Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://geophys.geol.msu.ru/berd/IPSE722.pdf Äàòà èçìåíåíèÿ: Tue Oct 28 15:00:00 2003 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:55:59 2012 Êîäèðîâêà:

722 (c) 0
0.3 2.7

BERDICHEVSKY et al.
CR
1 2 3 45 678

WV
9

WC HC
10 11 12 13 14 15

DP

69 54 180 132 33 8.7 3.4 21 19 22 22 68 139 108 93
4

13 26 59 15 4.2 6.3 124 6.5

3 3.5 10 37.5

23 13 3490 180 104 6110 2420 116 107 4230 14 8320

0 0.6 1.5

1.7 9840

4.0 23 34

9800

15

25
19 46

73

191 14

16 17

127 142 78 299

35 45 70

147
2520 2030

12 1280 18

36

215

46

612

110
22

22

4700 754
1190

110
24 42 1220 12

22

170
27 47

280 z, km ­ 200 ­ 100 (d) 0
0.3 5.1 41 171 46 40
3

280
30

, m
10 000 3000 1000 300 100 30 10 3 1

z, km 50 100
WV

0
CR
1 2 3

150
WC HC

200
DP

km

45

678

9

10 11 12 13 14 15
13 27 62 35 182 5.1 10 63

82 18 313 150 70 1.2 4.8 20 8.7 42 12 111 8.8 1620

3 3.5 10 37.5

0 0.6 1.5
2290

1.8 14 130

4.4 20 8.1

1650

504 1440 2320 734 13 38 57 18 47

190 90 1660 12 468 66 11

25 35 45 70

13 100

281 95 176 244 502 244 2110 27 91 1340 119 59 6.7

110
69

30

258 2210
2610

110
36 47 5180 25

25

170
589 183

280 z, km ­ 200 ­ 100 0
5

280 z, km 50 100 150 200 km

Fig. 20. (Contd.)

agreement of the model with observations. A remarkable feature of the TP model is the conducting continental asthenosphere and the vertical low-resistivity zone that branches from the asthenosphere and crosses the continental crust in the High Cascade region. This feature distinguishes the TP model from EMSLAB-I and EMSLAB-II and makes it similar to the predictive CASCADIA model; in the High Cascades, the latter delineates a vertical high-temperature zone of wet and dry melting that is evidently characterized by low resistivities. Inversion of j||. At this stage, we checked the tipper inversion results. To avoid difficulties related to near-

surface distortions of the curves ||, we restricted ourselves to the inversion of the curves ||, which satisfy the dispersion relations. The TP model, obtained from the tipper inversion, was used as a starting model. The inversion of longitudinal phases yielded the TE model, shown in Fig. 20c. The phase misfit (the rms deviation of model phases from observed values) in this model is 5­10 times smaller than the phase amplitude (the difference between the maximum and minimum phase values), indicating good agreement of the model with observations. As distinct from the TP model, the TE continental crust includes a better delineated conducting
Vol. 39 No. 9 2003

IZVESTIYA, PHYSICS OF THE SOLID EARTH


MAGNETOVARIATIONAL SOUNDING: NEW POSSIBILITIES CR
V IV III II I 12 34 5
8 50 7 20 2000 2000 4500 20 60 20 20 20 20

723

WV
6 78 9
13 3

WC

HC

DP

10 11 12 13 14 15
5 10 200 20 10

50 350 25 8 0.3 3 50 250

7 20 4.5 12 35 14 20 40

0.6 1.5 , m 7.0 25 40
10 000 3000 1000 300 100

3.0 3.5

2

3

37.5
1500 500 30 500

30

30 10

110
55

20 30 500 (?) 15 55

100 155

3 1

280
4

280 z, km ­ 200 ­ 100 0 50 100 150 200 km

z, km

Fig. 21. Final EMSLAB-III model (resistivity values in units of m are shown within blocks): I­V, soundings on the ocean floor; 1­15, soundings on the continent. CR, Coast Range; WV, Willamette Valley; WC, Western Cascades; HC, High Cascades; DP, Deschutes Plateau.

layer ( = 14­46 m) in a depth interval of 35­45 km, whereas the subvertical conducting zone ( = 12­46 m) in a depth interval of 45­110 km, bounded by layers with resistivities of 147­1260 m to the west and 215­ 612 m to the east, is localized with a higher contrast. One may state that the TE model is an updated TP model. Inversion of r^ and j^. At this stage, we inverted the TM-mode, which is less sensitive to conducting zones in the crust and mantle but is more effective in resolving the structure of the junction zone between the slab and crustal conducting layer and provides more reliable estimates of the resistivity in the upper consolidated crust. In inverting the TM-mode, the TE model, obtained from the inversion of phases ||, was taken as a starting model. The inversion of transverse apparent resistivities and phases of the transverse impedance yielded the TM model shown in Fig. 20d. In this model, the misfits of transverse apparent resistivities at most points vary within 6­12%, and the phase misfits are 7­10 times smaller than the phase amplitude (the difference between the maximum and minimum phase values). The TM model inherits the main features of the starting TE model (albeit with some deviations). The following implications of the TM model are noteworthy. First, no well-conducting junction is present between the conIZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 39

