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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 241 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 external-origin inducing field is often poorly known, particularly 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 compared 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 telluric current studies to take into more explicit account the relations of measured currents to the specific tectonic and geomorphological features of the regions under study. Approaches toward such a viewpoint have been presented recently by Hermance (1983). In general, 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 remarkably 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 sedimentary basins, such as in the Seine Basin and in the northern German anomaly (see Appendix; for other references see, e.g., Gregori and Lanzerotti, 1982). Shallow telluric currents are responsible for a component of the coast effect or magnetic signals, where the geometrical orientation of the magnetic variations at higher frequencies 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 (Figure 16.7). At shorter periods, when the skin depth is shallower, the coast effect reflects the coast shape. At longer periods, electromagnetic induction evidence suggests a dependence on the downward bending of the lithospheric slab where it approaches the Japanese subduction 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 Japanese 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 discontinuityâthe Moho (see, e.g., global map presented by Soller et al., 1982)âhas no obvious correspondence 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) derived 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-velocity channel or of the thickness of the high-velocity lid overlying the channel. They identified the lid as synonymous 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 experimental observations used, four different definitions can be distinguished: the elastic or flexural, the thermal, the seismic, and the chemical or mineralogical (U.S. Geodynamics 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 second. Hence, in this simple context, Figure 16.8 can provide an idea of the depth where a high electrical conductivity 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 geomagnetic 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. [A warning must be given
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 242 Figure 16.7 (a) The âZ/âH value distribution in Japan for geomagnetic variations corresponding to geomagnetic bays. The profiles AA' and BB' have been investigated in detail, and their results are shown in the subsequent min. figures. (b) Parkinson vectors along the profile AA' of part (a), for geomagnetic variations with period of 60 an Contours indicate the sea depth in 103 m. The Parkinson vectors are consistent with an interpretation in terms of asthenosphere bending and deepening in the subduction zone. (c) The same as for profile AA' in part (b), less but referring to the profile BB'. The downward bending of the asthenosphere in the subduction zone appears much pronounced in this region. (d) Parkinson vectors on the Miyakejima island for periods (a) 120, (b) 60, (c) 30, (d) the 15, and (e) 5 min, respectively. The coast effect is quite evident at the shorter periods, while at the longer periods (b), effect of the bending of the asthenosphere is predominant over the coast effect. The vectors appearing in the (c), and (d) sections of the figure are "Parkinson arrows" or "vectors," defined in the following manner. Consider a the deepest surface layer to which the incident electromagnetic wave of a given period can penetrate. Consider line plane (the "Parkinson plane") tangent to such a surface, directly beneath a given recording site. Construct a of perpendicular to this plane and oriented downward. Project this line in the horizontal plane: this is the direction thearrow. The length of the arrow is equal to the sine of the tilt of the Parkinson plane with respect to the horizontal a plane. Therefore, a vanishing Parkinson arrow implies a horizontal Parkinson plane, a unit length arrow implies the vertical Parkinson plane. A "normal" coast effect on an island shows that Parkinson arrows point outward from is island. (For other details on "induction arrows'' refer to the review by Gregori and Lanzerotti, 1980.) Figure adapted from Honkura (1974). Figure 16.8 Thickness of the lithosphere derived from a spherical harmonic (12 degree) representation of the global heat flow (see Figure 16.6) and continental and oceanic geotherm families. Contours are in kilometers, with variable intervals. From Chapman and Pollack (1977).