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The Telluric-Magnetotelluric Method

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The Telluric-Magnetotelluric Method

Abstract

The telluric‐magnetotelluric method uses magnetotelluric measurements at the base site, but only telluric measurements at remote sites. It thus combines the economy, simplicity, and speed of the traditional telluric method with the quantitative advantages of the traditional magnetotelluric 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 with the electric field recorded at each remote site. One needs only enough magnetic data to adequately determine the base tensor. A telluric transfer tensor coupling electric field measurements at the base site and each remote site can be used to transfer the base impedance tensor to an impedance tensor at each remote site. By being much more selective of the magnetic data used in the analysis, one can significantly improve the signal‐to‐noise ratio. Third, the data are analyzed to determine each element of the complex impedance tensor so that important phase information as well as amplitude information is available for interpretations which are more sophisticated than those currently attempted in conventional telluric surveys. Finally, in making the ultimate interpretation in terms of the impedance tensor rather than the telluric tensor used in conventional telluric surveys, one essentially refers the interpretation of remote electric field observations to the magnetic field at the base site rather than to the electric field. Both experience and model studies demonstrate that the magnetic field is much more homogeneous than the electricfield in the vicinity of lateral heterogeneities; thus the selection of a proper base site is not as critical in the combined method as it is in the conventional telluric method.
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 wit11 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 to 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,
Downloaded 05/14/19 to 128.148.254.57. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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 the 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
selected 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 vicinity
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
earths 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 the 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 the 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 the 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
[ZI =
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
IzJl2
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
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668
Hermance and Thayer
5 percent, a more than satisfa.ctory tolerance for
many applications.
CONCLUSIONS
The dominant advantages of the combined
telluric-magnetotelluric method 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 geophysicalprospecting: 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 earths 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 earths 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
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... Because a uniform, dense grid of sites can be prohibitively expensive to collect, an attractive alternative is complementing a coarser, largescale grid of sites with more densely spaced sites in regions of primary interest. Data acquisition can further be optimized with the telluric-magnetotelluric (T-MT) method (Hermance & Thayer 1975), whereby the magnetic field is recorded only at a subset of locations (Yungul 1977;Iliceto & Santarato 1986;García & Jones 2005;Melosh et al. 2010;Campanyà et al. 2014). From a methodological perspective, handling T-MT data requires only modest modifications to the data processing and inversion tools to take full advantage of simultaneously recording arrays (Egbert 2002). ...
... Conventionally, electric and magnetic fields are recorded at the same location r l . The T-MT method (Hermance & Thayer 1975) introduces an inter-site impedance, Z i , defined as ...
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... Conventionally, electric and magnetic fields are recorded at the same location r l . The T-MT method (Hermance & Thayer 1975) introduces an inter-site impedance, ...
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... Measurements were carried out in the nominal bandwidth 128 Hz-40s with Pb/PbCl 2 electrodes, CM11E induction coils and the MkIIb model of the Short Period Automatic Magnetotelluric (SPAM), developed in the U. of Edinburgh by Dawes (1984). Given that SPAM enabled simultaneous two-station data acquisition, the Telluric -Magnetotelluric field procedure (Hermance and Thayer, 1975) was implemented, for which information is provided in Section S1 of the Supplementary Information. The data were acquired using a 5-component Magnetotelluric configuration at a Base site (denoted by the suffix 'b') and a 2-component Telluric configuration at the Remote site (denoted by the suffix 'r'). ...
