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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

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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

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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

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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

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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

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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'

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238 10 r ~too ~ / _ / o ~ i ~ ( OoW~ , ~/~/: ,, o , ,, ,, Ace, _-Jo , ~ \ ((// of into/ ~ - \J ~ _. 5 - _ o 3 ._ ^ Cot X ~ ~ _ C) - Cat o . ~ ~ o o I= - ~5 o ~ US Ct ._ 5 - ~ 41) ._ >h ~ ~> I;., ._ C) ~ ~ o 8 o ~ C) 4 - b4 ~ ~ Cot 4= ~ , o -t =5 ~ Do, ~ o to 3 ~ .~> ._ I= o .m "S ~ ' X 8- - 5- oc' _ r. ;> O O C~ ~ ~ V) _ ~ C~ (L) 4 - ._ ~ Ct 3 ~ ~ C) .~: o.5 . =, .> C,) C~ o o ~ C) C) o _ ~ 3 o ~ ~ C - . ~ ~o ~5 U' C~ ~ ~ ~ =. - s ~ '~ c.:~ o ~o U] I, _ ~ o _ ._ ~ ~o ~ ~5 . C) o CD ~ C _ Cd ~; L~ .= ~ - C: V~ ~-4 -o X

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239 ,3 it\ ~ ~ - ~ Vat 1 / ._ ~ ~1 1 ,~ jp~. ~ \/ - \ ;~ I' ~ ' o 7N i- If / 1 ~ = , ~

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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

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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 ~ ~ ~

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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~.

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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

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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

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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. REFERENCES Achace, J., J. L. LeMouel, and V. 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Branch, Department of Energy, Mines, and Resources, Ottawa, pp. 56-67. La Cour, D., and E. Hoge (1937~. Note on the effects of electrification of railways passing in the area of the Copenhagen Observatory, Bull. Int. Assoc. Terr. Magn. Atmos. Electr. 10, 302-306. Langel, R. A. (1982~. Results from the MAGSAT mission, Johns Hopkins APL Tech. Dig. 3, 307-324. Lanzerotti, L. J. (1979a) . Impacts of ionospheric/magnetospheric pro- cesses on terrestrial science and technology, in Solar System Plasma Physics (3), L. J. Lanzerotti, C. F. Kennel, and E. N. Parker, eds., North-Holland, Amsterdam, pp. 317-363. Lanzerotti, L. J. (1979b). Geomagnetic influences on man-made sys- tems, J. Atmos. Terr. Phys. 41, 787-796. Lanzerotti, L. J. (1979c). Geomagnetic disturbances and technologi- cal systems, National Comm. Conf en (Nov. ), 7. 1. 1 -7. 1.4. Lanzerotti, L. J. (1983~. Geomagnetic induction effects in ground- based systems, Space Sci. Rev. 34, 347-356. Lanzerotti, L. J., C. G. Maclennan, and C. 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TELLURIC CURRENTS physical effects, NOAA Space Environment Laboratory Report, December. Meloni, A., L. J. Lanzerotti, and G. P. Gregori (1983~. Induction of currents in long submarine cables by natural phenomena, Rev. Geophys. Space Phys. 21, 795-803. Meunier, J. (1969~. Sondage electrique de le croute utilisant des cou- rants de retour industrials, C. R. Acad. Sci. Ser. B 268, 514-517. Mikerina, N. V. (1962~. The study of interference at the Voyeykovo magnetic observatory, Geomagn. Aeron. 2, 941-944. Miller, S. L. (1974~. The nature and occurrence of clathrate hydrates, in Natural Gases in Marine Sediments, I. R. Kaplan, ea., Plenum Press, New York, pp. 151-177. Miyakoshi, J. (1980~. Electrical conductivity structure beneath the Ja- pan island arc by geomagnetic induction study, Adv. Earth Planet. Sci. 8, 153-161. Moore, F. R. (1977~. Geomagnetic disturbance and the orientation of nocturnally migrating birds, Science 196, 682-684. Moureaux, T. H. (1896~. 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254 Stormer, C. (1955~. The Polar Aurora, Clarendon Press, Oxford, 403 PP. Summers, D. M. (1982). On the frequency response of induction anomalies, Geophys. J. R. Astron. Soc. 70, 487-503. Thakur, N. K., M. V. Mahashabde, B. R. Arora, and B. P. Singh (1981~. Anomalies in geomagnetic variations on peninsular India near Palk Straight, Geophys. Res. Lett. 8, 947-950. Thomson, K. S. (1983~. The sense of discovery and vice-versa, Am. Sci. 71, 522-524. Tsikerman, L. Ya. (1960~. Insulation of buried metal pipes to prevent corrosion, Gosstroiizdat, Moscow. U.S. Geodynamics Committee (1983~. The Lithosphere-Report of a Workshop, National Academy Press, Washington, D.C., 84 pp. Varentsov, I. M. (1983~. Modern trends in the solution of forward and inverse 3D electromagnetic induction problems, Geophys. Surv. 6, 55-78. Varley, C. F. (1873~. Discussion of a few papers on Earth currents, J. 1821 Soc. Telegr. Eng. 2,111-114. Walker, M. M., J. L. Kirschvink, S.-B. R. Chang, and A. E. Dizon (1984~. A candidate magnetic sense organ in the yellowfin tuna, Thunnus albacores, Science 224, 751-753. Ward, S. H. (1983). Controlled source electrical methods for deep ex- ploration, Geophys. Surv. 6, 137-152. Wertheim, G. K. (1954~. Studies of electrical potential between Key West, Florida, and Havana, Cuba, Trans. AGU35, 872. Williams, D. J. (1979~. Magnetosphere impacts on ground-based power systems, in Solar System Plasma Phys~cs, L. J. Lanzerotti, C. F. Kennel, and E. N. Parker, eds., North-Holland, Amsterdam, pp. 327-330. Winch, D. E. (1981~. Spherical harmonic analysis of geomagnetic tides, 1964-1965, Phil. Trans. R. Soc. London A303, 1-104. Winckler, J. R., L. Peterson, R. Hoffman, and R. Arnoldy (1959~. Auroral x rays, cosmic rays, and related phenomena during the storm of February 10-11, 1958, J. Geophys. Res. 64, 597-610. Wollaston, C. (1881~. Discussion of the paper by A. J. S. Adams Earth currents (2nd paper), J. Soc. Telegr. Eng. Electric~ans 10, 50-51; 85-87. Woods, D. V., and F. E. M. Lilley (1980~. Anomalous geomagnetic variations and the concentration of telluric currents in south-west Queensland, Australia, Geophys. J. R. Astron. Soc. 62, 675-689. Yanagihara, K. (1977~. Magnetic field disturbance produced by elec- tric railway, Mem. Kakioka Magn. Obs. (Suppl. 7), 17-35. Yanag~hara, K., and H. Oshima (1953~. On the Earth-current distur- bances at Haranomachi caused by the leakage current from the elec- tric railway Fukushima-Yonezawa [in Japanese], Mem. Kakioka Magn. Obs. 6, 119-134. Yanag~hara, K., and T. Yokouchi (1965~. Local anomaly of Earth- currents and Earth-resistivity [in Japanese], Mem. Kakioka Magn. Obs. 12, 105-113. Yoshimatsu, T. (1964~. Results of geomagnetic routine observations and earthquakes; locality of time changes of short period variations ~in Japanese], Mem. Kakioka Magn. Obs. 11, 55-68. 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

