DEPARTMENT OFMINERALS AND ENERGY

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS

0 14130

RECORD 1974/126

THE DUAL LOOP CONFIGURATION OF THE

TRANSIENT ELECTROMAGNETIC METHOD

by

BRIAN R. SPIES

The , ikfärmation contained in this report has been obtained by the Department of Minerals and Energy.p_dEts. of the policy of the Australian Government to assist in the exploration and development of

mineral; resources. It may not be published in any form or used in a company prospectus or statementwithouvithe permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

tateq ,

Rettled16710 6

Record 1974/126

THE DUAL LOOP CONFIGURATION OF THE TRANSIENT

ELECTROMAGNETIC METHOD

by

Brian R. Spies*

* Bureau of Mineral Resources, Geology & Geophysics,Australia. (presently on study leave at School of Earth

Sciences, Macquarie University, Sydney).

** Published with the permission of the Director,Bureau of Mineral Resources, Geology and Geophysics.

ABSTRACT

The transient electromagnetic method offersconsiderable advantages over conventional electromagneticsystems which make the method attractive under typicalAustralian conditions of conductive overburden. Mosttransient electromagnetic ground systems utilize a horizon-tal loop which is strongly coupled to horizontal conductors.By means of scale model studies and a field example it isshown that a dual loop configuration provides optimumdetectability of a vertical or steeply dipping conductor.In addition, the dual loop configuration reduces the problemof electrical interference

INTRODUCTION

Electromagnetic (EM) methods have been used inmineral exploration for more than fifty years and have beenparticularly successful in the search for sulphide -oredeposits. Most of the systems operate in the frequencydomain, involving continuous transmission at a fixedfrequency. A more recent development is that of time domainelectromagnetic systems, in which pulses are transmitted andthe transient decay of any resultant secondary field isrecorded during the interval between pulses. The namecommonly applied to such systemsis transient EM.

The concept of using electromagnetic transientsignals in prospecting for highly conductive orebodies wasproposed by Wait (1951). Since that time, Russianresearchers have been particularly active in the developmentand application of the method (Velikin & Bulgakov, 1967).In addition, much success has been acW.eved with an airborneversion of the method known as INPUT' ' (Boniwell, 1967).

Electromagnetic methods in general have met withvaried success in Australia; intensely weathered conductivenear-surface rocks and saline water commonly cause difficul-ties in producing and recording a significant response froman underlying conductor.

The transient electromagnetic (TEM) method offersa means of overcoming this problem and has been used inAustralia for a number of years. The INPUT system was firstflown in Queensland in 1964 and has since been used in otherparts of Australia. The first Russian-built groundtransient electromagnetic system was used in Australia in1970, and several such systems are in operation at present.Examples of case histories and scale model studies using theRussian-built MPPO-1 equipment, which is normally used witha combined transmitting and receiving loop, have been givenby Spies (1974). This paper presents field results andscale model studies of an alternative loop configurationusing the MPPO-1 equipment.

BASIC THEORY OF THE TRANSIENT ELECTROMAGNETIC METHOD

Any transient electromagnetic system basicallyoperates on the principle of inducing eddy currents in theground and analysing their decay, to provide information onsubsurface conductors. The changing current in the trans-mitting loop generates a magnetic field in the surroundingenvironment which in turn induces electric fields andresultant eddy currents in any nearby conductors.

(R)Registered tradename of Barringer Research.

-2-

These electric fields depend on the conductivity,shape, and size of the conductor, and on its position withrespect to the loop. After their initial establishment, theeddy currents in the conductor tend to diffuse inwardstowards the centre of the body, being gradually dissipatedas resistive heat losses. With a highly conducting body,the currents tend to circulate on the boundary of the bodyand to decay more slowly.

The decaying eddy currents produce a secondarymagnetic field, which decays within a relatively short timedepending on the physical and geometrical properties of theconductor. The receiving coil obtains an output voltageproportional to the time derivative (rate of decay) of thevertical component of the secondary field. In general, theresistivities encountered in the field produce transientdecays lasting from a fraction of a millisecond to 10 or20 milliseconds.

Analysis of the shape of the received transientwaveform is equivalent to measuring the response at a numberof frequencies for a harmonically varying source. Although,the transient curve involves the whole frequency spectrum,'different elements^contain differing proportions ofhigh- and low-frequency components.^Morrison, Phillips &O'Brien (1969) point out that at early times the response isdue to both low- and high-frequency groups whereas at latertimes only the low-frequency response remains and that thishypothesis, coupled with the fact that skin depth variesinversely as the square root of the frequency, suggests thatthe early part of the transient is governed by rapid decayat shallow depths, whilst the later part of the response isdue to lower-frequency energy which has penetrated togreater depths. This result has been verified by modelstudies by Velikin & Bulgakov (1967) and proved theoretic-ally by Wait (1956), Negi & Verma (1972), and others.

