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Applying the RiIegger Ground Tester in Electrical. Exploration

BY BELALOW* ND SHERWIN. KELLY,*NEW YORK,N. Y., AND WILLIAMB.

CREAGMILE,PHILADELPHIA,A.

(New York Meeting. February, 1931)

ELECTRICALethods and instruments for geophysical exploration

have been almost exclusively applied, during these years of development

of the ar t, by a few companies specializing in this field. The control of

patents, the cost of the instruments and the general lack of published

information on details of field procedure and office interpretation have

given these owners a monopoly in carrying out electrical surveys. The

development of the Megger Ground Tester1 seems likely to change t,his

situation, and to enable mining engineers and geologists, without special

training in geophysics, to carry out simple reconnaissance electrical'

surveys. Nevertheless, the need for such special training and experience fiin planning surveys and interpreting their results remains, and probably

always will remain, indispensable for a successful outcome.

The unique thing about, the Megger Ground Tester, which givcs rise to

this hope for its general use, is it s simplicity of operation. The four

binding posts of the instrumcnt are connected by insulated wires to four

stakes driven into the ground a t appropriate points. The crank of the

Megger is then turned, and the resistance of the ground between the two

inside stakes is read directly, in ohms, by the deflection of a pointer over a

scale. The apparatus is shown in Fig. 1. a

Fundamentally, the principle of operation is tha t of determining the

resistance of a four-terminal conductor, as applied by Frank Wenner to

the measurement of earth resistivity. "A four-terminal conductor is a

conductor provided with two terminals to which current leads may be

connected and two terminals to which pot,ential leads may be connected.

The resistance of such a conductor is the difference in potential between

the potential terminals divided by the current entering and leaving

* Consulting Mining Engineers, Geologists and Geophysicists.

t Electrical Engineer, James G. Biddle CQ.

The word "Megger" is ;t trade name which, for the past 25 years, has been

applied to instruments of a particular type for testing electrical insulation resistance.

The Megger Ground Tester is a recent development for testing the resistance to

earth of installed electrical ground connections, and also for quickly measuring

earth resistivity.

114

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through the current terminals. ""hen the crank of the Megger Ground

Tester is turned, i t drives a self-contained, direct-current generator. . The

current thus generated passes first through the current coil, or ammeter

element, of an ohmmeter. Thence i t goes to a commutator mounted on

the same shaft with the generator, where it is changed into alternating

current of a,bout 50 cycles per second. Finally, it is led from the current

binding posts C 3 and C z of the instrument to the two appropriate field

stakes. The other two field stakes are connected to the potential binding

FIG . 1b.-MEGGER APPARATUS FOR G E O P H Y S I C A L WORK.

At left, switchboard for facilitating th e connecting of th e M egger leads to fieldstakes. Center, the Megger Ground Tester . Right, carrying case, reel of wire andfield stakes.

posts P I and Pqof the instrument. The potential drop across these two

stakes is measured by leading the current picked up by them through a

second commutator, run synchronously with the first, where i t is con-

verted back into direct current.It

then goes to the potential coil, or

voltmeter element, of the ohmmeter. See Fig. 2.

The current coil and the potential coil of the ohmmeter are mounted on

a common spindle, so that their torques oppose each other. Thus they

automatically perform the division of volts by amperes. According to

F. Wenner: A Method of Measuring E ar th Resistivity. U. S. Bur. Stds. Sci.Paper 258 (1915).

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116 APPLY ING THE MEGGER IN ELECTRICAL EXPLORATION

Ohm's law, rosistailcu equals volts divided by amperes, so the male of theohmmeter is calibrated to read directly in ohms.

The arrangment described is doubly advantageous. It retains the

sensitiveness and accuracy of the direct-current ohmmeter. By using

FIG.2.-INTERNAL CONNECTIONS OF MRGGERG R O U N D ESTER.

alternating current ir " ground circuits, however, the errors that wouldbe introduced in a d' -current circuit by polarization and electrolysis atthe ground stakes, ly stray direct currents, are eliminated. Thus the

FIG. 3.-MEGGXR GROUNDTESTERCONNECTIONS TO FIELD STAKES.

