPPING DIKE

4

5

SEEGEND

GRADUALLY SLOPING

SURFACESPHERE RDIKE(VERTICAL SHEET) WIDE DIKE

1

2

3

4

5

SEE

GEND

t

riNARROW INTRUSIVE FAULT

( VERTICAL CYLINDER) (WITHIN ONEROCK UNIT)F A U L T

(INVOLVING T W O

ROCK UNITS)

TYPICAL ANOMALIES FROMVARIOUS GEOLOGIC BODIES(SHOWN FREE HAND - DO NOT

USE AS QUANTITATIVE REFERENCE)

ALL ANOMALY SOURCES EXCEPTSPHERE AND VERTICAL CYLINDER

ARE INFINITELY LONG NORMALTO THE PLANE OF THIS PAGE.

LEGEND

2) IF OR tF T R A V E R S E N - SIN4 (N-b

3) \F OR \F T R A V E R S E E - W(i.e., PROJECTION OF FIELD INTOPLANE OF THIS PAGE IS VERTICAL)

4) -F TRAVERSE N-SIN-I5) -F TRAVERSE E-W

(i.e., FIELD IS NORMAL TO PLANEOF THIS PAGE)

Figure 26. Anomalies for Geologic Bodies at Various Orientations and Different inclinations of the Field


25/58


geological exploration. Contour maps and selectedprofiles drawn across the anomaly are sketched inFigure 27.

Anomaly Amplitude

Amplitude Estimates for Common Sources

The large amplitude commonly observed anomalies(several hundred gammas or larger) are almost always

the result of a large magnetization contrast, i.e., changein lithology where one igneous rock is in juxtapositionwith another or with a sedimentary or metamorphicrock of much lower susceptibility. It must be rememberedthat magnetization of common rocks varies over 6 ordersof magnitude. Anomalies due to structure alone, i.e.,varying configuration of a uniformly magnetized rock, sel-dom produces anomalies larger than 10 or 100 gammas.

The relative amplitude of a given anomaly (signature)has been shown to be a function of the earths fielddirection, the configuration of the source and the rem-anent magnetization if any. The maximum amplitude ofan anomaly is, on the other hand, largely a function of

the depth and the contrast in the mass of magnetite (oriron, etc. in the case of search), and to a lesser extent,the configuration of the source. It is of interest to beable to estimate the maximum amplitude for a givensource in order to model it for the sake of interpreta-tion. This estimated amplitude can be used with thenormalized, i.e., dimensionless, anomaly signaturesabove and in Figure 26 to produce the anomaly onewishes for comparison with the observed. Estimation ofthe maximum anomaly amplitude is also useful in plan-ning a survey or planning the grid and coverage neces-sary in search applications.

For a few generalized configurations, it is relativelysimple to estimate the maximum anomaly amplitude

(at a single point above the source) assuming a depth,susceptibility and much simplified shape of the source.Expressions are given in the literature for calculation ofanomalies of more complex figures and later in thissection the calculation of the complete signature, i.e.,the amplitude as a function of distance along the pro-file for a few simple forms. The methods describedherein are merely order-of-magnitude techniques, butare useful for the applications covered by the Manual.

Estimation of the maximum anomaly for comparisonwith a given source requires first that the signature bestudied for the nature of the source; namely, whetherthe source can be approximated as an isolated dipole,

monopole, or line or sheet-like array of such. In thecase of the latter two, adjacent traverses or a contourmap may be required to determine if it is 2-dimensional,i.e., very long normal to the traverse. A depth is thenassumed or crudely estimated (according to proceduresthat follow). In addition, the susceptibility is assumedor if source rocks are accessible, it is measured follow-ing methods outlined in Chapter VI. The formulae belowcan then be used remembering that they are basedupon simplifications and assumptions and are often nobetter than a factor of two.

The basic expression for estimating the maximumamplitude of any anomaly is T=:where T is the anomaly, M the magnetic moment, r thedistance (depth) to the source, and n a measure of the

rate of decay with distance, or fall-off rate (n= 3 for adipole, n = 2 or a monopole, etc.).

Since the magnetic moment M (and k) is usually givenin centimeter-gram-second (cgs) units, r must be incentimeters, n is dimensionless and T is in gauss. Toexpress T in gammas, multiply M by 105; if r is in feet,multiply r by 30 and raise the quantity 30r to the expo-nent n, e.g., if the source is a dipole, then n = 3, and if

say, r = 2 feet, M = 1000 cgs,

then T = 1000 x 10s(2 x 30)3 = 460gammas.

