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OCR for page 232
16
INTRODUCTION
TeHuric Currents: The Natural
Environment and Interactions
with Man-macle Systems
LOUIS ]. LANZEROTTI
A TOT Bell Laboratories
GIOVANNI P. GREGORI
Istituto di Fisica dell 'Atmosfera, Rome
Telluric currents consist of both the natural electric
currents flowing within the Earth, including the
oceans, and the electric currents originating from man-
made systems. Telluric currents could also be consid-
ered to include geodynamo currents, i.e., the electric
currents that are presumed to flow in the Earth's core
and are responsible for the generation of the "perma-
nent" geomagnetic field. This review excludes geody-
namo considerations from its purview.
There has been an evolution (see Appendix) in the ter-
minology in the English-language scientific literature
related to telluric currents. A common former term used
for telluric currents has been "Earth currents," a term
that was widely used by Chapman and Bartels (1940) in
their classic work, whereas Price (1967) preferred "tel-
luric currents." A difference between the two terms can
be recognized in reading historical papers: an impres-
sion is obtained that Earth's currents was the name ap-
plied to the natural currents (or, more properly, volt-
ages) that are measured between two electrodes which
are grounded at some distance apart. Independent of
the cause, the observed current was termed an Earth's
current. It later became evident that electric currents
also flow in seawater. Therefore, the term telluric cur-
rents can be interpreted to include currents flowing both
232
within the solid Earth and within the seas and oceans.
However, we note that Earth currents and ocean cur-
rents do not form independent electric-current systems.
On the contrary, leakage currents exist between conti-
nental areas and oceans (see, e.g., references in Gregori
and Lanzerotti, 1982; Jones, 1983~. In the early French
and Italian scientific literature on the subject, however,
the term telluric (derived from the Latin tellus, for
Earth) was always used (e.g., Blavier, 1884; Battelli,
1888; Moureaux, 1896~.
The fundamental causes of telluric currents are now
believed to be understood. They are produced either
through electromagnetic induction by the time-vary-
ing, external-origin geomagnetic field or whenever a
conducting body (such as seawater) moves (because of
tides or other reason) across the Earth's permanent mag-
netic field. Both causes produce telluric currents,
which, in turn, produce magnetic fields of their own-
fields that add to the external origin geomagnetic field
and produce a feedback on the ionosphere current sys-
tem (a feedback that, however, is negligible; see, e.g.,
Malin, 1970~.
The complexities associated with telluric currents
arise from the complexities in the external sources and in
the conductivity structure of the Earth itself. Such com-
plexities have led earlier workers to make statements
such as "the simple laws of electromagnetic induction do
OCR for page 233
TELLURIC CURRENTS
not fully explain the cause of geoelectric and geomag-
netic activity" (Sanders, 1961), while Winckler et al.,
(1959), in discussing a 2650-V drop across a transatlan-
tic cable produced during a magnetic storm (see below)
concluded " . . . either the current circuit [in the Atlan-
tic] is in the horizontal plane or the currents are not the
result of the induced emf."
The mathematical modeling of telluric currents, un-
like the understanding of their physical causes, is still far
from a satisfactory solution. As far as Earth currents are
concerned, the investigations have been for the most
part carried out on a local or limited regional scale.
In contrast, the understanding of oceanic telluric cur-
rents (which cover a considerable fraction of the Earth's
surface) has, since the Ashour (1950) estimate of their
decay time in an ocean (order of a few hours), under-
gone substantial progress. The state of the art of ocean-
current modeling now takes into account coastlines, al-
though the ocean bottom is usually assumed flat either
nonconducting (although with a conducting mantle; see
e. g., Parkinson, 1975; Hobbs and Brignall, 1976; Hobbs
and Dawes, 1980; Beamish et al., 1980; Fainberg, 1980,
and references therein) or conducting (Hewson-
Browne, 1981; Hewson-Browne and Kendall, 1981;
Kendall and Quinney, 1983~. The ocean currents and
their related geomagnetic effects have been investigated
by, for example, Barber and Longuet-Higgins (1948),
Fraser (1965), Peckover (1973), Klein et al., (1975), and
Semevskiy et al. (1978~. Malin (1970, 1973), in consider-
ing the lunar tidal harmonic component M2 (which is
the most important one both in the atmosphere and in
the sea, with a period of half a lunar day), succeeded in
separating the effect of direct electromagnetic induction
from the ionosphere from the currents produced by oce-
anic tidal flow. He assumed that the geomagnetic varia-
tion associated with the tidal component should always
be observed, independent of local time, whereas the ion-
osphere component should be negligible at midnight. In
fact, he found that at Irkutsk, the geomagnetic observa-
tory farthest from any ocean, the ocean-produced effect
is negligible, unlike the situation at several other observ-
atories closer to a coast, where the ocean component is
present.
No equivalently sophisticated modeling, even for
long-period geomagnetic variations, can usually be
found for Earth currents. This situation exists princi-
pally because of the frustrating indeterminacies intro-
duced by local Earth conductivity anomalies. The basic
difficulty arises because of the nonuniqueness of the "in-
version problem," that is, the nonuniqueness of the eval-
uation of the underground conductivity structure in
terms of the surface geoelectromagnetic recordings.
Therefore, the problem is usually tackled in terms of the
233
"forward problem": an external-origin electromagnetic
field is assumed to impinge on an underground conduct-
ing structure of given geometrical shape, with only the
conductivities left to be optimized by a numerical fitting
of the model with the actual observational records. The
procedure can be worked out only for reasonably simple
geometrical shapes (Porstendorfer, 1976) for the con-
ducting bodies, which implies substantial limitations to
any attempt to extend such investigations to wider spa-
tial scales (see e.g., Rokityansky, 1982; Hohmann,
1983; Parker, 1983; Varentsov, 1983~.
In terms of planetary-scale currents, Gish (1936a,
1936b) presented the results of Figure 16.1, which he
deduced for the daytime western hemisphere from diur-
nal variation (24-h period) recordings of orthogonal
Earth current measurements collected during the sec-
ond International Polar Year (1932-1933) at a number
of sites around the world. The directions of current flow
were determined by taking the vector sum of the N-S
and E-W currents measured at the various sites. Large
errors could be expected because of the sparse number of
stations and the need, therefore, for large interpola-
tions.
A more recent picture has been provided by Mat-
sushita and Maeda (1965a, 1965b; see also Matsushita,
1967) from analyses of the worldwide geomagnetic field
on a planetary scale. These authors performed a stan-
card separation (by means of Gauss' spherical harmonic
expansion) of the external- and internal-origin field.