Notc, A must be at least 20 X B.

use of nonpolarizing electrodes is avoided, and copper or iron stakes canbe employed without having to introduce any manually operated device

to compensate for the unwanted currents. Stray alternating currentsare not bothersome, as by changing the speed with which the generator-commutator shaft is turned, the Megger Ground Tester circuits may bethrown out of phase with the strays, and so are no longer affected by them.

The four field stakes to which the Megger is connected may be of iron,brass or copper, for instance, about j5-in. dia. and driven into the ground

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to a depth of 1 ft. or more. For simplicity and convenience they arebest arranged in a straight line, at equal intervals A, Fig. 3. The

two end stakes are connected to the current leads from the Megger, andare correspondingly callcd C L and Cz. The two intermediate ones,

designatedp, andPz,are connected to the similarly named potential leads.

With the apparatus thus disposed, the resistance is obtained of aportion of the earth between the potential stakes. "Roughly speaking,

the effect of material a t distances equal to the distance between adjacent

terminals is found to be so small that the effect of materials beyond this

distance is negligible. Thus, the body of ear th involved in a single

determination has linear dimensions of the same order as the distancebetween terminals. "3 The depth of measurement consequently is equal

to the electrode, or stake separation A, Fig. 3.The actual resistance read on the instrument will depend on the dis-

tance separating the electrodes. Where this distance is not kept constant

throughout a given survey, the readings are not comparable. I t then

becomes necessary to determine the specific resistance, or resistance per

unit volume, called the resistivity, of the ground under investigation.This may be expressed as the ohms resistance of a cube measuring 1

meter on an edge, abbrcviated ohm-meter. As long as resistivities are

expressed in the same units, the figures are evidently comparable. In

this paper, because of the small scale of the experimental demonstrations,the unit ohm-inch is adopted for the laboratory experiments. The

Megger Ground Tester reading is related to the resistivity by asimple formula:

p = 27rARWhere

p = resistivity in ohm-meters,R = resistance in ohms, as read on the Megger,

A = electrode separation in meters.

If the electrode separation is given in inches, the resistivity will be inohm-inches. A full development of the derivation of the formula maybe found in the article by Wenner already mentioned.

In perfectly homogeneous ground the resistivity, obviously, must beconstant, consequently the resistance reading will vary inversely as the

electrode separation. If the ground is not homogeneous, this inverseproportion is interfered with. The application of the above formula in

such a case gives the average, or apparent, resistivity of the entire volumeinvolved in the measurement. In other words, the presence of a layer orbody of better conductivity, between the potential electrodes and within a-

W. 0. Hotchkiss, W. J. Rooney and James Fisher: Earth-resistivity Measure-

ments in the Lake Superior Copper co un try . Geophysical Prospecting, A. I. M . E.

(1929).

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118 APPLYING THE MEGGER I N ELECTRICAL EXPLORATION

radius A of the line joining them, will result in a lowered apparent

resistivity; conversely, a poor conductor within that volume will cause a

rise in the apparent, or average, resistivity. Evidently such material

must be of sufficient size to produce an appreciable effect in order tobe detected.

By starting with a small interval between stakes, and progressivelyincreasing their separation, successively greater depths of penetration are

obtained. Then the electrode separation a t which a markedly higher or

lower average resistivity is observed gives a rough measure of the depth

at which occurs a resistant body, as bedrock under drift, or a conductive

one such as a water table or orebody, capable of exerting an importantinfluence on the measurements.

This evidently does not provide a measurement of the true resistivityof the underlying, different layer or body. In order to calculate this

value from the observed readings it becomes necessary to utilize certain

formulas, or curves, with which the present paper will not deal. The

determination of the true resistivities of the formations under observation

is often useful, and frequent,ly indispensable for obtaining their depths

accurately. Those interested will find complete discussions of this matter

in papers by Weaver, Hummel, Ehrenburg and Watson, Lancaster- r

Jones, and Tagg.4 d

iAn alternative technique can be used where it is expected that forma-tions of different electrical characteristics may occur within a given depth

over a limited horizontal extent. These might be igneous intrusions, salt

domes, buried hills and valleys, concealed anticlines or synclines, sinks,

and conductive or resistant strata dipping into, or out of, the zone ofobservation. For such cases the electrode separation can be fixed for the

desired depth of investigation, and the given area explored by a systematic

grid of observations. A map of resistivities is then drawn up from the

results of this ~ u r v e y . ~ his map will show islands of higher or lower' W. Weaver: Certain Applications of the Surface Potential Methods. Geo-

physical Prospecting, A. I. M. E. (1929).