Dipole and Monopole Signaturesin Vertical and Horizontal Fields

The very generalized expression for the maximum anom-aly one may expect from a dipole or monopole was pre-sented above in its very simplest form. It may be ofinterest, however, to construct the signature of a dipoleor monopole in a vertical or horizontal earths f ield aswould be observed by a total field magnetometer along

a traverse over the source.

Apart from any total field considerations, a dipole has afield with magnitude and direction given by the radialand tangential components, Tr and To, according to thefollowing expression and for the geometry shown.

2M cos 0Tr =-3

Where the earths field is vertical or nearly vertical (dip70 to go), the dipole, if induced, would also be verticaland the total field magnetometer would measure thecomponent, T,, along this vertical direction, where

TZ = Tr cos 6 + To sin 6= 2M cos*e- M sinZf3

r3M (222 -x2)

= (x2 + Z2)5/2T0

=TF=T

As before, T, = TF = T, the anOmalY.At x = 0, T =?&

230.175M

at x = +Z, T =Z3

at x =*J2z, T = Oat x = * 2Z T = -0.04M

Z3


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INTERPRETATION 25

DIPOLE, INCLINATION OF F, SO(FOR INCLINATION -SO, I.E., SOUTHERN HEMISPHERE, N IS S)

DIPOLE, HORIZONTAL FIELD

A

I

B

I VERTICAL PRISMA VERTICAL FIELD

B B

VERTICAL PRISM1 HORIZONTAL FIELDA

Figure 27. Contour Maps of Total Intensity


27/58


28/58


29/58

8 APPLICATIONS MANUAL FOR PORTABLE MAGNETOMETERSA narrow, vertical dike in steep field or the edge of ahorizontal sheet in a horizontal field can be consideredas a line of monopoles varying as l/r which is a lowerrate of fall-off than a singl emonopo e for the samereasons given above for a horizontal cylinder (see Fig-ure 37). The magnetic moment M = It where t = width ofdike. Since the anomaly varies as l/r, the dimensionsof t are simply length. As an example, a vertical dikemight be 100 feet wide, at a depth of 500 feet, with

k = 10-3 in a field of 50,000 gammas, orkft lO-.j x 5 x lo4 x 10T =-= = 10 gammas

r 5 x 10

kFtT=-r

WHERE ttr

Figure 31. Anomaly of Narrow Vertical Dike

A common point of ambiguity arises with such simplifiedschemes as these in the case of a dike which is nearlyas wide as it is deep. In this case, the anomaly is approxi-mated as something between a line of monopoles asabove and a sheet of monopoles as shown in the follow-

ng. Moreover, as the dike is even wider than its depth,t can be approximated simply by 2 faulted contacts withno anomaly in between.

For a semi-infinite slab of material such as a rock sur-ace of great thickness and breadth in a non-horizontalfield, the flux lines do not vary in direction or densityabove the slab, therefore the field does not vary at allwith distance to its surface (similar to the limit of thespherical dipole above where R=r) so that

M 2711T Z-Z

r0 -, or T = 27rkF1which is useful in estimating the magnitude of the anom-

aly at a vertical fault (see Figure 32). For example, con-sider two rock types at a vertical contact of k = lo-9 andk = 10-S for an effective susceptibility contrast of k =10-310-5 E3 0 relative to 10-s) and whe re F = 50,000 gammas.

Thus

T = 27rX 10e3 X 5 X lo4= 300 g a m m a sf the rocks had k = 10-4 and 10-3, the effective suscepti-

bility contrast would be

IO- _ 10-J = 1 0 X 1O-4 -1O-4 = 9X 10e4 a n dT = 277 X 9 X lO-4 X 5 X lo4 = 270gammas

This simple example of two adjacent rock types is prob-

ably applicable in more instances in interpretation thanany of the other geometries discussed above.

T = 2nkF

I

Figure 32. Anomaly of Semi-infinite Slab

Anomaly Depth Characteristics

In a very approximate fashion, the wavelength, or, effec-tive width (or half-width described in the following) ofthe anomaly and, with more accuracy, the width of cer-tain characteristics of the anomaly such as slope, aremeasures of the depth to its source. However, recogni-

tion of the anomaly, the anomaly zero and certain slopeswould not only appear as different values as determinedby different interpreters, but they also depend uponwhat is removed as the regional gradient. More objec-tive criteria are used in some cases such as the nearlystraight portions of a slope, and distances and anglesbetween inflection points, peak values and other anomalycharacteristics.

