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Interpretation of a deep electromagnetic sounding of the ocean floor near the Californian coast
Physics of the Earth and Planetary Interiors, 13 (1976) 119—122 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
119
INTERPRETATION OF A DEEP ELECTROMAGNETIC SOUNDING OF THE OCEAN FLOOR NEAR THE CALIFORNIAN COAST L.L. VAN’YAN, E.P. KHARIN, I.L. OSIPOVA and V.A. SPIVAK Soviet Geophysical Committee, Moscow (U.S.S.R.)
1
(Received January 5, 1976; revised and accepted June 4, 1976)
Van’yan, L.L., Kharin, E.P., Osipova, I.L. and Spivak, V.A., 1976. Interpretation of a deep electromagnetic sounding of the ocean floor near the Californian coast. Phys. Earth Planet. Inter., 13: 119 122. Amplitude and phase curves of magnetotelluric (MTS) and gradient magneto-variational (MVS) soundings in the period range 0.5—24 h obtained by Cox, Filloux and Larsen are interpreted. After some corrections the results of both kinds of sounding are practically the same. These results indicate high-conductivity oceanic asthenosphere at the 70—170-km depths.
1. Introduction Though deep geoelectrics has made significant proon the continents, very little is known of the electrical conductivity of the ocean floor. Technical difficulties still preclude a wide use of sea-floor magnetotelluric arrays and magnetovariational gradient arrays which have specially been developed to be employed in under-sea conditions. The only example of the complementary utilization of the two techniques over a wide range of periods (0.5—24 h) is provided by Larsen and Cox (1966) and Cox et al. (1971). The measurements of the east magnetic component ‘~Yand the horizontal components of the electric field were carried out in 1965 at a depth of 4.3 km, 630 km west of the Californian coast. By interpreting the magnetotelluric sounding in the period range from 0.5 to 8 h, the authors of the above investigation propose a two-layered model: a practically nonconducting layer, 25 km thick, is underlain by a medium having an electric resistivity of about 2.0 ~2 m. Greenhouse (1972) estimates the resistivity gress
1
Address: Soviet Geophysical Committee, Molodezhnaya 3 Moscow 117296, U.S.S.R.
of the conductor about 20.0 ~ m. Marderfeld et al. (1970) interpreted the results of the gradient magnetovariational sounding using the data obtained by Larsen and Cox (1966) and Cox et al. (1971). They found that at a depth of about 120 km the resistivity abruptly decreased to very low values, which according to an estimate by Trofimov (1975)were smaller than 0.19 ~ m. Finally, in 1974 Launay, who made use ofthe convolution equation relating the horizontal magnetic variations at the sea bottom to those at the surface, obtained a conducting layer occurring within the depth range from 40 to 50 km with the integrated conductivity of 2~iO~ The purpose of the present paper is to obtain a model which would be consistent both with magnetotelluric and magnetovariational observations over the whole range of periods including daily variations.
2. Analysis of field data In our interpretation we use the values of the modulus and phase of two experimentally observed quantities published by Larsen and Cox (1966) and Cox et al. (1971). The impedance of the ocean floor
120 TABLE I Mean amplitude and phase values and deviations
T (h)
0.5 1 2
4 8 6 8
it is calculated using the cornplex magnetic ratio (A) (Trofimov and Fonarev, 1972). These calculations are simplified for periods exceeding 0.5 h, because then the skin-effect in water may be neglected. Since in such a case the difference between the horizontal component of the magnetic field at the netovariational sounding
~s.Z Z (%) 300 100 140 150 37 41 29
~O~4
(0)
8
5 8 14 6 22 16
A (%)
(0)
25 20 16 17
16 13 10 10
41 26
24 14
surface and the corresponding component measured at the sea floor is equal to the integrated current density in water (i) we have: YO—Y=iN=SEN
(Z = EN/fl and the ratio of the east component of magnetic variations measured at the surface to that measured at the ocean floor (A = Y0/fl. It will be recalled that instead of the ~‘0 at the surface of the ocean, the data of continental observatories were used. Published experimental data~containthe mean amplitude and phase values and deviations. It provides a possibility to calculate the relative accuracy both for Z and A. Calculated values and phase deviations are shown in Table I (note ça~= arg EN! Y and ~PA= arg Y0/ Y). Relative deviations vary from 16 to 41% for the magnetic ratio and from 17 to 300% for the electric to magnetic field ratio (deviations exceeding 100% being noted for periods of 0.5—4.0 h). It is interesting to note that the phase measurements are characterized by a higher accuracy. With the exception of the period of 6 h, the phase error does not exceed 16°,including that for the impedance in the period interval from 0.5 to 4.0 h.