J. N. Hummel: A Theoretical Study of Apparent Resistivity in Surface Potential

Methods. See page 392.

D. 0. Ehrenburg and R. J. Watson: Mathematical Theory of Electrical Flow in

Stratified Media. See page 423.

E. Lancaster-Jones: The Earth-resistivity Method of Electrical Prospecting.

Min. Mag. (1930) 43, 19.

G. F. Tagg: The Ear th Resistivity Method of Geophysical Prospecting. Some

'I'heoretical Considerations. M l n . Mag. (1930) 43, 150.6 E . G. Leonardon and S. F. Kelly: Some Applications of Potential Methods

to Structural Studies. Geophysical Prospecting, A. I. M. E. (1929).

E. G. Leonardon: Electrical Exploration Applied to Geological Problerns iri

( h i 1 Engineering. See pagc 99.

C. and M. Schlumberger: The Mctllod of the G ~ ~ o u ~ ~ c lesistivity Map arid 11s

I'ractical Applications. Can. Min. & Met. Bull. 226 (1931).

G. F. Tagg: E ar th Resistivity Surveying. E ng . $ M i n . Jnl. (1931) 131, 325.

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I-esistivitieswhere formations of poorer or better conductivity than their

surroundings are included within the depth of investigation.

The two field procedures described can be demonstrated by means of a

simple laboratory experiment. A glass tank measuring approximately

18 in. long, 7 in. wide and 6 in. deep6 is filled with water to represent

a section of the earth. Contact with the water surface is achieved with

four nails attached to the Megger Ground Tester leads. The nails are

thrust through holes, in which they fit tightly, bored a t intervals of 1 in. in

a wooden rod arranged to rest on the ends of the glass tank. A thin sheet

of metal serves for the conductive stra ta , and a piece of glass, bakelite or

phenolite for the resistant beds. These sheets fit in the tank, and are

$4 to 54 in. thick.

Keeping the electrode separation fixed at, say, 2 in., the conductive

stratum may be brought from a depth of 4 in. or more to positions

progressively nearer the surface, Megger Ground Tester readings being

taken at each step. The same process may be repeated with the resistant

sheet. These readings are representative of what would be expected in a

gridwork of Megger observations, where a conductive or resistant bed

dips up into the zone of investigation.Table 1 gives figures obtained in two laboratory demonstrations

similar to the one just described. The effect produced by the resistant

and the conductive sheets on the resistivities is apparent . This effect is

TABLE .-Figures Obtained i n T w o Laboratory Demonstrations of Megger

Ground Tester

Electrode Interval A, 2 Inches

i Experiment 1' Experiment 2 b

1 __ ~ L - - - - -epth ofImneraion 1 Conductive Sheet Re.atant Sheet 1 : Conductive Sheet Reeiatmt Sheet

of Sheet*In . __- ---I- L L - - - - -- --

I l 1 1 I Ia Performed at Haward University in the course of a lecture on Geophysics as a

New Tool for the Geologist, February, 1931.

* Performed during the course of a lecture on geophysics at Cornell University,

March, 1931.

R.Ohme p , ohm-in.:R. O h m s p. Ohm-in. p , ohm-in.\R , 0 h m e p. Ohm-in.I !4 110 1 1383 i 140 1 6 0 125 1572 145 1820

6 For demonstration at the February meeting, 1931, of the A. I . M. E. a glass tankwas provided through the courtesy of Eimer & Am end, New York C ity, and a MeggerGround Tester was loaned by James G. Biddle Co. f Philadelphia.