Anomaly Width

In general, the anomaly width as shown i nFigure 33 ison the order of 1 to perhaps 3 times the depth. Thus,when an anomaly appears to have a width as such of100 feet, it is definitely not produced by a source at

1000 feet or at 10 feet, but more likely by a source be-tween 30 and 100 feet deep (or distant). Such criteria,approximate as it is, is nevertheless useful for cursoryinterpretation of profiles and maps.

Anomaly Depth Estimation

Much is written on the variety and relative merit of meth-ods for estimating the depth to the source of anomalies.Since the magnetometer is primarily a tool for subsur-face mapping and detection, it follows that determinationof the depth as well as edges of bodies is important inits application to geological exploration and search. Thebasis for depth determination is presented here in briefwhich, together with the foregoing background on anom-

aly behavior, should allow one to at least appreciatehow a variation in depth affects an anomaly. In mostcases, one needs only to apply this knowledge quali-tatively through visual inspection of a profile. Whateverthe requirement, depths may be estimated by visualinspection, several rules of thumb, modeling (i.e., calcu-lation of assumed source and comparison with observed),measured gradient techniques (see Chapter VIII), orvarious computer-oriented procedures. As was demon-strated earlier, a given anomaly could have an infinitenumber of possible sources and source depths, but therealistic models that are assumed usually produce maxi-mum depth estimates.

Knowledge of the depth of a particular formation or

source may have considerable geological significanceas it determines the nature oconfiguration of a fo-ma-orma-


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INTERPRETATION 29

Figure 33. Anomaly Width

tion, the slope of its surface and its discontinuities. Thedepth to various points on the surface of crystalline rockor magnetic basement allows one to map that surfaceand its topography and structures to depths exceeding30,000 feet and to infer thickness of sediments or con-formable sedimentary structures above it for explorationof petroleum, sedimentary ores, placer deposits orgroundwater. Areas underlain by pediment or othersedimentary deposits may be ruled economic or non-economic according to depth. The depth to ore depositsassociated with pyrrhotite, magnetite or ilmenite may

be estimated as an aid to a drilling program or evenfor estimation of total tonnage of magnetic iron oredeposits. Black sand deposits of rutile, zircon, monazite,diamonds, gold, platinum, etc. are often associated withother high density, very resistant yet magnetic minerals,namely, magnetite or ilmenite. The depth to objects ofsearch whether buried iron or man-made structures isinvaluable in guiding the subsequent excavation efforts.

Identification of Anomaly

The anomaly of interest must be identified and discrim-inated against the obscuring effects of others. Recogni-tion of the anomaly itself is usually the most difficultaspect of depth determination because of the composite

effects of multiple sources, sources at various depthsand at various distances in any direction from the mag-netometer. Only the net effect of all anomalies are meas-ured by the magnetometer since it has no inherentdiscrimination ability at the disposal of the operator. Theanomaly should be inspected to ascertain the probablesource and, if complex, the possible combination ofsources. For example, a wide, shallow dike will appearas two anomalies which may or may not coalescedepending upon the relative width and depth. A verybroad anomaly or regional gradient (described in Chap-ter IV). is usually caused by anomalies which are ex-tremely deep or distant or by the normal variation in theearths magnetic field. If one wishes to remove this

gradient, it can be done either by drawing a straightline through the non-anomalous portions of the profile(away from the anomaly of interest) or by drawing avery smooth but broad wavelength curve through thedata of much longer wavelength than any anomaliesof interest. This regional gradient or background is thensubtracted from the anomaly and the remaining, orresidual anomaly, replotted. It is this anomaly which isthen interpreted for either depth or for amplitude orgeneral configuration of sources as described inChapter IV.

Fall-Off Rate

The variation of anomaly amplitude with distance, or

fall-off rate, is important in the interpretation of anom-alies for it relates the anomaly to depth, it describes ina general way the configuration of the source, and it

assists in determining susceptibility and mass of thecausative magnetite. Recall that the anomaly from a

dipole varies as 1 and that of a monopole as r+ .Ther3

fall-off rate, in actual practice, does not involve PreCiSelYsuch factors or exponents but, in fact,iS typically +5&r r*etc., or even1 as described above. In other words,

rvarious configurations of dipoles, monopoles, lines and

sheet-like distributions of these poles constitute a con-tinuous series of fall-off rates even in the vicinity o

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