erelimlnary report on electromagnetic model studies f. c ... · the present curves show ' the...

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Erelimlnary report on electromagnetic model studies

F. C. Friachknecht and G. B. Mangan

U. S. GEOLOGICAL SURVET

Beleased to open files

This report and accompanying illustrations have not been edited or reviewed for con formity to Geological Survey standards or nomenclature.

Table of Contents

Page

Abstract^. . ............................................................. 5*

Introduction. ........................................................... k

Acknowledgment a . ............................................ f ........... li

no, ""'. VTneopy . *» * * » * * * * » * * . * * 5

Sllngrais and tmrani oetnods. * * \

and tecnnlo^ues. » . * «.» **«*«r« »* « o

» » « *

Results and conclusions. ........................................ % . ...... 10

Selected DiDlJLofprapny * * . .. » * »*. * 12

List of Illustrations

Page

Figure 1* Block diagram of equipment for slingram model experiments...... 8a

Figure, 2 to 6l. Slingram coils horizontal and co-planar..................

Figure 62 to 70. Slingram coils vertical and co-axial.................... o&& $£ £ Figure 73- to 72. Slingram coils vertical and co-planar................... .

'.-,-',-;,-.. -- ..- ". .;".';' '-,, *W

Figure 73. Slingram coils perpendicular.................................. .-g

Figure 7U- to 80. Turam coila horizontal.................. -v.............. %:*?

Preliminary report oa electromagnetic model studies

by

F. C. Frischknecht and G. B. Mangazx

Abstract .s.

More than. 70 response curves for various models have been, obtained

using the si Ingram and turam electromagnetic methods. Results show that for

the allngrfun method, horizontal co-planar coils are usually more sensitive than

vertical, co-axial or vertical, co-planar coils. The shape of the anomaly

usually is simpler for the vertical coils*

Introduction

Electromagnetic measurements furnish, a useful means of prospecting for

ore deposits and are becoming increasingly important for geologic mapping./

Results are usually interpreted by comparing the data with reference curves

established by previous experience or with curves obtained from model studies

in the laboratory, but few such curves have been published.

Since 1955, the U. S. Geological Survey has experimented with various\

types of electromagnetic methods in many areas. The present curves show '

the results of typical model studies undertaken for interpreting field

observations.

Some of the results were obtained by Friachknecht in the laboratories

of Geofysisk Malmleting, in Trondheim, Norway. The others were obtained*by

Frischknecht and Mangan in the Survey laboratories in Denver, Colorado.

Acknowledgments

The authors wish to thank Geofysisk Malmleting, Trondheim, Norway for

the use of their model laboratory, and the staff of Geofysisk Malmleting

for their advice and assistance.

Theory

The conditions of electrodynamic similitude are veil known. (Sinclair,i

. If displacement currents can be neglected, as is usually permissible

in low-frequency prospecting, the only requirement for similitude is that2 the conductivity parameter, apai > shall have the same value in the model

and in the earth, that is,

< where:

or is the conductivity

[i is the magnetic permeability

co is the angular frequency

L is the length, of a- selected segment, and

index 1 refers to the field condition and '

index 2 refers to the model.

On the assumption that in both, cases, 1jhe magnetic permeability is the same

as that of free space, ( y.-^ » ̂ *2

l (0l *! * a 2

If the linear dimensions in the field are 1,000 times those in the model,

(L^ » 1,000 L2), then:

o 2 ̂ 2 * a 1 ̂ l ̂ "^

For a thin conducting sheet and low frequencies, the electromagnetic

response depends on the parameter at \i a>L (Wait, 1953 )i where t is the

thickness of the sheet and the other symbols have been defined. Within pro

per limits, a conducting sheet may be substituted for another if at is the

same for both.

For purely inductive electromagnetic methods where the primary field

is set-up non-galvanically, the effect of country rock and overburden is

often small. Most model studies, therefore, have been made using metallic

models placed in air. The conductivity of metallic ores covers a wide range

A reporesentative value -for sulphide ores might be taken as 100 mhos/meter t

(Parasnls, 1956). Tfce conductivity of aluminum, which is commonly used in

model experiments, is about 3. ̂ 5 x 10? mhos/meter or 3.^5 x Kr times that\

of ore. If the frequency used in the model studies is the same as that

used in the field, then the dimensions of an aluminum model should be ..i

l/(3.^5 x 105 )1' 2 or 1:590 that of the field situation. Scales of from

1:500 to 1:1,000 are convenient, and with the use of field frequencies in

the model studies some of the components used in the field can be utilized.