Some of the results are presented in Figure 16.2. Notice
the obvious, substantial differences compared to the
currents of Figure 16.1. However, even the more recent
work is obviously unable to recognize the effects, where
important, of localized anomalous conductors, such as
mid-oceanic ridges, or even the differences between
ocean basins and continents. In fact, spherical harmon-
ics vary too smoothly to be able to account for such local-
ized features, particularly if a reasonably limited num-
ber of terms is used in the expansion (for recent reviews
of spherical harmonic techniques, see Winch, 1981;
Fainberg, 1983~.
The first 75 to 100 years of Earth current work pro-
duced considerable debate as to causes and disagree-
ments among researchers as to the magnitude of the ef-
fect at given times. Gish (1936a, 1936b) noted that
better agreement between independent measurements
often occurred when relatively long lengths of wire were
used. Today it is clear that such a situation could easily
arise from experimental procedures such as improper
grounding of a wire and an insufficiently high impe-
dance in the measuring system (e. g., see Hessler, 1974~.
For example, in the work of Airy (1868) the wires were
grounded to water pipes, which themselves obviously
OCR for page 234
234
FIGURE 16.1 Planetary-scale distribution
of telluric currents according to Gish (1936a,
1936b) at 1800 GMT.
80°
70C
6oc
40'
2oc
C)C
2oc
40C
60'
70C
could carry currents flowing in the entire region over
which the relatively short lengths of wire extended. In
this case, the pipe network was the receiving "antenna,"
even more than the lengths of wire. Recent evidence of
the effect of telluric currents, integrated over a plane-
tary scale, has been provided by analysis of MAGSAT
data. Langel (1982) reported an analysis of the data in
terms of separation, by spherical harmonic expansion,
of the external- and internal-origin geomagnetic field.
The analysis was done for different sets of data, depend-
ing on the value of the Dst index (a measure of the parti-
cle ring current in the Earth's magnetosphere and,
therefore, of the level of disturbance of the geomagnetic
field). Figures 16.3(a) and 16.3(b) show that the lowest
order and degree terms (i.e., dipole terms), denoted g°
and q° (external and internal, respectively) change with
the level of Dst. The internal term increases with de-
creasing Dst, unlike g°, a consequence of the fact that
induced currents must flow in the direction opposite to
the inducing currents.
Summarizing, accurate knowledge of telluric current
patterns in the Earth on a planetary scale still remains a
basically open problem even though the subject has a
long history. In addition to the actual role of ocean wa-
ter and sediments, largely unknown is the influence of
localized conductivity anomalies (such as fold belts,
mid-oceanic ridges, and trenches and subduction zones)
on such patterns. The current patterns will obviously be
different for different periods of the external inducing
field. The higher-frequency patterns will be highly time
LOUIS J. LANZEROTT] and GIOVANNI P. GREGOR!
, . . . . . . . .
90° 120° 1 50° 180° 150° 120° 90° 60° 30° Go 30O 60° 90°
r ~I ~I ~I I I I I I I
00 04 08 NOON 16 20 24
l 80O
70°
60°
40°
20°
o°
20°
4oo
60°
7oo
variable because of the temporal and spatial variability
of the external-origin fields, variabilities that are not yet
amenable to accurate predictive modeling. Neverthe-
less, given all the foregoing caveats, we present in the
following sections additional discussions of many of the
relevant issues, as well as some implications for practical
concerns.
THE NATURAL ENVIRONMENT
The Physical Problem: Hydrology, Geology,
Geothermics, and Tectonics
Except during a lightning strike to Earth, essentially
negligible electric current flows between the air and the
ground (integrated over the Earth, the fair-weather
current amounts to some 1000-2000 A). Therefore, the
Earth's surface is a natural surface across which electro-
magnetic coupling occurs via an electromagnetic field.
This implies that it is possible in many cases to treat the
coupling problem in terms of scalar potentials (at least
for frequencies lower than those used in audio magneto-
telluric studies). An attempt by Berdichevsky and Fain-
berg (1972,1974) to evaluate, on a global scale, possible
currents between ground and air suffered large uncer-
tainties from the approximations used. As noted briefly
in the Introduction, the cause of telluric currents is ei-
ther electromagnetic induction by the time-varying geo-
magnetic field produced by the ionosphere and/or mag-
netosphere or by water movement across the permanent
OCR for page 235
TELLURIC CURRENTS
IMP
No
ooo
D MONTHS E MONTHS
I3J
C, 60°
SP
J NP
60°
3oo
ooo
Boo
60°
IMP
NP
60°
3oo
ooo
3oo
L1J
C, 60°
- SP
or
J MONTHS AVERAGE
. __
--` 1 -'I
=
.
_ .
00 03 06 09 12 15 18 21 24 00 03 06 09 12 15 18 21 24
D MONTHS
E MONTHS
J MONTHS AVERAGE
53.4 ~
00 03 06 09 12 15 18 21 24 00 03 06 09 12 15 18 21 24
LOCAL TIME
FIGURE 16.2 (a) External Sq current systems averaged worldwide
for D months (northern winter; top left), E months (equinox, top
right), and J months (northern summer, bottom left), and their yearly
average (bottom right). The current intensity between two consecu-
tive lines is 25 X 103 A; the thick solid curves indicate the zero-intensity
lines. The numbers near the central dots are the total current intensi-
ties of these vortices in units of 10 A. (b) Internal Sq current systems
averaged worldwide for D months (top left), E months (top right), and
J months (bottom left), and their yearly average (bottom right). Notice
the disagreement with Figure 16.1; the rotational senses of the vorti-
cies are opposite. This figure adapted from Matsushita (1967~.
geomagnetic field. Considering only the former cause,
the longer the period of the time-varying field, the
greater the depth in the Earth where the induced cur-
rents can be expected to flow. A quantitative criterion
can be given in terms of electromagnetic induction in a
half-space of uniform conductivity (note that this is a
highly idealized case that practically never occurs in re
235
ality). The "skin depth" (i.e., the depth at which the
external field is damped by a factor 1/e) is given by S =
0.5 (T/~0 5 km, where is the conductivity in mhos/meter
and T is the period of the variation in seconds. A signal
with period of about 24 h is generally believed to have a
skin depth of 600 to 800 km (Hutton, 1976, Cough and
Ingham, 1983~. (The skin-depth only provides, how-
ever, a rough approximation of the depth at which ac-
tual telluric currents of a given period are flowing. In
fact, the actual conductivity structure underground is
most often a matter of considerable indeterminacy.)