32

1 . 5

105 1320 14 7 , 1850 , , 115 1 1447 7 165 2075

80 ' 1005 168 2112 85 : 1070 1 185 2325i57 717 1 215 2705 1 65 817 j 230 1 2890

I I 26 327 325 1 4080 I ! 25 314 375 1 4720

0 . 5 5 . 5 I 69 I 420 ! 5280 1 1 1 i

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120 APPLYING THE MEGGER IN ELECTRICAL EXPLORATION

~ l lgh t l depths grcVatel i , l~an hc: clcc,trod(~ epal.:ttlon, hril I~c'c'o111~'s

incrrasi~lgly ronourlced :IS shallower t l~pths re re:~ched.

Such data as the above may also

be expressed in the form of curves, in

which resistance, or resistivities, are

plotted as ordinates, and the depths

to the stratum as abscissas (Fig. 4).

This has the advantage of show-

ing more graphically the change

brought about in the resistivity

when the disturbing factor comes

within the depth of investigation.Not infrequently the curves show

a break at or near this point, butthey may be quite smooth. Both

types are shown. Because no break

may appear, or if present may not

coincide exactly with the depth of

transition, it is safer to base the Iinterpretation on the type curves

given by Hummel, Lancaster-JonesR ~ , ~ ~ + S ~ ~ ~ ~

nd Tagg in the works already cited.

To demonstrate the process of

determining the depth to a given

stratum beneath a certain point,

the metal or glass sheet is kept ata depth of 2 or 3 in. The nail sepa-

ration is then increased from 2 in.

to 3 and then 4 in., Megger readings

being taken a t each stage. Thefigures duplicate what would hap-

pen in a survey designed to deter-

mine the depth to a given conduc-4 5 .

DepShto ~heet,;nches tlve formation, such as a clay bed,

PIG. 4.-EFFECT ON RESISTANCE ,(EAD- water table or orebody, or to 3.

INoS OF VARYING DEPTHS OF CONDUCTNE one, such as bedrock be-AND RESISTANT SHEETS.

Since electrode spacings were kept con- neath soil or glacial Or a lime-stant, resistances read, instead of resistivi- stone or sandstone st ratum in shale.tles, are given in the ordinate scalc.

Curves N1 and N 2were obtained duringIn this case it is necessary to

experiments a t Northwestern University. calculate the resistivities in orderCurves R I and R2 were obtained during

experiments a t Missouri School of Mines, get as the~ o l l a , O. electrode separationisnot constant.

An important fact to be kept in mind is that in a laboratory experiment

on such a small scale the resistance of the glass walls and bottom of the

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BELA LOW, SHERWIN F. KELLY AND WILLIAM B. CREAGMILE 121

tank interferes with obtaining true values. This may be verified byusing nothing but water in the tank and progressively increasing the nail

separation as described. Obviously, the water is homogeneous, but in

spite of this fact the apparent resistivity rises, owing to the effect men-tioned. Tanks of larger dimensions would lessen this error, but are hardto obtain and difficult to handle.

The results of two such experiments are set forth in Table 2.

Although the data should properly be drawn up in the form of curves,

this has not been done because of the impossibility of obtaining a sufficient

number of points when experimenting on such a restricted scale. The

TABLE%.-Results of Two Experiments in Determining Depth to Strata~ - I

Electrode Separation 1 2 In. 1 3 In. 4 In.

i Cond~lctive heet 1 R, ohms 45 1 20 1 12

1 at depth of 3 in. I1 p, ohm-in. 315

Conductive sheet

a t depth of 3 in.

' Resistant sheet a t

( depth of 3 in.

1Experiment 3 1

iWater

Dividing by effect

in water only

Resistant sheet a t

depth of 2.5 in.

R, ohms 60 60 -p , ohm-in. 1 818 1130 1510---.

ohms1 58 1 48 1 50I

P , ohm-in.

1 R, ohms 1 4 0 1 24 / 19.2

I -482

R, ohms 1 65 1 72 87

1 p , ohm-in. 818 1355 1 2185

Experiment 4

1 Water

1 R, ohms 1 58 62 1 75

1 p, ohm-in. j 729 1 1169 ) 1888

Experiment 3 was performed during a course of lectures on the Application of

Geophysics in Geology a t Northwestern University, Match, 1931.