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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

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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. APPENDIX REFERENCES Airy, G. B. (1868~. Comparison of magnetic disturbances recorded by self-registering magnetometers at the Royal Observatory, Greenwich, with magnetic disturbances described from the corre- sponding terrestrial galvanic currents recorded by the self-register- ing galvanometers of the Royal Observatory, Phil. Trans. R. Soc. 158A, 465-472. Ashour, A. A. (1950~. The induction of electric currents in a uniform circular disk, Q. J. Mech. Appl. Math. 3, 119-128. Baird, H. F. (1927~. A preliminary investigation of some features of four magnetic storms recorded at seven magnetic observatories in the Pacific Ocean region during 1924, Thesis, Univ. of New Zealand. Banno, N. (1953~. On the Earth-current potentials at the Memam- betsu magnetic observatory tin Japanese], Mem. Kakioka Magn. Obs. 6, 114-118. Barlow, W. H. (1849~. On the spontaneous electrical currents ob- served in the wires of the electric telegraph, Phil. Trans. R. Soc. 61A, 61-72. Barraclough, D. R. (1982~. Historical observations of the geomagnetic field, Phil. Trans. R. Soc. A306, 71-78. Bartels, J. (1957~. Erdmagnetische Tiefen-sondierungen, Geol. Rdsch. 46, 99-101. Bauer, L. A. (1922~. Some results of recent Earth-current observations and relations with solar activity, terrestrial magnetism and atmo- spheric electricity, TeTT. Magn. Atmos. Electr. 27, 1-30. Berdichevsky, M. N., and M. S. Zhdanov (1984~. Advanced Theory of Deep Geomagnetic Sounding, Elsevier, 408 pp. Blavier, E. E. (1884~. Sur les courants telluriques, C. R. Hebd. Seanc Acad. Sci. 98,1043-1045. Bossier, J. (1912~. Sur les relations des orages magnetiques et des phenomenes solaries, These de specialite, Paris. Bossolasco, M. (1936~. Sur la nature des perturbations magnetiques, C. R. Hebd. Seanc. Acad. Sci. 203, 676-678. Burbank, J. E. (1905~. Earth-currents and a proposed method for their investigation, Terr. Magn. Atmos. Electr. 10, 23-49. Chapman, S. (1919~. The solar and lunar diurnal variations of terres- trial magnetism, Phil. Trans. R. Soc. A218, 1-118. Chapman, S. (1963~. Solar plasma, geomagnetism and aurora, in Geophysics, The Earth's Environment, C. DeWitt, J. Hieblot, and A. Lebaeu, eds., Gordon and Breach, pp. 371-502. Chapman, S., and J. Bartels (1940~. Geomagnetism (2 volumes), Clarendon Press, Oxford, 1049 pp. Chapman, S., and A. Price (1930~. The electric and magnetic state of the interior of the Earth as inferred form terrestrial magnetic varia- tions, Phil. Trans. R. Soc. A229, 427-460. 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