Saltwater has a conductivity of about 4 mhos/m, hy-
drated sediments have a conductivity of about 0.1 mho/
m, and dry rock has a conductivity of about 0.0001
mho/m. Practically all the materials of the usual geo-
logic environment (see, e.g., ACRES, 1975; Keller,
1966) can be placed between these extremes. Nomo-
grams by which T. a, and S can be evaluated for differ-
ent materials and for the "actual" Earth are shown in
Figure 16.4. The conductivity of water is largely af-
fected by salinity (and to a minor extent by tempera-
ture). The conductivity of soil is largely affected by the
state of hydration. Porous materials and sediments can
easily be hydrated (see below) by considerable amounts.
Hence it might eventually be possible, by electromag-
netic means, to distinguish materials of equal density
but with different porosities, and hence different hydra-
tion (and electrical conductivities), that cannot be dis-
tinguished by seismic techniques.
The distributions of sediments, particularly impor-
tant for shorter-period variations, should be considered
on local or regional scales, because minor details in the
distributions can be relevant to telluric current flow. A
worldwide pattern of sediments has been given by
Hopkins (reproduced in Green, 1977, and in Gregori
and Lanzerotti, 1982~. Fainberg (1980) provided a
worldwide model map of the total conductivity of the
water shell plus sedimentary cover tFigure 16.5a.
Such a map is the result of a more detailed mapping
given by Fainberg and Sidorov (1978~. For example,
Figure 16.5(b) shows the conductivity profile for Eu-
rope. Clearly shown are the sedimentary structures re-
sponsible for the North German conductivity anomaly
and for the channeling in the Seine Basin. The North
German anomaly, with a depth-integrated conductiv-
ity ~ 3000 mhos, is equivalent to-750 m of seawater.
Another physical factor affecting conductivity, and
thus telluric currents, is temperature. Since the temper-
ature increases with depth in the Earth, the conductiv-
ity is higher with increasing depth. However, the effect
is not uniform; the heat flux through the Earth's surface
is greater in certain regions than in others, providing
thermal anomalies. Whenever a larger geothermal flux
OCR for page 236
236
FIGURE 16.3 (a) The spherical harmonic
coefficient of lowest degree and order describ-
ing the magnetic field originating external to
the Earth, as a function of the global Dst in-
dex used to describe temporal variations of
the equatorial horizontal magnetic field rela-
tive to magnetically quiet days. (b) The
spherical harmonic coefficient of lowest de-
gree and order describing the field originating
within the Earth as a function of the lowest
degree and order magnetic-field coefficient
describing the magnetic field originating ex-
ternal to the Earth (adapted from Langel,
1982).
occurs, there is an upward warping of isothermal sur-
faces. In such a case, telluric currents of a given period
will flow in shallower layers. A worldwide mapping of
the geothermal flux averaged over a 5° X 5° mesh (Fig-
ure 16.6) has been provided by Chapman and Pollack
(1975) . (This map has largely been obtained using about
5000 direct borehole measurements and, where un-
avoidable, indirect information. For example, since the
heat flow from the ocean floor is a well-defined function
S (km}
20
15
10 ~ .
5
O , 1 1 1
20 10 0 -10 - 20
DSt (nT)
LOUIS J. LANZEROTT} and GIOVANNI P. GREGOR]
-12
LL
~ -10
z - 8
go
-6t
q0= 20.2-
1 0.63Dst,nT
rms=0.66nT
~ _ 4
-
of - 2
cad
+
0-
1 1 1 1
\ 90 = 29989.4 +
~ \ 0.24 qO,nT
:~s=0.28nT
~ ,
O I 1 1 1
0 10 20 30 40 50
qO ( nT,EXTERNAL F IELD)
of the floor's age, the flow can be approximated even
when it has not been directly measured. Analogously, a
different function relates the continental heat flow to
age.)
Three additional aspects of the conductivity structure
of the Earth affect the flow of telluric currents spatial
gradients, temporal variations, and channeling.
The spatial gradients of telluric currents strongly de-
pend, in shallow layers, on geochemical composition,
~ = ~- Quaternary & Tertiary
(Granulite) ~ Metamorphic Rocks _ (Gneiss)
(Quartz Porphyry) ~Igneous Rocks -, (Basalt) Ocean Water
Dry Rocks (Limestone) ,Sed e y ( ) ~ Natural Water _
- - - . Deep Earth's Interior
1n-8 in-7 1n~6 1n~5 in-4 in-3 in-2 1n-1 1
103
1o2
10
1.
10 ~:~ ;~ . - - . - - _ ;_ _ 10 102 o (mhos m~, )
103
104
105
106
~ Q
_ Q
~ O
Z
'107 ~
~ loo
no
~ ._
_ 1010
1 10 1o2 103 104 lob `/T (see/')
, , , , , . . ~ . ~it,, . · . j . ~. . j . · . . . ~. ~ . · . - · ' ' ' - · ' ' !
(a) 1 min. lh ld 7d 30d 0.5y ly 2y 5y 11yl5y 20y 22y T
t
OCR for page 237
TELLURIC CURRENTS
FIGURE 16.4 (a) Skin-depth no-
mogram, indicating depth probed
as a function of period and mate-
rial conductivity. Representative
materials for given conductivities
are shown, taken from the ACRES
report (1975), which was adapted
from Keller (1966~. The central
solid line represents the skin depth
estimate for the actual Earth, as-
suming a planar half-space of uni-
form conductivity equal to the con-
ductivity of the lowest evaluated
depth of penetration; the upper
and lower solid lines are explained
below. The central line represents
a lower limit on the depth. The al-
most equivalent dot-dashed lines
have been drawn using the model
of Achace et al. (1981~. (b) Ex-
panded version of the central por-
tion of (a), detailing the depth
range 100 km c S c 3000 km. (c)
Profiles of the conductivity of the
Earth versus depth: full line, ac-
cording to Rokityansky (1982~;
dash-dot line according to Achace
et al. (1981~. The lines above and
below the estimated average pro-
files are indicative of the 95 percen-
tile probability error distribution
for the data of Rokityansky and of
some level of uncertainty in the
case of Achace et al. Rokityansky
shows that practically all of the
previous estimates of such a profile
by different authors basically fall
within his limits.