Experiment 4 was performed during a course of lectures on Geophysics a t the

Missouri School of Mines, April, 1931.

1

' Dividing b y effect

1 in water only

Ratio of drop, p

IRatio of rise, p 1 1.161 1.16

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122 AP P L YI NG T H E ME G G ER IN ELECTRICAL EXPLORATION

figures given suffice t o show th e d iminution of resistivity bro ugh t a bo ut j

by a conductive stratum when the electrode separation is extended to

include it w ithin th e de pth investigated. Conversely, th e increased

resistivity brought about under the same circumstances by a resistantformation also is demo nstrated.

T he effect of th e insulating walls of t he ta nk app ears in the figures ofresistance an d resistivity when only the water was measured. Fo r its

resist ivity t o have remained constant a t 729 ohm-in., a s would normally

1be the case, it is readily calculated th at the resistance should have read

39 ohms a t 3 in. separation, and 29 ohms a t 4 in. separation. Th e depar-

ture from these figures was less pronounced in experiment 3 than in

experiment 4, because the ta n k used in the former was considerably larger

th an the one used later. I n experiment 4 it was about the size mentionedon page 6 .

T h e figures in the rows headed "Ratio of D rop, p" are obtained bydividing the resistivities when using the conductive sheet by those with

wa ter only. Those und er "Ra tio of Rise, p" show the resistivities with

th e resistant sheet divided by t he resistivity with wa ter only.

An other pha se of t h e d em onstra tion, conc erning which no figures were

ke pt, de alt w ith th e effect of condu ctive and resistan t sh eets in a verticalposition. Fro m th is it readily developed tha t conductive veins, dikes or

vertically inclined strata would have the greatest effect when near and .

parallel to the line of electrodes. On th e oth er han d, resistant dikes or

sharply tilted beds exert the strongest effect when perpendicular to the

line of electrodes, lyin g across it between P 1 and P z .

The data from these experiments, it should be distinctly understood,

do n ot represent th e results of an y at te m pt to c arry ou t a series of scien-

tific investigations. Th e ap pa ra tus used was too crude and too hastily

assembled, an d th e work to o sketchily done, to w arrant putt ing any such

interp retatio n upon them . Th ey do show, however, the simplicitywith which it is becoming possible to apply th e resistivity technique, f or

these results could be dup licated b y anyon e with a M egger Gro und Tester,

a good-sized metal ash tr ay , a dinner plate an d a bat h tu b of w ater.

I n field work, of course, ce rta in prec aution s m ust be observed. For

example, i t m ay be necessary t o m easure the stak e resistances of P I and

P2, if it is expected th a t the y will lie outside a certa in range. By stakeresistance is mea nt the resistance th e current encoun ters a t the contact of

the stak e with th e earth . This value is easi ly determined by connecting

C1 a n d PI of the Megger to the sta ke where th e resistance is to be meas-

ured. P Zmu st be some distance aw ay, say 25 ft. or more, and Cz a s f a r

away again beyond P2 . T he reading thu s obtained is the sta ke resistance,

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which can then be used in a simple forlrlula to correct the ground-

resistance reading. The error introduced by high or low stake resistancesI is seldom large enough to warrant bothering about, save in excep-

\

!! tional cases.The question naturally arises, ''How can the Megger Ground Tester be

applied in civil engineering, mining engineering or geology?" The

answer must be somewhat roundabout. First and foremost, the sim-

plicity of the instrument puts it almost in the class with the magnetic

dip needle. For many years the dip needle has been used, as a mat ter of

course, to aid in working out geological questions where the formations

involved possess markedly different magnetic permeabilities. Similarly,

the Megger Ground Tester now gives the engineer and geologist the

opportunity to utilize certain electrical methods as everyday tools, in theirattacks on earth problems.