SCALE MODELLING RELATIONS

A description of the general theory of scalingelectromagnetic models is given by Frischknecht (1971). Itis sufficient to state here that the requirements forsimilitude between a field (full-scale) situation and alaboratory model are:

?O'm^Lm

2 = O'f /4f^uff^Lf -

where the subscript f indicates a quantity measured in thefield situation and the subscript m indicates a quantitymeasured in the model, and:

-3--

CY' . conductivity (S/m)

= magnetic permeability (H/m)

211 frequency (Hz)

L = linear dimensions (m)

The magnetic permeability is assumed to be that offree space for both the field situation and the model. Fora time domain system the time T can be substituted for-6-.The parameter T was not scaled in the model studies des-cribed in this paper.

Thus the modelling relation is reduced to

Crm = Lf 2

O'f^Lm2

Measurements were made using the standard field equipment,and models consisted of aluminium in air.. Results have beennormalized according to the relation:

e(t)^e(t)m^S.F.^microvolts/amp

Im^2500

where e(t) is the reading in microvolts at sampletime t after primary current cutoff.

Im is the current flowing in the model loop,in amps

S.F. is the scaling factor.

THE DUAL LOOP SYSTEM

The conventional horizontal loop used in the TEMmethod is maximally coupled to horizontal conductors asshown in Figure 1. However, many ore deposits are dyke-likeand dip steeply; for these, it is preferable to use a loopgeometry that will provide maximum coupling with verticalconductors and minimum coupling with shallow horizontalconductors. Vertical loops give optimum coupling withvertical conductors but are impracticable to use in thefield.

An alternative coil configuration involves a dualloop system. In this, two adjoining loops are connected inparallel as shown in Figure 1 such that the current flowthrough the two adjoining centre wires is in the samedirection, producing a strong local horizontal magneticfield.

-.4-

A comparison of the energizing primary magneticfield produced by the single and the dual loop coil config-urations is also shown in Figure 1. The lines of magneticflux are not influenced by the presence of underlyingconductors if a constant primary current is considered.This will be the case for a transient electromagnetic systemif the energizing current pulse is long enough for themagnetic field to become stable. At the instant when theprimary current is cut off, the primary field vanishes, andthe lines of magnetic flux present in the conductor will'collapse', causing eddy currents to flow. These time-vary-ing eddy currents will in turn produce time-varyingsecondary magnetic fields which are sensed by the receivingloop.

Thus, by examination of the flux linkage providedby a particular loop/conductor geometry it is possible topredict the expected response from a conductor. By referr-ing to Figure 1 it can be seen that if a thin vertical plateis ! at the centre of the single loop, the flux linkage isnil, which explains why the TEM response is zero directlyover the plate irrespective of depth to the top (Velikin &Bufgakov, 1967, pp. 16-17). With the dual loop system, onthe other hand, there is a strong horizontal field at thecentre of the coil configuration which has maximum couplingwith a vertical conductor.

To test the relative merits of the single and dualloop systems in an environment involving conductive over-burden, a model (shown in Fig. 2) was set up with a verticalconductor of 20 S/m conductivity overlain by a horizontalconducting sheet. The sheet had a conductivity-thicknessproduct of 16 Siemens, and could thus simulate a layer 16 mthick with 1 S/m conductivity. Such a sheet would effectiv-ely mask any response from a deeper body for most EMmethods. Profiles recorded from both configurations arepresented in Figure 2. The profiles for the single loopsystem show that for early sample times after primarycurrent cut off the response is primarily due to thehorizontal sheet, and only at later sample times can theresponse of the underlying conductor be resolved as atwo-peak anomaly, the peaks being about twice the amplitudeof background values at t = 10.1 ms.

The profiles for the dual loop system show thatthis coil configuration is more loosely coupled to thehorizontal sheet, the response at 1.1 ms being only onefifthof that recorded with the single loop. At later sampletimes the effect of the underlying vertical plate is clearlyrecognized; at 10.1 ms the response is effectively due onlyto the deeper conductor. The peak of the anomaly at 10.1 mshas an amplitude well above background values.

1

-5--

To test the relative merits of the two loopconfigurations a field test was conducted at Dobbyn, 115 kmnorth-northeast of Mount Isa in northwestern Queensland.The area contains steeply dipping sulphide lodes in meta-morphosed rhyolite and dolerite. The results of a TEMsurvey using 100 m, 50 m, and 25 m loops over one suchmineralized zone are shown in Figure 3 for both single anddual loop configurations. The profile shape for a singleloop has two maxima separated by a distance equal to theloop size. The dip of the body can be computed from theratio of the amplitude of the larger peak to the smallerpeak by the relation:

e (t) Max,

= e2.70 (Velikin & Bulgakov, 1967)

e (t) Maxs

where 0 is the angle between the plane of theplate and the vertical,

e (t) max i is the amplitude of the largerpea,

e (t) max is the amplitude of the smallerpea.

Substituting in values from the profiles, 0= 20 ° (dip = 70 ° )for both 108 in and 50 m loop sizes. This agrees with a dipvalue of 80 estimated from drilling results (Fig. 3).