237
G. ( mhos ~ m '}
3tXXler 10 10 10- 3 10
2000
1500
S (km)
000
500
400
300.
200.
arts
~104 aid)
1 10
· . . . .. I-. . . ~. . . ....
ln4
.6
10
(b) 1 min. 1 h
S (km]
ld Id 30d OSLO 1y 2y 5~ n~,79~ 20y 22y T
6 (mhos. nix)
1~` 1~2 ~
_
_ .
i\. \\
1 \.. \
_
1n'
OCR for page 238
238
10
r ~too ~ / _ /
o
~ i
~ ( OoW°~
, ~/~/:
,, o , ,,
,, Ace,
_-Jo ,
~ \ ((// of into/
~°° - \J ~
_. 5 -
_
o 3
._
^
Cot
X ~
~ _
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Cat o
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o
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US Ct
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~ Cot
4= ~
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to 3
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O
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~ =.°
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OCR for page 239
239
,3
it\
~ ~ - ~
Vat 1
/
._ ~
~1
1
,~
jp~. ~
\/
-
\
;~ I' ~ '
o 7N
i- If
/ 1 ~ =
, ~
OCR for page 240
240
FIGURE 16.6 Spherical harmonic repre-
sentation (degree 12) of global heat flow from
observations supplemented by predictor.
Heat-flow contour lines are in milliwatts per
square centimeter. Adapted from Chapman
and Pollack (1975~.
geological structure, and hydration. "Hydration in this
context can be taken just in terms of water content (pro-
ducing an increase in conductivity) or in terms of the
formation of particular compounds (clathrate hydrates)
that can decrease the conductivity (although there are
no reports of this in the telluric current literature); see
Miller (1974~.] Deeper in the Earth, it is believed that a
more or less thick layer of dry rocks (having reduced
conductivity) is further underlain by layers of increasing
conductivity, which is a function of the increasing tem-
perature with depth. In such deep layers it has generally
been assumed that the Earth becomes increasingly ho-
mogeneous with greater depth. More realistically, how-
ever, the increasing difficulty (if not impossibility) of
recognizing spatial gradients at greater depths must be
acknowledged. Differently stated, telluric currents as a
means of remote sensing of the underground conductiv-
ity provide ever-diminishing spatial (horizontal) resolu-
tion with depth.
The problem of spatial gradients of the telluric cur-
rents is also related to the state of knowledge of the spa-
tial gradients of the external-origin inducing field. In
fact, the diurnal and the lunar variation fields (Sq and L
fields, respectively) have a planetary scale, albeit show-
ing strong spatial gradients related to the auroral and
equatorial electrojets for quiet conditions (e.g., Sch-
lapp, 1968; Riddihough, 1969; Greener and Schlapp,
1979~. For disturbed conditions, the planetary-scale de-
scription still plays a relevant, though not singular, role
(e.g., Sato, 1965; Campbell, 1976~. Therefore, the ex-
ternal-inducing source at these low frequencies can be
approximately described in terms of a planetary-scale
field, occasionally with strong spatial gradients.
On the contrary, for higher frequencies (magnetic
storms, geomagnetic pulsations) the source can often
appear quite localized (see, e. g., Davidson and
Heirtzler, 1968; Lanzerotti et al., 1977; Southwood and
LOUIS J. LANZEROTT] and GIOVANNI P. GREGOR}
If_
V,~ I\
\1 ~/~
`17 1
~\~\
i.\60-\ ]\
~ _~ ~r
I'
Hughes, 1978; Reiff, 1983) and is highly time dependent
as well. At the Earth's surface the spatial extent of the
source for pulsations (period of a few to a few hundreds
seconds) is believed to be not smaller than the height of
the ionosphere.
Temporal variations in the Earth's conductivity
structure can be caused by such effects as seasonal cli-
matic changes affecting water salinity and temperature,
ice extension, permafrost and hydration content, and
tectonic processes. The tectonic processes can be either
slow (i.e., those involving the geologic time scale), inter-
mediate (as in earthquake precursors; e.g., Honkura,
1981), or rapid (as-in volcanoes).
Channeling of telluric currents in specific, higher-
conductivity regions is an actively debated area at
present. Some recent research papers, without pre-
sumption of completeness, include Lilley and Woods
(1978), Babour and Mosnier (1980), De Laurier et al.
(1980), Miyakoshi (1980), Srivastava and Abbas (1980),
Woods and Lilley (1980), Camfield (1981), Chan et al.
(1981a, 1981b), Kirkwood et al. (1981), Kurtz et al.
(1981), Sik et al. (1981), Srivastava (1981), Thakur et al.
(1981), Booker and Hensel (1982), DeBeer et al. (1982),
Le Moue! and Menvielle (1982), Nienaber et al. (1982),
and Summers (1982~; see also extensive review and dis-
cussion by Jones (1983~. The issue revolves around the
interpretation of the measured telluric currents. Should
the measurements at some given site be interpreted in
terms of electromagnetic induction on a local (or in any
case on a small-scale) spatial extent, or should they be
considered as the result of a large-scale (i.e., regional,
continental, or planetary scale) induction phenomenon,
whereby telluric currents are channeled from more re-
mote areas within some relevant conducting body not
far away from the recording site? While specific cases
can be discussed (such as the North German anomaly;
see Appendix', a generally valid reply is difficult to give
OCR for page 241
TELLURIC CURRENTS
basically because (1) the planetary-scale response of the
actual Earth in terms of telluric currents is poorly
known and (2) the temporal and spatial scale of the ex-
ternal-origin inducing field is often poorly known, par-
ticularly for shorter-period variations represented by
magnetic storms and geomagnetic pulsations.
It is interesting to note that the current channeling
was addressed early on in studies of telluric currents.
Varley (1873) discussed current channeling from the sea
in telegraph wires between the coastal town of Ipswich
and London. He also claimed that enhanced currents
were seen in the line between Glasgow and Edinburgh,
which connected the sea across the British Isle, as com-
pared with a line solely on land.
Summarizing, telluric currents depend on several
physical parameters and, if properly interpreted, can be
used for studies of the underground electrical structure
at both shallow and great depths. It is important for tel-
luric current studies to take into more explicit account
the relations of measured currents to the specific tec-
tonic and geomorphological features of the regions un-
der study. Approaches toward such a viewpoint have
been presented recently by Hermance (1983~. In gen-
eral, such investigations can best be tackled by means of
large arrays of instrumentation (Alabi, 1983~.