Naturally, the actual geophysical technique employed must depend on

the type of information desired, and on the geological circumstances of the

problem. Sometimes two or more methods must be conjointly applied in

order to obtain the desired information. Before deciding to use the

Megger Ground Tester, or any other lnethod or combination of methods,

the situation must be carefully exa ined to determine the proper geo-

physical course of action.' If th :ological circumstances warrant abelief in the probable efficacy of re vity (or conductivity) or potential

methods, the Megger Ground Tester ,robably could be applied. Excep-

tion must be made for studies to great depths, say about 600 ft.or over,

for which the instrument as now designed is not sufficiently powerful.

The conditions that make possible the application of resistivity

methods have been detailed in numerous geophysical articles18and need

not be repeated here. There is some literature, however, outlining actual

field applications of the Megger Ground Tester.

'S. F. Kelly: Geophysics in Exploration; Prospect and Retrospect. Eng. &

Mi n. J n l . (1931) 131,11.

E. G. Leonardon and S. F. Kelly: O p . cit.

E. G. Leonardon: O p . cit .

H. Lundberg and T. Zuschlag: A New Development in Electrical Prospecting.

See page 47.

S. F. Kelly: Electrical Methods for Sub-soil Investigation. Proc. Brooklyn Engi-

neers' Club (April, 1930).

I. B. Crosby and S. F. Kelly: Electrical Subsoil Exploration and the Civil Engi-

neer. Eng. News-Rec. (1929) 102.0. H. Gish and W. J. Rooney: Measurements of Large Masses of Undisturbed

Earth. Terrestrial Magnetism (1925) 30.

C. and M. Schlumberger: O p . cit.

F. W . Lee, J. W. Joyce and P. Boyer: Some Earth Resistivity Measurements.

U.S. Bur. Mines, Inf. Circ. 6171 (1929).

H. A. Buehler: Geophysical Prospecting. Biennial Report of St ate Geologist

of Missouri, Appendix 111 (1931).

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124 APPLYING THE MEGGER IN ELECTRICAL EXPLORATION

The study of subdrift contours is described by Lancaster-Jone~,~ho

also goes into theoretical discussions of the technique of interpretingthe results. Professor Gilchrist,'of Toronto, has used the Megger Ground

Abana mine in Quebec.lo Eve and Keys discuss the Megger GroundTester and some of its appIications.l1

ester to study geological structure as well as sulfide orebodies a t the !

FIG.5.-MEGGER OBSERVATIONS OF DEPTH TO BED ROCK, APRIL 22, 1931.

a. At Gasconade Bridge, Route 63, Missouri.b. At Roubidoux Bridge, Route 66, Missouri.

The curves drawn up from some measurements made to determinedepth of bedrock are given in Fig. 5. The work was done in cooperation

with F. C. Farnham of the Missouri Geological Survey, through thecourtesy of the director, H. A. Buehler. Two bridge locations were

chosen, where the depth of overburden was known from drilling records.

The close correspondence of the break in each curve with the depth to

bedrock is a striking feature of these graphs. The fact that the observa-tions a t each place occupied but a couple of hours makes this an interest-

¶ E. Lancaster-Jones: Op. cit.

'0 L. Gilchrist: Measu rements of Resistivity by th e Central Electrode M ethod a t theAbana Mine, Northwestern Quebec, Canada. A. I. M. E. Tech. Pub. 386 (1931).

'1 A. S. Eve and D. A. Keys: Applied Geophysics in the Search for Minerals.Cam bridge, 1929. University Press.

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DISCUSSION 125

ing procedure for engineers concerned with obtaining information

regarding subsoil conditions a t proposed bridge or dam Sites.

CONCLUSION

The study of certain types of geological problems may often be

facilitated and speeded, with a saving in expense, by employing theMegger Ground Tester for making reconnaissance, resistivity studies.

Problems involving such things as mapping subsurface contours and

determining depth of overburden, tracing faults, contacts and intrusions,

following key horizons and investigating simpler questions of ore occur-

rence come to mind as being frequently amenable to treatment by elec-trical studies, to which the Megger Ground Tester might profitably be

applied on many occasions.

DISCUSSION

G . F. TAGG, ondon, England (written discussion).-I have read this paper with

considerable interest, particularly those parts which deal with the interpretation of

the results. There are one or two points in connection with this which I should like

to mention. In dealing with the effective depth to which the current will penetrate,

the authors make the statement: "The presence of a layer or body of better con-

ductivity between the potential electrodes, and within a radius A of the line joining

them, will result in a lowered apparent resistivity; conversely, a poor conductor within

that volume will cause a rise in the apparent or average resistivity!'