On the profiles for the dual loop configurationthe main peak occurs directly over the top of the body, withsmaller peaks either side, and corresponds in position tothe Turam anomaly. The larger of the two minor peaks islocated on the down-dip side of the conductor. The fieldresults agree with the profiles expected o from model studiesfor a thin conducting plate dipping at 70 .

An important advantage in using the dual loopconfiguration is the inherent noise rejection. One of themain difficulties in using TEM is electrical interference.However, many noise sources (such as power lines) producecoherent noise, which can be effectively cancelled by a dualloop system.

Thus, advantages in the dual loop system are:

(1) Reduction of electrical interference whose phaseis identical in the two loops.

(2) A peak occurs directly over the top of the conduc-tor instead of being displaced to the side.

-6-

Disadvantages are:

(1) Electrical interference from impulse sources, e.g.wind moving the loop wire, may be increased.

(2) Areal coverage is reduced because of the extratime needed to lay out a dual loop.

CONCLUSIONS

The transient electromagnetic method offers con-siderable advantages over conventional electromagneticmethods. Most TEM ground systems employ a horizontal loopwhich is strongly coupled to horizontal conductors. It hasbeen demonstrated by scale model studies and a field examplethat the dual loop configuration provides advantages whenthe target is a steeply dipping conductor. In addition, useof a dual loop reduces some troublesome electrical inter-ference.

ACKNOWLEDGEMENTS^ 1.

I wish to thank the Director of the Bureau ofMineral Resources for permission to publish this paper. Thepossibility of using a dual loop configuration was firstpointed out to me by Professor L.V. Hawkins.

REFERENCES

BONIWELL, J.B., 1967 - Some recent results with the Inputairborne EM system: Canad. Min. Metallurg. Bull., V. 60,p. 325-332.

FRISCHKNECHT, F.C., 1971 - Electromagnetic scale modelling,in: Electromagnetic probing in geophysics (Wait., J.R.ed.): Boulder, Golem Press.

MORRISON, H.F., PHILLIPS, R.J., and O'BRIEN, D.P., 1969 -Quantitative interpretation Of transient electromagneticfields over a^layered half space: Geophys. Prosp.,v. 17, p. 82 - 101,

NEGI, J.G., and VERNA, S.K., 1972 - Time domain electro-magnetic response of a shielded conductor: Geophys.Prosp., v. 20, p. 901-909.

-7-

SPIES, B.R., 1974 - Experience with the transient electro-magnetic method in Australia: Bur. Miner. Resour. Aust. Report (in prep.).

VELIKIN, A.B., and BULGAKOV, J.I., 1967 - Transient methodof electrical prospecting (one-loop version): Inter-national seminar on geophysical methods of prospectingfor ore minerals. Moscow, 1967.

WAIT, J.R., 1951 - A conducting sphere in a time varyingmagnetic field: Geophysics, v. 16, p. 666-672.

WAIT, J.R., 1956 - Shielding of a transient electromagneticdipole field by a conducting sheet: Canad. Jour. of Physics, v. 34, p. 890-893.

SINGLE LOOP

T .

50-

■•‘..

-

100

LOOP CONFIGURATION

150

IFig. I

A

LOOP CONFIGURATION

TiLFGEND

MAGNETIC FIELD DIRECTION(Lines of FLUX)

LOOP CURRENT iNTO PAGE

LOOP CURRENT OUT OF RAGECONTOURS OF EQUAL FIELD STRENGTH

TESLA1r7;:-.1 MORE 1HAN DO

0 - 20

ED,^5 —10

2-5

= LESS THAI: 2

Record No. /974//26

G 29-211—IA

LEGEND Conductivities -Siemens

d4.16-5rn6.3 --• 0

No /974426

Fig 2

f5000—

.^ 1.11 ms

2 - 3

1000-

500 -- ---- _^---- -----

2 - 3

4-I

100

0

10 1•—■

10- 1

200^meNes

SHEET, IhicknEss j,

■50 ' 40 100 150

MIA WI VIII C7,7 IRIL WORM% %MOIL VIIIMILILIM

PROFILES FOR SINGLE LOOP Sheet: = 16S d2

PROFILES FOR DUAL LOOP 1:52=20S/m d 3 35m

I .

1

G 29-213-1A

Fig. 3

SINGLE LOOP

1 00 m loop

r 90 m

• 25m

150

50

DUAL LOOP

100 m loop

7\0 50m'---.

. •

/ 25rp-

`..--..-^..------^....... ..—

Turorr4onomoly

IP anomaly

\ Drill hole---- Model study results ,100 m loop

0^Field results , 100 m loopField results , 50 m loop

.---• Field results , 25 m loop

GOSSAN

Rhyolite^11

Dolerite

SULPHIDES' .,:ts,.\

Rhyolite

Pocord No 1974/126

25^0^25^50^75^100

Metres

E54/R7 - 78 . i