Shallow Telluric Currents
Effects on shallow telluric currents (generally shorter
period) can be found whenever a mineral has some re-
markably different electrical conductivity compared
with that of the surrounding materials. This gives rise to
a localized conductivity anomaly that can be studied by
means of a dense network of recording instruments.
Shallow currents have also been reported in several sedi-
mentary basins, such as in the Seine Basin and in the
northern German anomaly (see Appendix; for other ref-
erences see, e.g., Gregori and Lanzerotti, 1982~. Shal-
low telluric currents are responsible for a component of
the coast effect or magnetic signals, where the geometri-
cal orientation of the magnetic variations at higher fre-
quencies are correlated with the shape of the coast. The
coast effect has been reviewed by Fischer (1979),
Parkinson and Jones (1979), and Gregori and Lanzerotti
(1979b).
The difference between shallow and deep effects (the
latter arising from local tectonic features) has been
shown by Honkura (1974) for the Japanese islands (Fig-
ure 16.7~. At shorter periods, when the skin depth is
shallower, the coast effect reflects the coast shape. At
longer periods, electromagnetic induction evidence sug-
gests a dependence on the downward bending of the
lithospheric slab where it approaches the Tananese .~h
241
auction zone. Similar effects have been reported by
Honkura et al. (1981) for a small island in the Philippine
Sea ("regular" coast effect) and by Beamish (1982) for
the island of South Georgia (Scotia Arc, South Atlantic).
The threshold period discriminating between shallow
and deep effects appears to be about 20 min in the Japa-
nese area, a result obtained from a reinterpretation by
Gregori and Lanzerotti (1982) of data published by
Yoshimatsu (1964~.
Deep Telluric Currents
The best recognized, by seismic waves, underground
discontinuity the Moho (see, e.g., global map pre-
sented by Soiler et al., 1982) has no obvious correspon-
dence in geoelectromagnetic phenomena. In fact, the
behavior of deep telluric currents is largely controlled
by the shape of the isotherms. An idea of the trend of
such isotherm surfaces is given by Figure 16.8, which
plots isocontours of the thickness of the lithosphere
(Chapman and Pollack, 1977), based on the heat-flux
results of Figure 16.6. Chapman and Pollack (1977) de-
rived the lithosphere results by determining the depth at
which both continental and oceanic geotherms intersect
the mantle solidus. They showed this to be a consistent
estimator of the depth to the top of the seismic low-ve-
locity channel or of the thickness of the high-velocity lid
overlying the channel. They identified the lid as synony-
mous with the lithosphere.
A similar discussion, limited to the Soviet Union, is
given by Cermak (1982~. Oxburgh (1981) presented a
critical discussion of the method employed for such
analyses. For the sake of completeness, however, it
should be noted that the concept of the lithosphere is
actually more complicated. Depending on the experi-
mental observations used, four different definitions can
be distinguished: the elastic or flexural, the thermal, the
seismic, and the chemical or mineralogical (U.S. Geo-
dynamics Committee, 1983; Anderson, 1984, Maxwell,
1984~. In the context of telluric currents, the thermal
structure of the deep Earth is likely the most relevant
factor, with the chemical/mineralogical being the sec-
ond. Hence, in this simple context, Figure 16.8 can pro-
vide an idea of the depth where a high electrical conduc-
tivity can be expected at a given site. A very general and
approximate statement is that the thickness estimates of
Figure 16.8 are in reasonable agreement with geomag-
netic depth-sounding and magnetotelluric estimates of
the depth of the "ultimate conductor": about 200 km
below continents (cratons), about 100 km under stable
continental areas, about 60-70 under rifts and grabens,
and about 10-20 km (or even shallower) under volcanic
areas and mid-ocean ridges. tA warning must be given
~< ~ ~ _ al LEA_ J ~r ~--eve ~ ~ ~
OCR for page 242
242
FIGURE 16.7 (a) The /`ZI^H value distri-
bution in Japan for geomagnetic variations
corresponding to geomagnetic bays. The pro-
files AA' and BB' have been investigated in
detail, and their results are shown in the sub-
sequent figures. (b) Parkinson vectors along
the profile AA' of part (a), for geomagnetic
variations with period of 60 min. Contours in-
dicate the sea depth in 103 m. The Parkinson
vectors are consistent with an interpretation
in terms of an asthenosphere bending and
deepening in the subduction zone. (c) The
same as for profile AA' in part (b), but refer-
ring to the profile BB' . The downward bend-
ing of the asthenosphere in the subduction
zone appears much less pronounced in this re-
gion. (d) Parkinson vectors on the Miyakejima
island for periods (a) 120, (b) 60, (c) 30, (d)
15, and (e) 5 min. respectively. The coast ef-
fect is quite evident at the shorter periods,
while at the longer periods the effect of the
bending of the asthenosphere is predominant
over the coast effect.
The vectors appearing in the (b), (c), and
(d) sections of the figure are "Parkinson ar
rows" or "vectors," defined in the following
manner. Consider the deepest surface layer to
which the incident electromagnetic wave of a
given period can penetrate. Consider a plane
(the "Parkinson plane") tangent to such a sur
face, directly beneath a given recording site.
Construct a line perpendicular to this plane
and oriented downward. Project this line in
the horizontal plane: this is the direction of the arrow. The length of the arrow is equal to the sine of the tilt of the Parkinson plane with respect to the
horizontal plane. Therefore, a vanishing Parkinson arrow implies a horizontal Parkinson plane, a unit length arrow implies a vertical Parkinson
plane. A "normal" coast effect on an island shows that Parkinson arrows point outward from the island. (For other details on "induction arrows"
refer to the review by Gregori and Lanzerotti, 1980.) Figure is adapted from Honkura (1974~.
LOUIS J. LANZEROTT! and GIOVANNI P. GREGORI
,~.o
A 0.4 O 30Okm
N >
J iff
0.5 ' /' (iii)
(C)
ta) ~)2 ~ `b,
N
(i) (ii)
( iv) (V)
(d )
FIGURE 16.8 Thickness of the lithosphere
derived from a spherical harmonic (12 de-
gree) representation of the global heat flow
(see Figure 16.6) and continental and oceanic
geotherm families. Contours are in kilome-
ters, with variable intervals. From Chapman
and Pollack (1977~.
OCR for page 248
248
methods are well suited for shallow prospecting but can
hardly be applied to deeper layers, because of the skin-
depth phenomenon. Such required long-period electro-
magnetic waves cannot be generated practically. Long
powerlines have been used for generating electromag-
netic induction fields for prospecting purposes (e.g.,
Gill and MacDonald, 1967~.
