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GEOPHYSICS, VOL. 40, I\‘O. 4 (AUGUST 1975). P. 664-668; 1 FIG., 1 TABLE

THE TELLURIC-MAGNETOTELLURIC METHOD

.JOHiX F. HERMANCE”

AND

RICHARD E. THAYER”

The telluric-magnrtotelluric method uses

magnetotrlluric measurements at the base site,

but only telluric mrnsurements at remote sites.

It thus combines the economy, simplicity, and

speed of the traditional telluric method with the

quantitative advantages of the traditional mag-

netotelluric method. The dominant features of

the combined method are the following: First,

the time required to set up a telluric site is less

by a factor of at least, 5 than the time for a

complete magnetotelluric site. Second, one does

not need to record magnetic field data at the

base site simultaneously wit’11 the electric field

recorded at each remote site. One needs only

enough magnetic data to adequately determine

the base tensor. A tclluric transfer tensor cou-

pling electric field measurements at t#he base site

and each remote site can be used to transfer the

base impedance tensor to an impedance tensor

at each rem&e site. By being much more selec-

tive of the magnetic data used in the analysis,

one can significantly improve the signal-to-noise

rat#io. Third, the data are analyzed to determine

each element of the complex imlwdance tensor

so that, important, phase information as well as

cLm#plitude information is availablr for interpre-

tations which are more sophisticated than those

currently attempted in convent ional telluric

surveys. Finally, in making the ultimate inter-

pretation in terms of the impedance tensor

rather than the telluric tensor usrd in conven-

t,ional telluric surveys, one essentially refers t#he

interpretation of remote electric field observa-

tions to the ma.gnefic field at the base site rather

than to the electric field. Both rsperience and

model studies demonstrate t#hat the magnetic

field is much more homogeneous than the elec-

tric field in the vicinity of lateral hricrogeneities;

thus the selection of a proper base site is not

as critical in the combined method as it is in

the conventional telluric method.

INTRODUCTION

The

telluric

n&hod has evolved over a num-

ber of years through the notable efforts of

Schlumberger (1939) in France, Berdichevskiy

(19G5) in Russia, and Yungul (1966) and

Yungul et al (1973) in the United States. The

method involves the comparison of horizontal

electric field measurements simultaneously re-

corded at a base site and a remote site; the

measurements WC grounded electrode lines.

After sufficient data arc acquired from one

remote site, the portable instruments from that

site are moved to a new location while the base

station is kept fixed (see Figure 1). After a

large number of remote sites are occupied, a

patt,ern may emerge that reveals the distortion

of regional electric current systems around

local geolrogic structures of interest, provided,

of course, that these structurrs exhibit an

elect,rical contrast with their surroundings.

Alt#hough many features of telluric current,

fields make them potentially attractive can-

didates for regional reconnaissance studies (for

example, the fact that one usch broad-scale

natural

sources rather than localized, high-

powered artificial sources), the application of

these measurements, as in most geophysical

techniques, is restricted by the tools one has at

hand for t,heir interpretation. In fact, it appears

from the current lit,erature (Kellrr and Frisch-

knecht, 1966; Yungul, 1966, 1968; Yungul et al,

1973) that the reduction and interpretation of

Manuscript received by the Editor May 29, 1974; revised manuscript received January 22,

1975.

* Brown University, Providence, RI 02912

@ 1975 Society of Exploration Geophysicists. All rights resew&.

664

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The telluric-Magnetotelluric Method 665

FIG.

1.

Plan view schematically representing the location relative to a suspected anomalous structure

of a magnetotelluric base site and telluric remote sites.

these

measurements is generally qualitative and

only becomes quantitative for severely restricted

field situations seldom encountered in practice.

In contrast, the magnetotelluric method,

which uses information from both the magnetic

and the electric field intensities, has evolved

considerably from the original concepts of

Cagniard (1953). These concepts were valid for

simple plane-layered situations, but the method

has reached a point where it is now a quantita-

tive tool for commercial exploration, even in the

presence of large scale lateral inhomogeneities

(Word et al, 1971; Vozoff, 1972).