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Geophysical methods of analysis were applied, in order to investigate the deep structure and the geothermal potential of the Methana Volcano (NE Peloponnesus, Greece). The study is based on a re-evaluation and reinterpretation of legacy magnetotelluric (MT) data with modern analysis methods, as well as 3-D inversion of aeromagnetic data constrained by in situ measurements of magnetic susceptibility. Magmatic systems are located in regions of active tectonic processes that often play a controlling role. The MT method is effective in delineating low resistivity functional elements of volcanic systems, such as magma chambers, vents, thermal fluid reservoirs and thermal fluid circulation conduits, the latter two of which are typically associated with active faults. The aeromagnetic data can assist in mapping the configuration, hence emplacement modes of volcanic rocks at depth. Accordingly, the joint interpretation of these lines of evidence, together with structural and geochemical information, is expected to allow insight into the influence of contemporary tectonics on the inception and evolution of the volcano. The contemporary stress field is mainly extensional, NNE-SSW oriented and overall homogeneous; in the area of Methana it allows for the formation of WNW-ESE north-easterly dipping normal faults, W-E faults consistent with the synthetic (dextral) R-shear direction of Riedel's shear theory and NW-SE faults consistent with the antithetic (sinistral) R′-shear direction; all such features have been mapped on Methana Peninsula. The magnetotelluric data imaged a significant geothermal reservoir developing around an intersection of the three active fault zones (normal, R and R′) at depths of 1–1.5 km below the centre of the peninsula, as well as elongate epiphenomenal conductivity anomalies associated with the circulation of thermal fluids along all three fault zones. The 3-D magnetic susceptibility model strongly suggests that the intrusion and emplacement of magmas were guided by the same active fault zones, with particular reference to the R and R′ shears whose influence is imprinted on the configuration of volcanic rocks at depth. The joint interpretation of all lines of evidence indicates that magmatism and volcanism at Methana are almost completely controlled by tectonic activity in a manner analogous to the situation of the large Santorini Volcanic Complex. It also indicates that the reservoir is replenished through the weak permeable zone created by the intersection of the R and R′ shears, which is very probably collocated with the main vent of intrusive magmatic activity and may connect with a shallow magma chamber at depths greater than 4.5 km. The apparently common origin and similarities/differences in the circulation paths of thermal fluids may amply explain both the individual characteristics and similarities/differences in the chemical composition of thermal spring discharges, which have been reported by hitherto geochemical investigations.
... The developed algorithm can also use the intersite tensors as data for an inversion. The quasielectric tensor (Q) can be defined as (Hermance and Thayer, 1975) ...
Article
A new algorithm is developed for the inversion of magnetotelluric (MT) data. The developed algorithm is built to be fast, versatile, and accurate. A fast inversion algorithm has to include a fast forward-modeling routine. To achieve that, a hybrid approach consisting of finite differences (FD) and finite elements (FE) is used to benefit from both the speed of the FD method and the flexibility to add topographic features of the FE method. To reduce the number of cells, and thus reducing the size of the system to be solved in the forward and pseudo-forward solutions, different meshes for various groups of frequencies are used. These are then mapped onto the inversion mesh by a mesh decoupling technique to further accelerate the inversion. To build a versatile inversion algorithm, the capability of using different data types is implemented. In addition to the impedance tensor and the magnetic transfer function, the algorithm also computes the phase tensor and phase vector, which are distortion-free forms of MT data. It is also possible to invert inter-site data and their respective phase tensors using the developed code. Furthermore, the distortion matrix can also be estimated as a parameter. The new code is tested with different noisy and distorted synthetic data measured on a surface with topography to evaluate inversion accuracy and computational efficiency. The results indicate that the code is accurate and that the runtimes are reasonable for the large three-dimensional models considered. Employing four graphical processing units, the hybrid forward modeling approach and the mesh decoupling technique all together resulted in a 12 times speed-up for the examples presented in this study.
... Calculations are performed using the APPL 3-D conductivity model. 3-D MT intersite impedances Z i (r s , r b , ) (Campanya et al., 2019;Hermance & Thayer, 1975;Kruglyakov & Kuvshinov, 2019) that relate the surface horizontal electric field at each grid point r s with the surface horizontal magnetic field at a fixed base ...