A distinction should be made between prospecting
techniques that use the static geomagnetic field of the
Earth and techniques that use electromagnetic induc-
tion effects. In handling the standard aeromagnetic and
oceanographic magnetic surveys, whose purposes are to
understand the static fields, the time-varying fields re-
corded at a ground-based site "close" to the area of the
survey must be subtracted from the air or the ocean sig-
nal. This introduces some errors, whose actual values
are often difficult to estimate (e.g., Reford, 1979~. The
time-varying component from such surveys can be used
for geomagnetic depth-sounding studies (Gregori and
Lanzerotti, 1979a).
Deep-Earth Studies
Telluric currents are likely eventually to be important
tools for prospecting the deep structure of the Earth,
thus providing valuable complementary information to
that provided by seismic waves. A great advantage of
GDS methods is that, while many studies concentrate
on magnetic storm events, the studies can also be carried
out using inducing signals during more quiet times, sig-
nals that are always in existence. The use of induced cur-
rents from natural electromagnetic waves for deep-
Earth research is being pursued actively in a number of
countries, particularly the Soviet Union. Recent reviews
include books by Rokityansky (1982), Patra and Mallick
(1980), Parkinson (19823, and Berdichevsky and
Zhdanov (1984~.
Tidal Phenomena and Water Flows
There are three types of tides: atmospheric, oceanic,
and solid Earth. Atmospheric tides generate a large part
of the external-origin inducing field of long period; oce-
anic tides produce a time-varying geomagnetic field as-
sociated with water flows; and solid-Earth tides can
similarly produce a geomagnetic field because they can
produce an eventual water flow that will produce a
magnetic field. Theoretical and observational aspects of
the phenomena have been discussed by Meloni et al.
(1983~.
Within the past decade extensive use of shorter under-
sea cables (such as those across the Dover Strait; e.g.,
Prandle, 1978) has been made for studies of tidal oscilla
LOUIS J. LANZEROTT! and GIOVANNI P. GREGOR!
1 1 1 1 1 1 1 1 1 1 1
_ ~ ~ Poripatrick
_ t~\,6
_._ _
-0.4 _
LL _
~ O _
J _
° 0.4 ~
0.8 _
. 1 1 1 1 1 1 1 1 1 1 1 1
18 24
21 MARCH 1974
FIGURE 16.13 Cable voltage on the Donaghadee-Port Patrick cable
on a geomagnetically disturbed day (from Prandle and Harrison,
1975).
lions and water flow. Cables across the Irish Sea have
been used for such studies (for example, Prandle and
Harrison, 1975; Prandle, 1979~. Geomagnetic distur-
bances can affect the measurement capabilities and,
hence, results of such a cable-monitoring system. The
data presented in Figure 16.13 are from chart record-
ings of the cable voltage on the Donoghadee-Port Pa-
trick cable on a day of geomagnetic disturbances (Pran-
dle and Harrison, 1975~. The low-frequency variation
in the voltage, spanning the record, is produced by tidal
flow. The higher-frequency variations, produced by
geomagnetic storm induction of currents in the cable,
obscure the variations in such a manner that the data
cannot be used reliably for water-flow information on
such a day.
Earlier, Wertheim (1954), in studying water flow
across the Florida straits using the Key West-Havana
cable, found occasional rapid variations in the cable
voltage. He attributed these to geomagnetic effects and
tried to model them using magnetometer data from the
San Juan Observatory. Recent work in studies of the
Florida current were reported by Larsen and Sanford
(1985~.
Earth's Astronomical Motion
The variation in the length of the day and the dis-
placements of the positions of the geographic poles are
among the most precise and fascinating topics in
geophysics and have now become a vast discipline. The
problem, however, of a possible role of telluric currents
in producing a braking or an acceleration in the Earth's
rotation or in displacing the Earth's poles appears still
basically unsolved and/or is not considered important
by many. This is discussed in some detail by Meloni et al.
(1983) (and references therein), where also the possible
OCR for page 249
TELL URIC C URRENTS
use of a transatlantic communication cable is discussed
as a possible experimental device to detect such an ef-
fect.
Earthquakes, Voicanoes, and Geodynamics
Since telluric currents are excellently suited for deep-
Earth investigations, they are in principle also suitable
for monitoring long-scale time variations as well. The
most investigated aspect from this viewpoint is con-
cerned with earthquake precursors (e.g., review by
Honkura, 1981, and references therein). A clear distinc-
tion should be made among three different, possible
types of phenomena: (1) geomagnetic effects that can
presumably be said to be "very shallow" and are likely
related to piezomagnetism, following changes in local
stresses in the upper crust, which is an effect strictly lo-
cal and can completely change in a distance of a few
kilometers or less; (2) "shallow" effects that can be de-
tected by ground resistivity changes or by suitably short-
period MT or GDS investigations; and (3) "deep" or
"very deep" effects that can be most suitably detected by
means of long-period GDS investigations. This latter
category of effects is more strictly related to telluric cur-
rents than are the first two types of effect.
Additional possible applications in this area include
(1) slowly varying effects correlated with geodynamic
and tectonic features, (2) shallow effects related to
magma migration in volcanic areas, and (3) the moni-
toring of temporal variations in underground structures
as related to fluid extraction (or reinfection). The shal-
low effects could, however, possibly be better detected
by use of man-made electromagnetic fields than by
means of natural fields. Telluric currents are also well
suited for investigating ocean-bottom and geothermal
areas (see Law, 1983; Berktold, 1983~.
Communications
Within the last 10 to 15 years suggestions have been
made that a natural waveguide in the Earth's crust,
composed of the insulating layer of dry rocks sand-
wiched between the upper hydrated conducting and the
underlying conducting hot layer, could be used for com-
munication purposes. This suggestion, however, does
not seem to have been followed by any known applica-
tion. Existing literature is referenced in Gregori and
Lanzerotti (1982~. A practical problem is certainly con-
cerned with the spatial nonuniformity of such a wave-
guide, the width and depth of which is undoubtedly
widely varying (e.g. compare a cratonic area with a
mid-oceanic ridge area) and is essentially unknown in
many regions.
249
In a similar fashion, magma chambers in mid-oceanic
ridges can be considered the natural "equivalent" of
man-made submarine communication cables. The con-
ductivities are such that the mid-Atlantic ridge is equiv-
alent to about 1000 such cables in parallel (see Gregori
and Lanzerotti, 1982~.