These effects are not limited to conducting or resistant bodies a t a depth equal

to or less than the electrode separation, or to bodies located between the potential

electrodes. The effects are present to a greater or lesser extent in any case where

such bodies are existing in the earth. The effect, of course, is dependent on the

size of the body and its distance from t he electrode system relative to the electrode

separation. The effect increases gradually for bodies nearer and nearer t o the elec-

trode system, reaching a maximum when the electrodes are actually in the body

concerned. This is clearly shown for one particular case in the curves given in my

paper dealing with theoretical considerations. 2

The method used to interpret the results obtained is still open to discussion.There is a vast amount of experimental da ta upholding the theory that if a sudden

change occurs in the apparent resistivity curve it indicates the presence of a body

having a resistivity different from that of its surroundings, a t a depth equal to the

electrode separation at which the change occurs. On the other hand, all theoretical

and mathematical considerations show that such a sudden change cannot occur.

In an experimental survey I carried out to tes t one thing, no sudden changes in the

resistivity were apparent.13 It thus appears to be safer to base the interpretation

on theoretical considerations where this is possible. The method of interpretation

based on theory was quite successful in the case quoted above.

Up to the present, however, the theory has been developed for only one or twosimple cases of earth structure, and for the more complicated forms of structure

it is necessary for the present to be content with some form of empirical method,

which has been shown to be successful in many cases.

l a Min.Mag. (1930) 43, 150.

l aProc. Phys. Soc., London (1931) 43, 305.

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126 APPLYIKG THE MEGGER IN ELECTRICAL EXPLORATION

W. . CREAGMILEwritten discussion).-Mr. Tagg begins his criticism by calling

attention to a statement on page 117. He explains that the effects described are

not limited strictly to materials within the volume mentioncd. This is well recog-

nized, but Mr. Tagg seems to have overlooked our quotation from Hotchkiss, Rooney

and Fisher, earlier on the same page.We agree th at the method of interpreting results, as referred to on page 118, lines

8 to 12, in the sentence beginning with "Then the electrode separation," has no theoret-

ical or mathematical foundation but has worked out in many cases, and therefore

we feel that it must be given some weight.

Naturally, it will be best to work from curves plotted bctween resistance or resis-

tivity and the electrodc spacing, as referred to in the first paragraph of page 120.

If a curve shows a break, t he spacing of the electrodes a t which the break appears

may be taken as the depth a t which a change in strata occurs. Mr. Tagg scems to

have overlooked that part of the paragraph in question, when he says "Not infre-

qucntlg the curves show a break at or near this point, but they may be quite smooth.Both types are shown. Because no break may appear, or if present nlay not coincide

exactly with the depth of transition, i t is safer to base the interpretation on the type

curves given by Hummcl, Lancaster-Jones and Tagg in the works already cited."

Of course, if a sharp break occurs in a curve, the approximate depth a t which the

change in stra ta occurs can be read off the curve more easily than any of the thcoret-

ical methods of interpretation can be applied.

S. F. KELLYwritten discussion).-Mr. Tagg's comments cniphasize a point which

may not have been sufficicntly stressed in our paper, although it was certainly brought

out, as Mr. Creagmile has shown. In actual field work, the curves plotted with

resistivities as ordinates against electrode separation as abscissas frequently fail toshow a break corresponding t o the change of formation a t a given depth. Even Iwhen thcy do show such a hreak, the electrode separation does not necessarily cor-

respond closely to the depth of transition from one formation to another. Therefore,

iit should be clearly understood that an attempt to deduce thc depth to the lower

formation fro111 such a brcak is admittedly a rough estimate, and not t o be applied

where accuracy is desired. I t will often suffice in reconnaissance work, however.

The only way of obtaining a reasonably accurate idea of the thickness of the overlying

formation is to utilize some such set of graphs as has been worked out by Mr. Tagg

These should be applied whether the curaes show a break or not.

elogs megger 097-11

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