However, there are several problems involved

in using the conventional magnetotelluric

method for high density coverage of a survey

area. First’, from one to several hours are neces-

sary t’o set up a complete magnetotelluric sound-

ing experiment, whereas a telluric system can be

set up in ten or fifteen minutes. Second, for a

given source field strength, it is generally more

difficult to obtain high-quality magnetic field

data by the use of convent,itrnal induction-coil

sensors or fluxgate magnetometers than it is to

obtain high-qualit,y electric field data, with

grounded electrode lines. Although adequate

electric field data, are available a great deal of

the time high amplitude magnetic field data

occur much less frequently. This means, of

course, that one may have to record magnetic

field variat,ions for a longer period of time to

obtain adequate data. On the avera.ge, a sig-

nificantly longer time is required for each mag-

netotelluric measurement than for a conven-

t,ional telluric measurement. Clearly, the very

feat,ure that makes the magmtotelluric method

attractive, namely, that it incorporates magnetic

field information into the anal)&, may serve to

detract from its usefulness for rapid reconnais-

sancc surveys.

This, then, was our motivaiion for develop-

ing a technique which combines the economy,

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666 Hermance and Thayer

simplicity, and speed of the traditional telluric

method with the quantitative characteristics of

the tradit,ional magnetotellwic method. Al-

though from time to time various workers have

alluded to this idea (Srivastava et, al, 1963;

Vozoff et al, 1964; Yungul, 1966; Yungul et al,

1973), very little quantitative work seems to

have been done toward actually combining the

two methods, except for a series of measure-

ments reported by Madden and Swift (1969)

;

they used a fixed base magnetic observatory and

remote telluric. field observations. In their ex-

periment, however, the telluric field was not

recorded at the base site as we advocate in the

following discussion. We might, classify their

experiment as a telluric-magnetic experiment

rat,her than a telluric-magnetotelluric experi-

ment, such as is described in this paper.

THE TELLURIC-MAGNETOTELLURIC METHOD

Base impedance tensor

The tensor impedance is defined as the matrix

of linear coupling coefficients relating the mag-

netic field components measured at a point t,o

the elect,ric field components measured at the

same point. In other words, one postulates the

existence of a relation of the form

or, in a more condensed form,

Eb =

[Z”].H’,

where the superscript b indicates that the field

components are recorded at a base site; hence

[Zb] is called the base-site magnetotelluric im-

pedance tensor, or simply t’he base impedance

tensor.

The electric field E consisting of two horizontal

components (E,,E,) and the magnetic field H

consisting of the two horizontal components

(HZ,&,) are monitored more or less continuously

at the base site throughout the survey. Actual

recordings of E and H are made during periods

of appropriately high magnetic activity. From

select’ed samples of E and H the tensor im-

pedance elements are determined by the use of

one of a number of available techniques (see, for

example, the review by Hermance, 1973).

The telluric transfer tensor

The behavior of electric fields in the vicinit’y

of laterally inhomogeneous structures can be

calculated in closed form at the dc limit for a

variety of simple two- and three-dimensional

theoretical models (Berdichevskiy, 1965). These

solutions can then be used to develop for a par-

ticu1a.r model a linear relation between the

electric vectors observed at two points on the

earth’s surface. If we let one of there points cor-

respond to our base site and the other point to

our remote site, as shown in figure 1, we have

in general a linear coupling relation of the form

[:::I = [$ ::;]Q:] , (3)

or in a more condensed form

E’ = [T].E~, (4)

where [T] is the telluric transfer tensor. Except

for the dc limit, the tensor elements in [T] will

be complex functions of frequency as well as

position.

It turns out that (3) is not, only valid for

simple t,heoretical models

;

it can also be used

as a conventZion for expressing the relation be-

t,ween telluric field measurements made in na-

ture. The usefulness of the relation has been

verified experimentally many times by demon-

strating the st,abilit#y of estimates of t’he t,ensor

elements derived for different record sections.

However, as might be expected, certain cases

could arise in which (3) is not valid, and one

must be cautious in applying it. Our experience

in the field suggests that the telluric transfer

tensor as defined above is generally a valid

relation within a precision of 5 percent and

that situations in which it is not valid are more

easily conceived in t,heory t,hnn realized in

practice.