An interesting communication experiment related to
artificial telluric currents was reported by Fraser-Smith
et al. (1977~. They operated, as a transmitting antenna
(using a simple car battery), a circuit loop composed of
seawater encircling a small peninsula in a nearly en-
closed area.
Biological Effects
The response of living species to electromagnetic
fields (such fields being either responsible for, or a con-
sequence of, telluric currents) is a difficult but impor-
tant problem. Several examples discussed in the litera-
ture include the induced currents in a tree produced by
geomagnetic fluctuations (Fraser-Smith, 1978) and the
use of magnetic fields for orientation by aquatic bacte-
ria (e.g., Blakemore, 1975) and by migrating birds
(e.g., Moore, 1977; Larkin and Sutherland, 1977; Aler-
stam and Hogstedt, 1983; Beason and Nichols, 1984~.
Telluric currents could play a role in some control of fish
(e.g., Leggett, 1977; Kalmijn, 1978; Brown et al., 1979;
Fainberg, 1980; Fonarev, 1982~. Magnetite crystals
have been reported as isolated from a sinus in the yel-
lowfin tuna (Walker et al., 1984~. Enhanced DNA syn-
thesis has been reported for human fibroblasts exposed
to magnetic-field fluctuations with frequencies and am-
plitudes similar to many geomagnetic occurrences (Li-
boff et al., 1984~. The entire area is fraught with contro-
versy, particularly that related to magnetic effects, and
has been reviewed by Parkinson (1982) and commented
on by Thomson (1983~.
CONCLUSIONS
Historically, telluric currents were intensively inves-
tigated in the second half of the nineteenth century, par-
ticularly because of their influences on long telegraph
conductors (see Appendix). The intrinsic difficulties en-
countered in obtaining fundamental understanding,
basically related to the several causes that can be core-
sponsible for the observed effects, discouraged geophys-
icists from pursuing such investigations vigorously.
However, with respect to a century ago, large improve-
ments have been made in a number of areas, including
recording techniques, density of available observations,
international data exchanges, computational facilities,
mathematical methodologies, and general geophysical
OCR for page 250
250
understanding. Hence, research in this area would ap-
pear to be poised for achieving significant new under-
standing.
Investigations using, and studying, the telluric cur-
rents suffer from three principal drawbacks: (a) the spa-
tial coverage by the recording equipment is often too
sparse compared to the extent and spatial gradients of
the phenomena uncler investigation; (b) there is not gen-
eral agreement on the experiment and analysis metho-
dologies, often leacling to difficulties when comparing
the results from different investigations; and (c) the use,
within the general understanding of deep geophysical
structures, of the information provided by electromag-
netic techniques is often neglected. It is hoped that these
deficiences will be ameliorated in future telluric current
work.
Telluric currents are a relevant part of our electro-
magnetic environment, both as a consequence of the
time-varying natural electromagnetic field and as a con-
sequence of moving seawater. Human interactions with
telluric currents and their related effects are definitely
important, even though such interactions are only in-
completely understood. A considerable effort on geo-
magnetic source characteristics and Earth conductivity
characteristics are required before a satisfactory and
comprehensive understanding can be achieved. The
critical role of telluric currents within the geophysical
environment, encompassing such areas as the Earth's as-
tronomical motion, to current channeling, to eventual
implications for ocean-water motion, still appears as a
challenging frontier. As well, further assessments must
be made on the actual role of telluric currents, as well as
of the geomagnetic field in tote, on biological systems.
The field is multidisciplinary and fascinating. Substan-
tial achievements can be expected in the near future.
ACKNOWLEDGMENTS
We thank the referees and F. I. Lowes for helpful
comments and references.
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APPENDIX: HISTORICAL DEVELOPMENT
A selective sketch of the historical development of the un-
derstanding of telluric currents follows. It is essentially impos
sible for the present authors to attempt to give full justice to all
authors of the most recent investigations. It is particularly dif
ficult to evaluate these recent works in a historical context.
For more extensive general aspects of the subject and for re
cent literature references, the interested reader should refer to
LOUIS J. LANZEROTTI and GIOVANN! P. GREGORI
Dosso and Weaver (1983~. A recent excellent review of pri
marily American work is contained in Hermance (1983)
while Rokityansky (1982) and Berdichevsky and Zhdanov
(1984) contain many references to Eastern literature.
1540 First reported measurement of geomagnetic declina
tion and dip in London (as discussed, for example, in
Malin and Bullard, 1981; Barraclough, 1982~. For
the early history of geomagnetism, including the
works of Gilbert and Gauss, refer also to Mitchell
(1932a, 1932b; 1937), Chapman (1963), Mattis
(1965, Chap. 1), Parkinson (1982, Chap. 6), and
Merrill and McElhinny (1983~.
1600 First modeling of the geomagnetic field by Gilbert's
(1600) terrella (Malin, 1983~.
Davy (1821) suggested the existence of Earth cur
rents that, he argued, could be responsible for varia
tions in the geomagnetic declination (Burbank,
1905~.
1832 Faraday (1832) envisaged for the first time the exis
tence of induced currents in water, related to water
flows and tides. He also attempted, without success,
to detect, from the Waterloo Bridge, such currents
flowing within the Thames. Gauss (1833) reported
the first measurements, on May 21, 1832, of the ab
solute value of the geomagnetic field (Malin, 1982~.
1846- Barlow (1849) made the first observations, in En
1847 gland, "on the spontaneous electric currents ob
served in the wires of the electric telegraph."
1848 Matteucci detected induced currents in the telegraph
wire between Florence and Pisa, while Highton ob
served the same effect in England (see section on
Communication Cables).
1850 Similar effects were reported in the United States.
1859 A telegraph line in the United States was reported
operated by means of the natural induced currents
during geomagnetic disturbances on September 2.
1862 Lamont (1862) reported one of the first experiments
to specifically address Earth currents (carried out in
the Munich Alps).
1865 Experiment by Airy (1868) on two wires of 13 and 16
km from Greenwich.
1867 Secchi (1867) reported measurements on two almost
orthogonal telegraph lines of lengths 58 km (Rome
Arsoli) and 52 km (Rome-Anzio).
1881 The Electrical Congress, meeting in Paris, recom
mended that certain short lines be set apart in each
country for the study of Earth current phenomena
and that longer lines be used as frequently as possible
(Burbank, 1905~.
