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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 234 Figure 16.1 Planetary-scale distribution of telluric currents according to Gish (1936a, 1936b) at 1800 GMT. 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 planetary 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, depending on the value of the Dst index (a measure of the particle 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 and (external and internal, respectively) change with the level of Dst. The internal term increases with decreasing Dst, unlike , 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 water 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 variable because of the temporal and spatial variability of the external-origin fields, variabilities that are not yet amenable to accurate predictive modeling. Nevertheless, 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 electromagnetic 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 Fainberg (1972, 1974) to evaluate, on a global scale, possible currents between ground and air suffered large uncertainties from the approximations used. As noted briefly in the Introduction, the cause of telluric currents is either electromagnetic induction by the time-varying geomagnetic field produced by the ionosphere and/or magnetosphere or by water movement across the permanent
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 235 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 currents 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 reality). 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/Ï) 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; Gough and Ingham, 1983). (The skin-depth only provides, however, a rough approximation of the depth at which actual 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, hydrated 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 geologic environment (see, e.g., ACRES, 1975; Keller, 1966) can be placed between these extremes. Nomograms by which T, Ï, and S can be evaluated for different materials and for the "actual" Earth are shown in Figure 16.4. The conductivity of water is largely affected by salinity (and to a minor extent by temperature). 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 electromagnetic means, to distinguish materials of equal density but with different porosities, and hence different hydration (and electrical conductivities), that cannot be distinguished by seismic techniques. 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 consecutive lines is 25 Ã 103 A; the thick solid curves indicate the zero-intensity lines. The numbers near the central dots are the total current intensities 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 vorticies are opposite. This figure adapted from Matsushita (1967). The distributions of sediments, particularly important 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 [Figure 16.5(a)]. 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 Europe. Clearly shown are the sedimentary structures responsible for the North German conductivity anomaly and for the channeling in the Seine Basin. The North German anomaly, with a depth-integrated conductivity >3000 mhos, is equivalent to 750 m of seawater. Another physical factor affecting conductivity, and thus telluric currents, is temperature. Since the temperature increases with depth in the Earth, the conductivity 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
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 236 occurs, there is an upward warping of isothermal surfaces. 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 5Â° Ã 5Â° mesh (Figure 16.6) has been provided by Chapman and Pollack (1975). (This map has largely been obtained using about 5000 direct borehole measurements and, where unavoidable, indirect information. For example, since the heat flow from the ocean floor is a well-defined function 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.) Figure 16.3 (a) The spherical harmonic coefficient of lowest degree and order describing the magnetic field originating external to the Earth, as a function of the global Dst index used to describe temporal variations of the equatorial horizontal magnetic field relative to magnetically quiet days. (b) The spherical harmonic coefficient of lowest degree 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 external to the Earth (adapted from Langel, 1982). Three additional aspects of the conductivity structure of the Earth affect the flow of telluric currentsâspatial gradients, temporal variations, and channeling. The spatial gradients of telluric currents strongly depend, in shallow layers, on geochemical composition,
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 237 Figure 16.4 (a) Skin-depth nomogram, indicating depth probed as a function of period and material 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, assuming a planar half-space of uniform conductivity equal to the conductivity 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 almost equivalent dot-dashed lines have been drawn using the model of Achace et al. (1981). (b) Expanded version of the central portion of (a), detailing the depth range 100 km â¤ S â¤ 3000 km. (c) Profiles of the conductivity of the Earth versus depth: full line, according to Rokityansky (1982); dash-dot line according to Achace et al. (1981). The lines above and below the estimated average profiles are indicative of the 95 percentile 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.
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 238 Figure 16.5 (a) Model maps of total conductivity of the water shell plus sedimentary cover. The isolines give the depth-integrated conductivity in units of mhos. The exceptionally solid lines are regions of rapid gradients in conductivity. From Fainberg and Sidorov (1978). (b) Expanded view of the European sector, from Fainberg and Sidorov (1978), where expanded models for other regions of the world are also provided.
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 239
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 240 Figure 16.6 Spherical harmonic representation (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 (producing 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 temperature with depth. In such deep layers it has generally been assumed that the Earth becomes increasingly homogeneous with greater depth. More realistically, however, 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 conductivity provide ever-diminishing spatial (horizontal) resolution with depth. The problem of spatial gradients of the telluric currents is also related to the state of knowledge of the spatial 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 showing strong spatial gradients related to the auroral and equatorial electrojets for quiet conditions (e.g., Schlapp, 1968; Riddihough, 1969; Greener and Schlapp, 1979). For disturbed conditions, the planetary-scale description still plays a relevant, though not singular, role (e.g., Sato, 1965; Campbell, 1976). Therefore, the external-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 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 climatic 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), intermediate (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 presumption 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 MouÃ«l and Menvielle (1982), Nienaber et al. (1982), and Summers (1982); see also extensive review and discussion 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 remote 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