3. The apparent resistivity The most convenient mode of presenting the results of electromagnetic sounding is to express them through the complex apparent resistivity which can be calculated by the well-known formula: P a =Z 2/iwp
In magnetotelluric sounding the impedance of the sea floor (Z) is measured directly, whereas in gradient mag-
where S is the integrated conductivity of water. Hence: Z=EN/Y=S~(YO/Y—l) It will be noted that the calculation of the impedance from just one east magnetic component yields a stable result only in the case of horizontal layers. Otherwise, the impedance is a tensor. It is possible that the above-mentioned scattering in experimental values is simply due to the horizontal imhomogeneities.
4. Interpretation The mean values of Pa I and arg Pa calculated by magnetic ratio are shown in Fig. I (black circles). The amplitude decreases with period from 20 to 5 ~ m (minimum value at T 8 h) and after that increases to about 8 ~ m for T = 24 h. This value is close to that for global magnetovariational sounding obtained by spherical harmonics (Fiskina et al., 1976) which are shown by rectangles on Fig. 1. Phase values increase from —60 to —16°.For quantitative interpretation of both amplitude and phase data, a set of curves for 55 models was calculated. Three examples of them are shown in Fig. 1 (resistivity in ~2 m and thickness of each layer in km, see Table II). Model 1 (solid line in Fig. 1) with a low-resistivity layer in the depth range 70—170 km was chosen as the best fit for the data. In this model the bottom-sediment thickness (the first layer) was taken to be in accord with seismic data (Greenhouse, 1972). Let us consider now the MTS data (crosses in Fig. 1). Mean phase values agree well with the data discussed above and support the best-fit model. As to mean amplitude it agrees with MVS data for 8—24-h periods but is lower by about half of an order of magnitude for shorter ~-‘
121
IPaI~m
IP
0Hm
+
Phase
.~—
~b22
____
Phase
0.5
2.p
8p
~
2~1hours
0:5
2;O
8:0
2:4 hours
Fig. 1. Comparison of experimental sea-floor apparent resistivity with calculated curves (for indexes of curves see Table II)
Fig. 2. Comparison of experimental “surface” apparent resistivity with calculated curves (for indexes of curves see Table II).
periods (note that there is the most scattering of experimental data in this range). To prove the ‘Pa’ values are understated we calculated the MTS amplitude and phase curves for ocean surface using the sea-floor EN component and Y0. It is possible, since for T>~0.5 h, the skin-depth in water is sufficiently large and E at the surface is nearly the same as at the sea floor. “Surface” amplitude and phase data are shown in Fig. 2 (open circles). They are compared with curves calculated for models of Table II plus a water layer with p = 0.33 ~ m and thickness 4.3 km. Phase values agree well with the curve calculated for the best-fit model. The ‘na’ curve corresponding to this model (index 1 in Fig. 2) agrees with “surface” experimental data for T>~8 h but
differs from them at 0.5 h ~ T ~ 6 h. Note the integrated conductivity of the water and sediments determined by experimental amplitude “surface” curve is about 35 l0~~l’ while the real value for water is 13.2- iO~~2’ and for sediments is not more than some hundreds of ~21. It suggests that MTS pI values for 0.5 h ~ T~6 h are understated. To correct these values we must multiply them by some factor for agreement with curve 1. Since the electric field at the surface is practically equal to that at sea floor, the same factor was used for correction of sea-floor values of IPaI~Corrected values (crosses in circles in Fig. 1) are in agreement with those for MVS.