Determining the tensor elements of [T] from

simultaneous samples of EV and Eb is directly

analogous to determining the t,ensor elements

of [Zb] from simultaneous samples of E* and Hb,

as discussed in the previous section. Therefore

we can determine the elements of [T] by the use

of the same techniques described by Hermance

(1973).

In this way, a telluric transfer tensor that

relates the electric field components at, a given

remote site to the electric field components at

the base site can be calculated.

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The

Telluric-Magnetotelluric

Method 667

The transfer impedance tensor

Table 1. Comparison of the direct and indirect

At t,he base site, we have from simultaneous

methods for estimating the transfer impedance

tensor.

electric and magnetic field meas4rements~ Indirect. Direct,

E’ = [Zb].Hb. (5) Element estimate estimate Deviation

considered A 13 N%o)

Between the remote site and t’he base site, we

0.150

0

232 -4.8

have from simultaneous electric field measure-

-0.011 0.078 -5.2

1.222 1.‘291 -4.1

ments

0.167 0.

203

-2.1

-1.006 -1.011 to.3

E’ = [T] .E*. (6)

-0.385

-0.248 -8.4

0.323 0.298 +1.5

Substituting (5) into the right-hand side of (6),

0.140 0.050 +5.3

we obtain

E’ = [T].[Z*].H*. (7) method using the two-stage analysis described

above, which yields the tensor product [T]-[Z”].

We will call the product of t’he telluric trans- Table 1 summarizes the results of a comparison

fer t,ensor and the base impedance tensor the between the two methods using data from two

transfer impeda.nce tensor sites on the Reykjanes Peninsula in southwest

Iceland. The base site is located at the center of

[Z’I =

Pl.[Zbl,

the peninsula, and the remote site is 7 km to the

since it relates the electric field at, the remote south along the sea coast. We define the percent

site to the magnetic field at the base site. deviation for each tensor elemcut as

The remote impedance tensor

It seems plausible to assume that the horizontal

magnetic field is in fact reasonably uniform over where A corresponds to the indirect estimate, B

the survey area. For example, in Iceland we have corresponds to the direct Pstimate, and the

found from simultaneous measurements with Euclidean norm of the tensor

magnetometers separated by a distance of 30 km

that H is uniform to within IO percent, even

llzll = Pii

IzJl”2

beneath the auroral zone. In this case, if we

assume that over the survey area H’ G Hb, then is a measure of its magnitude. In Table 1, we

to a very good approximation the impedance compare the actual values of the tensor cal-

tensor at the remote site is simply given by the culated by each method and give the percent

transfer impedance tensor: deviation of each element. The agreement is

quite good. For interpretative purposes, it is

[zr] G [Z”]. often desirable to rotate the impedance tensor

We have, therefore, transferred an impedance int,o its principal coordinates, calculating the

measurement’ at the base site to an impedance maximum and minimum tensor resistivities For

estimate at the remote site using only electric example in the direction for which Z,, is maxi-

field recordings. We have not had to move our mized,

magnetometers, nor have we had to make mag-

netic field measurements simultaneous with our

remote telluric recordings. where f is the frequency in PC+. When this is

AN EXPERIMENTAL TEST

done for the direct and indirrct8 tensors in

We

now consider the precision with which the Table 1, the results are 17.2 and 16.3 ohm-m,

transfer impedance tensor can be estimated. Two respectively, for the maximum and 10.7 and

methods for estimating

[ZI]

are available: a 10.4 ohm-m for the minimunl resistivities. In

direct method using simultaneous measurements general, it. appears that the two methods can be

of the magnetic field at the base site and the expoctcd to agree, both in individual tensor

_.

electric field at the remote site and an indirect elements and auxiliary parameters, to within

668

Hermance and Thayer

5 percent, a more than satisfa.ctory tolerance for

many applications.

CONCLUSIONS

The dominant advantages of the combined

telluric-magnetotelluric met’hod outlined here

are the following:

First, the t,ime required to set up a telluric

site is less by a factor of at least. 5 than that

required for a complete magnetotelluric site.