1884- Four complete years of records on two telegraph
1887 wires in Germany (262 and 120 km) investigated by
Weinstein (1902) and Steiner (1908~.
1883- Blavier (1884) recorded, for 9 months, Earth poten
1884 tials on five long telegraph lines extending from
Paris, ranging in length from 200 to 390 km. See also
Counil et al. (1983~.
1886 Shyda (1886) reported an Earth current study on the
OCR for page 255
TELLURIC CURRENTS
land line plus ocean cable route from Nagasaki, la
1889
pan, to Fusan, Korea.
Schuster (1883, 1908) performed the first investiga-
tions on the diurnal variation of the geomagnetic
field. He concluded that the origin is external, that
the Earth must have an upper layer less conducting
than that deep in the interior, and he proposed the
"suggestive cause" of tidal motion in the atmosphere
for the origin of the observed diurnal variation.
1892- Two orthogonal Earth current lines, ~ 15 km each,
1985 were established at Saint-Maur-des-Fosses Observa
tory southwest of Paris (Moureaux, 1895, 1896;
Bossier, 1912; Rougerie, 1940; Counil et al., 1983~.
Moureaux (1893) found that the east-west Earth cur
rents in the Paris basin were "exactly" correlated
with the H-component of the geomagnetic field (i.e.,
the horizontal, north-south component), while this
did not appear to be true for the north-south Earth
current and the declination (east-west horizontal)
geomagnetic field. This was the first reported detec
tion of what is now interpreted in terms of telluric
currents channeled east-west in the Seine basin from
the Atlantic Ocean.
Burbank (1905) provided a comprehensive bibliog- 1949
raphy on Earth currents.
Van Bemmelen (1908) found that geomagnetic
storm sudden commencements (ssc's) have opposite
signs at Kew (close to London) and at St. Maur (close
to Paris). He correctly explained this in terms of elec
tric currents flowing in the English Channel.
Schmidt (1909) investigated geomagnetic storms at
Potsdam and at the Hilf Observatory (13 km south of 1953
Potsdam).
1912- Van Bemmelen (1912, 1913) investigated the lunar
1913 period magnetic variation at 15 observatories.
1917- Terada (1917) and Dechevren (1918a, 1918b) inves- 1954
1918 ligated Earth currents in Japan and in England (Ier
1893
1905
1908
1909
255
1927- Baird (1927) and Skey (1928) detected for the first
1928 time (at Watheroo in Australia and at Amberley and
Christchurch in New Zealand, respectively) the in-
tersection of what is now called the Parkinson plane
(see, e.g., Gregori and Lanzerotti, 1980) with the
1930
1931
1936
1950
1918
1919
1922
sey), respectively.
The British Admiralty succeeded for the first time to
detect electro-magnetic disturbances related to sea-
water flows (Young et al., 1920; figure reported in
Chapman and Bartels, 1940~.
Chapman (1919) performed a systematic (and still
quite valuable) analysis on the diurnal magnetic var-
iation at 21 observatories, based on records collected
in 1905.
Bauer (1922) reviewed the status of Earth current
studies.
Some historical points of interest in the past 60 years include
the following:
1923 Chapman and Whitehead (1923) appear to have
been the first investigators to be concerned with in
duction effects associated with the auroral electrojet
(a localized current system). They erroneously con
cluded that geomagnetic storm effects at low lati
tudes are produced by Earth currents induced by the
auroral electrojet.
1955
DZ plane (i.e., the vertical, east-west oriented
plane).
Chapman and Price (1930) reconsidered the Chap-
man and Whitehead (1923) analysis and clearly
stated that "the storm-time variations of the geomag-
netic field in low latitudes cannot be due to currents,
induced either the Earth or in a conducting layer of
the atmosphere, by varying primary currents in the
auroral zones."
Cooperative project between the U.S. Coast and Ge-
odetic Survey, the Carnegie Institution of Washing-
ton, and the American Telephone and Telegraph
Company initiated at Tucson magnetic observatory
to study Earth currents.
Bossolasco (1936) detected for the first time (from
measurements performed at Mogadiscio, Somalia,
during the second International Polar Year, 1932-33)
what is now called the Parkinson plane.
De Wet (1949) attempted a numerical computation
of the induction effects in oceans taking into account
the coastal shapes.
Ashour (1950) estimated the decay time of induced
telluric currents within oceans. Constantinescu
(1950) discovered what is now called the Parkinson
plane and draw a plot, which is quite similar to a
Wiese plot (see, e.g., Gregori and Lanzerotti, 1980~.
Rikitake and Yokoyama (1953) clearly stated the ex-
istence of the Parkinson plane. Banno (1953) de-
tected for the first time the coast effect on Earth cur-
rents at Memambetsu (Hokkaido).
Fleischer (1954a, 1954b, 1954c) hypothesized an
east-west electric conductor 70 to 100 km deep be-
neath Bremen. Kertz (1954) stated that it cannot be
lower than 80 km. Bartels (1957) estimated a depth
of 50 to 100 km. Schmucker (1959) estimated a cylin-
der 63 km in radius, 100 km deep. Porstendorfer
(1966) estimated high conductivity (0.2-0.5 mho/m)
down to 10 km depth, an insulator (0.0001 mho/m)
down to 100 km, a conductor (0.1 mho/m) between
100 and 130 km, an insulator (0.0001 mho/m) be-
tween 130 and 400 km, and 0.1 mho/m underneath.
Vozoff and Swift (1968) reported a sedimentary
layer (1.0 mho/m) 6 km deep in North Germany (8
sites from Braunschweig to Luebeck). The North
German conductivity anomaly is now believed to be
principally produced by surface-hydrated sedimen-
tary layers that channel electric currents from the
North Sea eastward to Poland. This is a classic exam-
ple of how difficult the inversion (interpretation)
problem is for geomagnetic measurements.
Rikitake and Yokoyama (1955) appear to be the first
authors to use the term "coast effect." In theoreti-
cally calculating a model of electromagnetic induc
OCR for page 256
256
lion in a hemispherical ocean, they noted an en
hanced magnetic field close to the coasts.
1958 Mansurov (1958) used the term "coastal effect" in
analyzing geomagnetic measurements made at
Mirny Station, Antarctica.
1959 Parkinson (1959, 1962a, 1962b, 1964), in a series of
classic papers, analyzed in detail what is now called
the Parkinson plane for geomagnetic measurements.
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OCR for page 258
Representative terms from entire chapter:
geomagnetic field