TABLE II Three examples of conductivity models Index of the curve in Figs. 1 and 2
P1
1 32
1.25 1.25
(t2
.
m)
h1
P2
(km)
(t2
0.2 0.2
i0 ~
3
h2 .
m)
(km)
P3 (12
60 70 40
2.2 4.0
.
m)
h3
/24
h4
(km)
(12 . m)
(km)
p~ (12 - m)
160 100 110
102 102
320 350 350
1.0 1.0
122
5. Conclusions A well-conducting layer of oceanic asthenosphere occurring at depth 70—170 km and having a specific resistivity of 2—3 ~ m is indicated by gradient magnetovariational sounding in the period range 0.5—24 h. This model is supported by MTS phase measurements, while amplitude values for 0.5 h t~T ~ 6 h are understated
References Cox, C.S., Filloux, J.H. and Larsen, J.C., 1971. Electromagnetic studies.of ocean currents and electrical conductivity below the ocean floor. In: A. Maxwell (Editor), The Sea, Vol. 4, Part I, Wiley, New York, N.Y. Fiskina, M., Rotanova, N., Feinberg, E. and Mershikova, N.,
1976. Master Curves for Depth Geomagnetic Soundings. Nauka, Moscow (in Russian). Greenhouse, J.P.,southern 1972. Geomagnetic time Thesis, variations on the sea floor off California. Ph.D. University of California, San Diego, Calif. Larsen, J.C. and Cox, C.S., 1966. Lunar and solar daily variations in the magnetotelluric field beneath the ocean. J. Geophys. Res., 71: 4441. Launay, L., 1974. Conductivity under the oceans: Interpretation of a magnetotelluric sounding 630 km off the Californian coast. Phys. Earth Planet. Inter., 8: 83. Marderfeld, B.E., Rodionov, A.V. and Fonarev, G.A., 1970. Depth determination for the conducting mantle layer using the observations of variations in the horizontal geomagnetic field components on the ocean floor and at the surface. Geomagn. Aeron., X: 1122 (in Russian). Troflmov, IL. and Fonarev, G.A., 1972. On the applicability of the gradient methods of sounding at sea. Geomagn. Aeron., XII: 301 (in Russian). Trofimov, I.L., 1975. Experiment of gradient magnetic sounding in the Pacific Ocean. Geomagn. Aeron., XV: 181 (in Russian).
119
INTERPRETATION OF A DEEP ELECTROMAGNETIC SOUNDING OF THE OCEAN FLOOR NEAR THE CALIFORNIAN COAST L.L. VAN’YAN, E.P. KHARIN, I.L. OSIPOVA and V.A. SPIVAK Soviet Geophysical Committee, Moscow (U.S.S.R.)
1
(Received January 5, 1976; revised and accepted June 4, 1976)
Van’yan, L.L., Kharin, E.P., Osipova, I.L. and Spivak, V.A., 1976. Interpretation of a deep electromagnetic sounding of the ocean floor near the Californian coast. Phys. Earth Planet. Inter., 13: 119 122. Amplitude and phase curves of magnetotelluric (MTS) and gradient magneto-variational (MVS) soundings in the period range 0.5—24 h obtained by Cox, Filloux and Larsen are interpreted. After some corrections the results of both kinds of sounding are practically the same. These results indicate high-conductivity oceanic asthenosphere at the 70—170-km depths.
1. Introduction Though deep geoelectrics has made significant proon the continents, very little is known of the electrical conductivity of the ocean floor. Technical difficulties still preclude a wide use of sea-floor magnetotelluric arrays and magnetovariational gradient arrays which have specially been developed to be employed in under-sea conditions. The only example of the complementary utilization of the two techniques over a wide range of periods (0.5—24 h) is provided by Larsen and Cox (1966) and Cox et al. (1971). The measurements of the east magnetic component ‘~Yand the horizontal components of the electric field were carried out in 1965 at a depth of 4.3 km, 630 km west of the Californian coast. By interpreting the magnetotelluric sounding in the period range from 0.5 to 8 h, the authors of the above investigation propose a two-layered model: a practically nonconducting layer, 25 km thick, is underlain by a medium having an electric resistivity of about 2.0 ~2 m. Greenhouse (1972) estimates the resistivity gress
1
Address: Soviet Geophysical Committee, Molodezhnaya 3 Moscow 117296, U.S.S.R.