ducting slab and the crustal conducting layer. Second, the upper consolidated crust of the continent has a resistivity of about 2000 m, indicating that it is fractured. Note that the TM-mode inversion is essentially dependent on the choice of the starting model. If the START model is taken as the initial one, the TM-mode inversion yields a model in which the continental asthenosphere is absent. This is evidently due to the low sensitivity of the TM-mode to deep conducting structures. In this case, active is the same mechanism that gave rise to the model EMSLAB-I, devoid of the continental asthenosphere, in [Wannamaker et al., 1989b]. Synthesis. At this stage, we analyzed the TP, TE, and TM models and constructed the general EMSLABIII model, smoothing insignificant details and enlarging blocks on the basis of information characterizing the effect of individual blocks on the MVS and MTS characteristics. All changes were made in an interactive regime, with the calculation of local misfits and with the correction of boundaries and resistivities. The resulting general model shown in Fig. 21 provides a coherent geoelectric image of the subduction zone. The extent of its agreement with observed data is seen from Fig. 22, where the model curves , ||, , ||, Re Wzy,
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724 MTS 1 10
3

BERDICHEVSKY et al. MTS 2


MTS 7

MTS 8


2



a, m

10

10
1

||



||



||




||





0 ­15 ­30 ­45 ­60 ­75 ­90 0.6 0.4 0.2 0 ­ 0.2 ­ 0.4 ­ 0.6 10











||



||

, deg



||



||









Re Wzy

Re Wzy

Re Wzy

Re Wzy

Wzy

Im Wzy
0 1 2

Im Wzy

Im Wzy

Im Wzy 10
2

10

10

10

1

10

2

10
1/2

1

10

1

10

2

T, s

Fig. 22. Comparison of the observed MTS and MVS curves with the curves calculated from the EMSLAB-III model: (1) observations; (2) EMSLAB-III model.

and Im Wzy are compared with the observed curves (the static distortion in the observed curves || was removed by a vertical shift of their low-frequency branches). The model curves agree well with the observed curves at the majority of points. In its oceanic part, the EMSLAB-III model is close to EMSLAB-I and EMSLAB-II and exhibits a thick oceanic asthenosphere in a depth interval of 37.5­ 110 km. The structure of the continental part of EMSLAB-III is distinguished by the following significant elements: a crustal conducting layer ( = 20 m, a depth interval of 25­40 km) and a conducting asthenosphere (30 m, 100­155 km) are distinctly resolved; the crustal and asthenospheric conductors are connected by a columnlike conducting body (20­30 m) crossing the lithosphere and reaching depths of about 7 km in the volcanic zone of the High Cascades;

the subducting slab, in a depth interval of 4­40 km contains a thin inclined conductor (20 m) separated from the crustal conducting layer by a higher-resistivity zone (60 m); apparently, the crustal conducting layer is unrelated to slab fluids and has a deep origin. The reliability of these elements is supported by the fact that the elimination of any of them noticeably increases the model misfits. These features of the continental section make the EMSLAB-III and predictive CASCADIA models similar. The fluid regime of the subduction zone is clearly observable here. The subducting slab entraps fluid-saturated low-resistivity rocks of the ocean floor. As the slab moves down, the released free water migrates through the shear zone (the contact zone between the subducting oceanic and stable continental plates). The dehydration (the release of bound water) developing in the slab at depths of 30­40 km supplies fluids to the mantle and causes the wet melting of asthenospheric material. The low-resistivity melts move upward
Vol. 39 No. 9 2003

IZVESTIYA, PHYSICS OF THE SOLID EARTH


MAGNETOVARIATIONAL SOUNDING: NEW POSSIBILITIES MTS 9 10
3

725
2

MTS 12


MTS 14

10

MTS II (ocean)
||

a, m 10
2



10
||

1

10
1

||



||

10


0





10 0 ­15 ­30 ­45 ­60 ­75 ­90 0.6 0.4 0.2 0 ­ 0.2 ­ 0.4 ­ 0.6 10

­1











||




||

, deg



||



||





10 Re Wzy Re Wzy Im Wzy Im Wzy Re Wzy
0

1

10

2

Wzy

1 2

Im Wzy

10

1

10

2

10

1

10

2

10
1/2

1

10

2

T, s

Fig. 22. (Contd.)

through the lithosphere and form a volcanic arc. The heating of the lithosphere activates dehydration in the lower crust, producing the crustal conducting layer. All this confirms the validity of the predictions underlying the CASCADIA model. CONCLUSION Theoretical and experimental results indicate that the MVS method can play a significant role in the development of geoelectric studies. Its main advantage consists in the fact that the magnetic field distortions caused by near-surface inhomogeneities attenuate with decreasing frequency and do not spoil the information on structures of the crust and upper mantle. This makes the results of electromagnetic studies more reliable and better constrained. The development of the MVS method should be regarded as a promising problem of modern geophysics.
IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 39