We are grateful to axel Bjiirnsson of the

Department, of Natural Heat, National Energy

Authority of Iceland, for his assistance in ac-

quiring the field data used above. This research

was supported by NSF grants G$-33444 and

GA-37092.

Second, one does not need to record magnetic

field data at t)he base site simultaneously with

each remote site. One needs only enough mag-

netic data to determine adequately the base im-

pedance tensor and can therefore be much more

selective in choosing the magnetic data used in

the analysis. This point is sometimes quite im-

portant, since by carefully selecting the data to

be used in the final analysis one can improve

the signal-to-noise ratio by more than a factor

of five.

Berdichevskiy, M. N., 1965, Elect.ricnal prospecting

with the telluric current method: Quart. Colo-

rado School of Mines, v. 60, no. 1. (English

translation of original 1960 Russian article.)

Cagniard, L., 1953, Basic theory of the mag-

netotelluric method of geophyslcnl prospecting

:

Geophysics, v. 18,. p. 605-635.

Hermance, J. F., 1973, Processing of magnetotel-

lurie data: Phys. Earth Plan. Interiors, v. 7,

p.

349-364.

Third, the dais arc analyzed to determino

each eiement of the cornpiex impedance tensor,

so that important phase information as well as

amplitude information is available for interpre-

t&urn- mmc sop~hisricated than those currentiy

attempted.

Keller. G. V.. and Frischknecht. F. C.. 1966. Elec-

t,rical methbds in geophysical’ prospecting’: New

York, Pergamon Press,

519

p.

Madden, T. R., and Swift, C. M.! Jr., 1969, Mag-

netotelluric studies of the electrical conductivity

st.ructure of the crust and upper mantle, &I

The earth’s crust and upper mantle: AGU

Geophys. Monogr.

13,

Pembroke J. Hart, ed.,

D. 469-479.

Finally, in making the final interpretation in

terms of the impedance tensor rather than the

telluric tensor used in conventional telluric sur-

veys, one essentially refers the interpretation of

remote electric field observations to the magn,etic

field at t)he base site rather than to the base-site

electric field, whirh itself may be strongly dis-

torted due to local inhomogeneous structures.

Experience and model studies demonstrate that,

tho magnetic field is much more homogeneous

in the vicinity of lateral discontinuities than is

the electric field; thus the selection of a proper

base site is not as critical in the combined

method as it is in the conventional telluric

method.

Schlumbrrger, M., 1939, The application of telluric

currents to surface prospecting: Trans. AGU,

p. 271-277.

Srivasta,va, S. P., Douglass, J. L., and Ward, S. H.,

1963. The application of the mannetotelluric

and ‘telluric Methods in central Alberta: Geo-

physics, v. 28, p. 426446.

Vozoff, K., 1972, The magnetotelluric method in

the exploration of sedimentary basins: Geo-

physics, v. 37, p. 98-141.

Vozoff, K., Ellis, R. M., and Burke. M. D., 1964!

Telluric currents and t.heir use in petroleum

cxplorat,ion

:

Bull. AAPG, v. 48, p. 1890-1901.

Word. D. R.. Smith. H. W.. and Boslick. F. X.. Jr..

197i,

Cruital invkstigations by the magnetbtel:

luric tensor impedance method,

in

The structure

and physical properties of the earth’s crust:

AGU Mcncr. 14. J. G. Heacnck 0~1

*. _U.,

D. !&J-E?.

Clearly, the t&uric-magnetotelluric method

allows one to retain the speed and economy of

the traditional magnetotelluric method as well

as the quantitative aspects of the traditional

magnetotelluric method.

Yungul, S. I?., 1966, Telluric so&d&-A mag-

netot,elluric method without magnetic measure-

ments: Geophysics, v. 31, p. 185-191.

1968, Measurement of telluric “relative

ellipse area” by means of “vectograms”: Geo-

physics, v. 33, p. 127-131.

Yungul, S. H., Hembree, M. F+, and Greenhouse,

J. P., 1973, Telluric anomahes associated with

isolated reefs in Midland Basin, Texas: Geo-

physics, v. 38, p.

545-556.

ACKNOWLEDGMENTS

REFERENCES