of the conductor about 20.0 ~ m. Marderfeld et al. (1970) interpreted the results of the gradient magnetovariational sounding using the data obtained by Larsen and Cox (1966) and Cox et al. (1971). They found that at a depth of about 120 km the resistivity abruptly decreased to very low values, which according to an estimate by Trofimov (1975)were smaller than 0.19 ~ m. Finally, in 1974 Launay, who made use ofthe convolution equation relating the horizontal magnetic variations at the sea bottom to those at the surface, obtained a conducting layer occurring within the depth range from 40 to 50 km with the integrated conductivity of 2~iO~ The purpose of the present paper is to obtain a model which would be consistent both with magnetotelluric and magnetovariational observations over the whole range of periods including daily variations.
2. Analysis of field data In our interpretation we use the values of the modulus and phase of two experimentally observed quantities published by Larsen and Cox (1966) and Cox et al. (1971). The impedance of the ocean floor
120 TABLE I Mean amplitude and phase values and deviations
T (h)
0.5 1 2
4 8 6 8
it is calculated using the cornplex magnetic ratio (A) (Trofimov and Fonarev, 1972). These calculations are simplified for periods exceeding 0.5 h, because then the skin-effect in water may be neglected. Since in such a case the difference between the horizontal component of the magnetic field at the netovariational sounding
~s.Z Z (%) 300 100 140 150 37 41 29
~O~4
(0)
8
5 8 14 6 22 16
A (%)
(0)
25 20 16 17
16 13 10 10
41 26
24 14
surface and the corresponding component measured at the sea floor is equal to the integrated current density in water (i) we have: YO—Y=iN=SEN
(Z = EN/fl and the ratio of the east component of magnetic variations measured at the surface to that measured at the ocean floor (A = Y0/fl. It will be recalled that instead of the ~‘0 at the surface of the ocean, the data of continental observatories were used. Published experimental data~containthe mean amplitude and phase values and deviations. It provides a possibility to calculate the relative accuracy both for Z and A. Calculated values and phase deviations are shown in Table I (note ça~= arg EN! Y and ~PA= arg Y0/ Y). Relative deviations vary from 16 to 41% for the magnetic ratio and from 17 to 300% for the electric to magnetic field ratio (deviations exceeding 100% being noted for periods of 0.5—4.0 h). It is interesting to note that the phase measurements are characterized by a higher accuracy. With the exception of the period of 6 h, the phase error does not exceed 16°,including that for the impedance in the period interval from 0.5 to 4.0 h.
3. The apparent resistivity The most convenient mode of presenting the results of electromagnetic sounding is to express them through the complex apparent resistivity which can be calculated by the well-known formula: P a =Z 2/iwp
In magnetotelluric sounding the impedance of the sea floor (Z) is measured directly, whereas in gradient mag-
where S is the integrated conductivity of water. Hence: Z=EN/Y=S~(YO/Y—l) It will be noted that the calculation of the impedance from just one east magnetic component yields a stable result only in the case of horizontal layers. Otherwise, the impedance is a tensor. It is possible that the above-mentioned scattering in experimental values is simply due to the horizontal imhomogeneities.
4. Interpretation The mean values of Pa I and arg Pa calculated by magnetic ratio are shown in Fig. I (black circles). The amplitude decreases with period from 20 to 5 ~ m (minimum value at T 8 h) and after that increases to about 8 ~ m for T = 24 h. This value is close to that for global magnetovariational sounding obtained by spherical harmonics (Fiskina et al., 1976) which are shown by rectangles on Fig. 1. Phase values increase from —60 to —16°.For quantitative interpretation of both amplitude and phase data, a set of curves for 55 models was calculated. Three examples of them are shown in Fig. 1 (resistivity in ~2 m and thickness of each layer in km, see Table II). Model 1 (solid line in Fig. 1) with a low-resistivity layer in the depth range 70—170 km was chosen as the best fit for the data. In this model the bottom-sediment thickness (the first layer) was taken to be in accord with seismic data (Greenhouse, 1972). Let us consider now the MTS data (crosses in Fig. 1). Mean phase values agree well with the data discussed above and support the best-fit model. As to mean amplitude it agrees with MVS data for 8—24-h periods but is lower by about half of an order of magnitude for shorter ~-‘
121
IPaI~m
IP
0Hm
+
Phase
.~—
~b22
____
Phase
0.5
2.p
8p
~
2~1hours
0:5
2;O
8:0
2:4 hours
Fig. 1. Comparison of experimental sea-floor apparent resistivity with calculated curves (for indexes of curves see Table II)
Fig. 2. Comparison of experimental “surface” apparent resistivity with calculated curves (for indexes of curves see Table II).