Presently, we are at the very beginning of this research, and many questions remain unanswered. What is the best way to organize qualitative analysis of MVS data that accounts most adequately for horizontal and vertical variations in the electrical conductivity? What is the resolution of tippers with respect to these variations? How rapidly do the magnetovariational near-surface effects attenuate at low frequencies? What is the sensitivity of tippers to deep structures in the crust and mantle? Which geoelectric conditions are favorable for the application of the MVS method? What are the conditions at which tippers admit a 2-D approximation of 3-D structures? Such is the list of problems, far from being complete, whose solution is crucial for the progress of MVS. Answers to many questions can be obtained with the further accumulation of experience in studies in various geological provinces. Here, we should emphasize that a necessary condition for the MVS application is the
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BERDICHEVSKY et al. Dmitriev, V.I., Inverse Problems in Electrodynamical Prospecting, Ill-Posed Problems in the Natural Sciences, Moscow: Mir Publishers, 1987, pp. 77­101. Dmitriev, V.I. and Mershchikova, N.A., Synthesis of the Magnetotelluric Field, Fiz. Zemli, 2002, no. 11, pp. 69­75. Groom, R.W. and Bailey, R.C., Decomposition of Magnetotelluric Impedance Tensor in the Presence of Local ThreeDimensional Galvanic Distortion, J. Geophys. Res., 1989, vol. 94, no. B2, pp. 1913­1925. Gusarov, A.L., On the Uniqueness of the Magnetotelluric Data Inversion in a Two-Dimensional Medium, Matematicheskie modeli v geofizike (Mathematical Models in Geophysics), Moscow: MGU, 1981, pp. 31­61. Jones, A.G., Static Shift of Magnetotelluric Data and Its Removal in a Sedimentary Basin Environment, Geophysics, 1988, vol. 53, no. 7, pp. 967­978. Nowozynski, K. and Pushkarev, P.Yu., The Efficiency Analysis of Programs for Two-Dimensional Inversion of Magnetotelluric Data, Izvestiya, Physics of the Solid Earth, 2001, vol. 37, pp. 503­516. Osipova, I.L., Berdichevsky, M.N., Vanyan, L.L., and Borisova, V.P., Geoelectric Models of North America, Geomagn. Issled., 1982, no. 29, pp. 117­130. Pous, J., Queralt, P., and Marcuello, A., Magnetotelluric Signature of the Western Cantabrian Mountains, Geophys. Res. Lett., 2001, vol. 28, no. 9, pp. 1795­1798. Rasmussen, J. and Humphries, G., Tomographic Image of the Juan de Fuca Plate beneath Washington and Western Oregon Using Teleseismic P-Wave Travel Times, Geophys. Res. Lett., 1988, no. 15, pp. 1417­1420. Roecker, S.W., Sabitova, T.M., Vinnik, L.P., et al., ThreeDimensional Elastic Wave Velocity Structure of Western and Central Tien Shan, J. Geophys. Res., 1993, vol. 98, no. B9, pp. 15 579­15 795. Rokityansky, I.I., Geoelectromagnetic Investigations of the Earth's Crust and Mantle, Berlin: Springer, 1982. Romanyuk, T.V., Mooney, W.D., and Blakely, R.J., A Tectonic­Geophysical Model of the Cascadian Subduction Zone in North America, Geotektonika, 2001, no. 3, pp. 88­110. Siripunvaraporn, W. and Egbert, G., An Efficient Data Subspace Inversion Method for 2-D Magnetotelluric Data, Geophysics, 2000, vol. 65, pp. 791­803. Trapeznikov, Yu.A., Andreeva, E.V., Batalev, V.Yu., et al., Magnetotelluric Sounding in the Kyrgyz Tien Shan, Fiz. Zemli, 1997, no. 1, pp. 3­20. Trehu, A.M., Asudeh, I., Brocher, T.M., et al., Crustal Architecture of the Cascadia Forearc, Science, 1994, vol. 265, pp. 237­243. Vanyan, L.L., Osnovy elektromagnitnykh zondirovanii (Fundamentals of Electromagnetic Sounding), Moscow: Nedra, 1965. Vanyan, L.L., Varentsov, I.M., Golubev, N.G., and Sokolova, E.Yu., Construction of Induction Magnetotelluric Curves from Profile Geomagnetic Data for the Study of Electrical Conductivity of the Continental Asthenosphere in the EMSLAB Experiment, Fiz. Zemli, 1997, no. 10, pp. 33­46. Vanyan, L.L., Varentsov, I.M., Golubev, N.G., and Sokolova, E.Yu., Derivation of Simultaneous Geomagnetic Field Components from Tipper Arrays, Izvestiya, Physics of the Solid Earth, 1998, vol. 34, pp. 779­786.
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presence of conducting inhomogeneities that play the role of deep local sources of electromagnetic field. The main goal of this paper was to attract the attention of geophysicists to the potentialities of the MVS method and to problems whose solution is indispensable to the practical realization of these potentialities. ACKNOWLEDGMENTS We are grateful to P. Weidelt and U. Schmucker for discussions stimulating this work. This work was supported by the Russian Foundation for Basic Research, project nos. 02-05-64079 and 03-05-64167. REFERENCES
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