Article
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The intensification of the fluctuating geomagnetic field during space weather events leads to generation of a strong electric field in the conducting earth, which drives geomagnetically induced currents (GICs) in grounded technological systems. GICs can severely affect the functioning of such infrastructure. The ability to realistically model the ground electric field (GEF) is important for understanding the space weather impact on technological systems. We present the results of three-dimensional (3-D) modeling of the GEF for the eastern United States during a geomagnetic storm of March 2015. The external source responsible for the storm is constructed using a 3-D magnetohydrodynamic (MHD) simulation of near-Earth space. We explore effects from conductivity contrasts for various conductivity models of the region, including a 3-D model obtained from inversion of EarthScope magnetotelluric data. As expected, the GEF in the region is subject to a strong coastal effect. Remarkably, effects from landmass conductivity inhomogeneities are comparable to the coastal effect. These inhomogeneities significantly affect the integrated GEF. This result is of special importance since the computation of GICs relies on integrals of the GEF (voltages), but not on the GEF itself. Finally, we compare the GEF induced by a laterally varying (MHD) source with that calculated using the plane wave approximation and show that the difference is perceptible even in the regions that are commonly considered to be negligibly affected by lateral nonuniformity of the source. Overall, the difference increases toward the north of the model where effects from laterally variable high-latitude external currents become substantial.
... During the last 10 years, the telluric method has regained favour, rather as an auxiliary method of MT itself (Hermance & Thayer 1975;Stodt, Hohmann & Ting 1981). This is because if the horizontal magnetic field does not vary laterally each telluric station may become a full MT sounding, with evident savings in time and equipment costs. ...
Article
The experience, that has generally to now been reached in digital processing of the Magnetotelluric (MT) time series, allows to apply the MT commonly used techniques of automatic computing to the telluric method also. This possibility allows to obtain from only the telluric time series more numerous and detailed information with respect to the customary analogue working out of the telluric signal. On the other hand it involves both a more appropriate setting of the telluric surveys and a more significant presentation of the results. -English
... The method consists of determining the quasi-MT transfer function at a local site from the multiplication of two tensor relationships: (1) The transfer function between the daytime telluric channels of the local site and a base site and (2) the transfer function of the night-time conventional MT impedance tensor at the base site. The obtained transfer function represents the ratio of the local-telluric to base-magnetic fields obtaining the telluric-magnetotelluric (T-MT; Hermance & Thayer 1975) AMT transfer function of the local site. Garcia & Jones (2005) also note that the quasi-MT transfer function can be a reasonable approximation of the real AMT impedance tensor of the local site if there are no major differences between the horizontal magnetic fields acquired at the two sites. ...
Article
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A new methodology to estimate magnetotelluric (MT) tensor relationships, called Estimation of Local transfer-functIons by Combining Interstation Transfer-functions (ELICIT), is proposed whereby the MT tensor relationships of the local site are derived using only interstation transfer functions. The MT impedance tensor and the geomagnetic transfer function at the local site are characterised by combining interstation tensor relationships between electric and magnetic fields at the local site with the horizontal magnetic fields acquired at a neighbouring site. The main property of the proposed method is that the employed interstation transfer functions are independently constrained, without the need to acquire the electric and the magnetic fields at the local site simultaneously to recover the local MT tensor relationships. Due to this property, the ELICIT method offers new possibilities for MT data acquisition and processing, providing significant improvements when the magnetic time-series at the local site are affected by local noise or are truncated. Error analysis shows that, even when magnetic fields are truncated, the quality of the results obtained following the ELICIT method are similar to those we would obtain if the magnetic fields had not been truncated. Another important property is that different neighbouring sites can be used to recover the tensor relationships at the local site. Averaging of results obtained using different neighbouring sites can be performed to improve the statistics. For our example data, when the ELICIT method is used to improve the statistics, errors of the estimates for periods between 1000 and 20 000 s periodicities are clearly reduced. All interstation transfer functions are calculated doing remote reference and the bootstrap method is used to compute the errors, when necessary. Long period magnetotelluric data acquired in the Pyrenees and in the Atlas Mountains in Morocco, and magnetic data provided by Furstenfeldbruck magnetic observatory have been used to test the proposed ELICIT method, with positive results. Due to the lack of requirement that the electric and the magnetic fields of the local site be acquired simultaneously, the proposed method also offers new possibilities for MT data acquisition, optimizing the available instrumentation.