Sllngram and turam methods

The slingram method utilizes two, small, portable coils; one serves

as a transmitter and the other as a receiver (frrisithknecht, 1959) A refer

ence voltage is brought from the transmitting coil to the receiving coil

with a cable. The ratio of the mutual impedance between the two coils isv

the presence of the earth, to their mutual impedance in free space is measured

by finding the complex ratio between the1 voltage Induced in the receiving

coil anri the reference voltage. The coils are moved together at a fixed

separation which, in practice is usually between 100 and 300 feet. Commonly,

the coils are oriented so that they are co-planar and horizontal; however,

occasionally the coils are held in perpendicular, vertical, co-axial, or

co-planar positions. The field frequency ranges from 500 to 8,000 cycles

per second.

In the turam method, a long grounded wire or a large, horizontal, in

sulated loop is used as the transmitter (Frischknecht, 1959)- Tne field-... -

frequency ranges from 100 to 800 cycles per second. Measurements are made

along traverses at intervals of 25 to 100 feet using two small receiving

coils, the lagging coil being placed at the position previously occupied by

the leading coil. The complex ratio of the voltages induced in the two coils

is measured. The ratios measured in the field are normalized by dividing

by theoretical ratios calculated from free space considerations. When

significant anomalies occur in the ratios, the actual normalized fields are

calculated by beginning with a measured, or an assumed value, for the field

at a point near the cable, and successively multiplying this value by the

normalized ratios.

1

Equipment and techniques

The studies were made using a frequency of 500 cycles per second.

The ratiometer and amplifier (fig. l) were made by Geofysisk Malmleting

Figure 1. Block diagram of equipment for slingram model experiments.

for turam field measurements. The alingram and the turam receiving coils

had several hundred turns of copper wire wound on plastic fbrms about 1 cm

in diameter. The reference coil was wound on the same form as the trans

mitting coil. Because the coils had a Q of only 2 at the selected frequency,

they were not tuned. It was not necessary to use electrostatic shielding

around the coils, as would be required at higher frequencies or with larger

coils. For the turam measurements, the transmitting loop was a single turn

of wire in a 100 cm x 1JO cm rectangle.

Under good conditions, the ratiometer could be read to 0.1 percent but

interference from the ninth harmonic of the power line frequency usually

prevented reading closer than 0.2 percent. The drift of the instruments

was small and could usually be eliminated by resetting the zero before

each run. If the drift was not negligible, it was assumed to be linear

during the run and the readings were .adjusted accordingly. The greatest

instrumental error in the measurement is probably less than 0.5 percent,

with either aign, although larger errors may occur for large anomalies be

cause of errors in determining the positions of the models.

8

RATIOMETER

AMPLIFIER

OSCILLOSCOPE

REFERENCE COIL

RECEIVING COIL

OSCILLATOR

POWER AMPLIFIER

TRANSMITTING COIL

MODEL

Figure I. Block diagram of equipment for slingram model experiments

8a

Data

Some of the curves presented in this report represent a systematic attempt

to study horizontal sheets of various dimensions. Others represent attempts*

to reproduce conditions found in field measurements. Frequently, dimensions

were selected according to the materials at hand.

Most of the curves were obtained for the slingram method, but a few

were obtained for the turam. method. The turam. curves correspond to normalized

field curves. Most of the slingram curves are for horizontal, co-planar

coil arrangements and all of the turam curves were for horizontal coils.

Profiles were observed for at least three heights above most of the models.

Most of the profiles were made along traverses which pass over the midpoint

and are perpendicular to the longest dimensions of the model. For all of

the profiles taken in this fashion, an end view of the model is shown. For

profiles where the traverse did not pass over the midpoint or was not per

pendicular to the longest dimension of the model, a plan view of the model

and the position of the traverse are shown.

Thin sheets are described in terms of their two largest dimensions and

the product, at, which has the dimensions of mhos and which was actually

measured in the laboratory. For the other models, all dimensions are speci

fied and no attempt was made to measure their conductivity.

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