periods (note that there is the most scattering of experimental data in this range). To prove the ‘Pa’ values are understated we calculated the MTS amplitude and phase curves for ocean surface using the sea-floor EN component and Y0. It is possible, since for T>~0.5 h, the skin-depth in water is sufficiently large and E at the surface is nearly the same as at the sea floor. “Surface” amplitude and phase data are shown in Fig. 2 (open circles). They are compared with curves calculated for models of Table II plus a water layer with p = 0.33 ~ m and thickness 4.3 km. Phase values agree well with the curve calculated for the best-fit model. The ‘na’ curve corresponding to this model (index 1 in Fig. 2) agrees with “surface” experimental data for T>~8 h but
differs from them at 0.5 h ~ T ~ 6 h. Note the integrated conductivity of the water and sediments determined by experimental amplitude “surface” curve is about 35 l0~~l’ while the real value for water is 13.2- iO~~2’ and for sediments is not more than some hundreds of ~21. It suggests that MTS pI values for 0.5 h ~ T~6 h are understated. To correct these values we must multiply them by some factor for agreement with curve 1. Since the electric field at the surface is practically equal to that at sea floor, the same factor was used for correction of sea-floor values of IPaI~Corrected values (crosses in circles in Fig. 1) are in agreement with those for MVS.
TABLE II Three examples of conductivity models Index of the curve in Figs. 1 and 2
P1
1 32
1.25 1.25
(t2
.
m)
h1
P2
(km)
(t2
0.2 0.2
i0 ~
3
h2 .
m)
(km)
P3 (12
60 70 40
2.2 4.0
.
m)
h3
/24
h4
(km)
(12 . m)
(km)
p~ (12 - m)
160 100 110
102 102
320 350 350
1.0 1.0
122
5. Conclusions A well-conducting layer of oceanic asthenosphere occurring at depth 70—170 km and having a specific resistivity of 2—3 ~ m is indicated by gradient magnetovariational sounding in the period range 0.5—24 h. This model is supported by MTS phase measurements, while amplitude values for 0.5 h t~T ~ 6 h are understated
References Cox, C.S., Filloux, J.H. and Larsen, J.C., 1971. Electromagnetic studies.of ocean currents and electrical conductivity below the ocean floor. In: A. Maxwell (Editor), The Sea, Vol. 4, Part I, Wiley, New York, N.Y. Fiskina, M., Rotanova, N., Feinberg, E. and Mershikova, N.,
1976. Master Curves for Depth Geomagnetic Soundings. Nauka, Moscow (in Russian). Greenhouse, J.P.,southern 1972. Geomagnetic time Thesis, variations on the sea floor off California. Ph.D. University of California, San Diego, Calif. Larsen, J.C. and Cox, C.S., 1966. Lunar and solar daily variations in the magnetotelluric field beneath the ocean. J. Geophys. Res., 71: 4441. Launay, L., 1974. Conductivity under the oceans: Interpretation of a magnetotelluric sounding 630 km off the Californian coast. Phys. Earth Planet. Inter., 8: 83. Marderfeld, B.E., Rodionov, A.V. and Fonarev, G.A., 1970. Depth determination for the conducting mantle layer using the observations of variations in the horizontal geomagnetic field components on the ocean floor and at the surface. Geomagn. Aeron., X: 1122 (in Russian). Troflmov, IL. and Fonarev, G.A., 1972. On the applicability of the gradient methods of sounding at sea. Geomagn. Aeron., XII: 301 (in Russian). Trofimov, I.L., 1975. Experiment of gradient magnetic sounding in the Pacific Ocean. Geomagn. Aeron., XV: 181 (in Russian).