Article
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A common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses.
Article
By studying the relation of electromagnetic fields and impedance of sill like medium between water surface and water floor, we have gained the attenuation coefficient and the attenuation law of electromagnetic fields in water area. It is obvious that the signals are attenuated worse when the frequency increases, but the liquid attenuation characters upon electric field and magnetic field are different. First, the attenuation amplitude of magnetic field is far greater than electric field in the same condition, and the frequency effect range is wider. Second, the attenuation of electric field not only lies on mostly the character of liquid self (the depth and resistivity of liquid), but also on the media property below the water, that is, the higher the resistivity is the more severe the attenuation of the magnetic field, the attenuation of magnetic field relates to the characters of liquid medium. The condition in which the water effect can be neglected in water MT exploration of electromagnetic signal sampling is as follows: the water depth is less than 3 m in MT, 2 m in CSAMT.
Article
It appears that strong but irregular positive telluric anomalies are geographically associated with three major, isolated, known, and deeply buried reefs. These are the Millican, Jameson, and IAB Reefs, covered by a telluric survey in the Midland Basin Texas. Dipole resistivity sounding and E-log analyses give information about the resistivity distribution in the post-reef sediments. The Millican Reef telluric anomaly was interpreted quantitatively by two-dimensional modeling. This anomaly apparently is almost totally due to lateral variations in resistivity within most of the "layer-cake" type post-reef sediments. This is possibly related to the unusual sedimentational and geochemical processes brought about by the presence of the reef.
Article
In 1950, Tikhonov in Russia and Kato and Kikuchi in Japan showed that quantitative information concerning the electrical properties of the subsurface could be obtained from simultaneous measurements of short-period fluctuations in the earth's electric and magnetic fields. This appeared to amount to a significant advance from qualitative telluric field studies of Schlumberger and his followers. However, it was not until 1953, when Cagniard presented his basic paper, that the potential of the magnetotelluric method was realized.
Article
The processing of magnetotelluric data involves concepts from electromagnetic theory, time series analysis and linear systems theory for reducing natural electric and magnetic field variations recorded at the earth's surface to forms suitable for studying the electrical properties of the earth's interior. The electromagnetic field relations lead to either a scalar transfer impedance which couples an electric component to an orthogonal magnetic component at the surface of a plane-layered earth, or a tensor transfer impedance which couples each electric component to both magnetic components in the vicinity of a lateral inhomogeneity. A number of time series spectral analysis methods can be used for estimating the complex spectral coefficients of the various field quantities. These in turn are used for estimating the nature of the transfer function or tensor impedance. For two dimensional situations, the tensor impedance can be rotated to determine the principal directions of the electrical structure. In general for real data, estimates of the apparent resistivity are more stable when calculated from the tensor elements rather than from simple orthogonal field ratios (Cagniard estimates), even when the fields are measured in the principal coordinates.
Article
The basic theory and objectives of telluric sounding (TS) are about the same as those of the well‐known method of magnetotelluric sounding (MTS) (Cagniard, 1953). Both methods make use of the natural electromagnetic phenomena known as geomagnetic micropulsations to obtain crude “resistivity logs” from the surface down to great depths, without drilling, if the subsurface has mild structures, low dips, and lateral continuity in the electrical resistivity. Let the x-y plane of the Cartesian coordinates represent the surface of the earth. With MTS, the field operation consists of simultaneously recording the time variations of an arbitrary x component of the electric field, E x ( t ) , called a tellurogram, and that of the y component of the magnetic field, H y ( t ) , called a magnetogram, both at the same point where the downward information is desired. The main difficulty is in the measurement of the magnetic field variations with sufficient accuracy. The measurement of the electric field variations is very simple and expeditious. TS bypasses this difficulty, because it does not require the measurement of the magnetic field. With TS, the field operation requires two electric field recording units. One of these units remains at a “base station” where the subsurface is known from a well log, while the second unit is placed at a “field station” where one wishes to explore the subsurface. Thus, for each sounding, one obtains two simultaneous tellurograms. These are Fourier analyzed. The ratios of the electric field amplitudes as a function of frequency, combined with the resistivity log at the base station, furnish the MTS‐type data at the field station that are interpreted in the usual manner to yield a crude resistivity log at the field station. The primary objective of TS is the exploration of sedimentary basins. It may be preferable to MTS in certain cases and vice versa; it is not meant to replace MTS. The theoretical basis and the procedures of TS are discussed in this paper.
Article
From Ampere’s Law (for a homogeneous earth) and from Maxwell’s equations using the concept of Hertz vectors (for a multilayered earth), solutions are obtained for the horizontal components of the electric and magnetic fields at the surface due to telluric currents in the earth. The ratio of these horizontal components, together with their relative phases, is diagnostic of the structure and true resistivities of subsurface strata. The ratios of certain other pairs of electromagnetic elements are similarly diagnostic. Normally, a magneto‐telluric sounding is represented by curves of the apparent resistivity and the phase difference at a given station plotted as functions of the period of the various telluric current components. Specific formulae are derived for the resistivities, depths to interfaces, etc. in both the two‐ and three‐layer problems. For two sections which are geometrically similar and whose corresponding resistivities differ only by a linear factor, the phase relationships are the same and the apparent resistivities differ by the same proportionality constant which relates the corresponding true resistivities. This “principle of similitude” greatly simplifies the representation of a master set of curves, such as is given for use in geologic interpretation. In addition to the usual advantages offered by the use of telluric currents (no need for current sources or long cables, greater depths of investigation, etc.), the magneto‐telluric method of prospecting resolves the effects of individual beds better than do conventional resistivity methods. It seems to be an ideal tool for the initial investigation of large sedimentary basins with potential petroleum reserves.
Article
The subject of this note is a method of acquisition and reduction of data in connection with what is commonly known as the “telluric method,” which primarily deals with the parameter known as “relative ellipse area.” Let us suppose that at an instant t the orthogonal components of the telluric (electric) field at a point B, called Base Station, are E x and E y . At the same instant, the components at another point F, called Field Station, measured in another orthogonal coordinate system, are E u and E v . It is assumed that these components are related to each other as follows: E u = aE x + bE y E v = cE x + dE y (1) where a, b, c, and d are real numbers, called the “correlation coefficients,” which depend only on the directions of the measuring axes and on the electrical properties of the subsurface. It follows that the Jacobian of the transformation from the x‐y system to the u‐v system, J = | ad - bc | depends only on the electrical properties of the subsurface; it is called the relative ellipse area at F with respect to B. For information concerning the validity of equation (1), and the geologic meaning of J, the reader may refer to a book by Berdichevskii (1960).
Article
The paper describes the theory of the magnetotelluric (MT) method, and some of the experimental, analytical, and interpretive techniques developed for its use in petroleum exploration in the past five years. Particular emphasis is placed on interpretation, since it is the area least amenable to routine treatment. Whereas present interpretation techniques are adequate, interpretation is the area of both the greatest progress and the greatest need for improvement. Field results are presented from traverses in South Texas bordering on the Gulf of Mexico, and the Anadarko Basin of southwestern Oklahoma. Wide station spacings were used, such as might typify basin evaluations. The South Texas results are compared directly with smoothed induction logs. No useable logs could be found for Oklahoma. Comparisons with known and inferred geology show that the surveys mapped resistivity successfully in the known parts of these basins as well as in portions inaccessible seismically. The capabilities and economics of the MT method justify its consideration for evaluating large unexplored blocks and "no record" areas.