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The comparison of the experimental MT data with the results of numerical modelling for the Kamchatka Peninsula
226
Physics of the Earth and Planetary Interiors, 34 (1984) 226—231
Elsevier Science Publishers By., Amsterdam
—
Printed in The Netherlands
Letter Section The comparison of the experimental MT data with the results of numerical modelling for the Kamchatka Peninsula P. Kaikkonen 1, L.L. Vanyan2 B.A. Okulessky
2
and A.M. Poray-Koshitz 2
Department of Geophysics, University of Oulu, SF- 90570 Oulu 57 (Finland) 2 pp• Shirshov Institute of Oceanology, Moscow 117218 (U.S.S.R.)
(Received January 4, 1984; accepted March 20, 1984)
Kaikkonen, P., Vanyan, L.L., Okulessky, B.A. and Poray-Koshitz, A.M., 1984. The comparison of the experimental MT data with the results of numerical modelling for the Kamchatka Peninsula. Phys. Earth Planet. Inter., 34: 226—231. The experimental MT data and theoretical data calculated by the thin sheet technique for a low-frequency telluric field are analysed and compared for the Kamchatka Peninsula. The comparison shows a good agreement between the experimental and theoretical data and results in a conclusion that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. The component of the telluric field which is parallel to the general direction of the Kamchatka Peninsula is less distorted and should be used for the estimation of deep geoelectric structures.
1. Introduction
2. The structure of the MT field according to numerical modelling
Inhomogeneities of the near-surface part of the Earth’s crust can distort the deep geoelectrical information. This distortion effect is especially strong, if the conductivity contrasts are large. At the Kamchatka Peninsula and the surrounding seas conductances of the sediments and water layer vary from 25 to 25000 S. Numerical modelling was used to investigate distortions owing to this greatly varying near-surface conductance. For two-dimensional structures distortion effects were analysed in detail by Berdichevsky and Dmitriev (1976). For three-dimensional structures there are many papers analysing these distortions mainly by means of scale modelling (Dosso, 1966; 1973) or of the thin sheet technique (e.g., Vasseur and Weidelt, 1977; Haak, 1978; Debabov, 1980; Fainberg and Zinger, 1981; Hermance, 1982). As we are interested in the deep geoelectrical structure the low-frequency solution of the Price’s equation (Price, 1949) was used. The algorithm and cornputer program are described in detail by Yegorov et a!. (1983a). 0031-9201/84/$03.00
© 1984 Elsevier Science Publishers B.V.
The map of the near-surface conductance (Fig. 1) was constructed using, among other things, the bathymetric data and MT results obtained by Moroz and Pospeev (1976) for the Kamchatka Peninsula. To reduce the computer time and storage in numerical modelling a hybrid technique was used combining the numerical and analytical solutions in an iterative sense (Yegorov et a!., 1983b). This calculation technique requires the constant conductance values on the boundaries of the mesh. Therefore at the most northern part of the peninsula the conductance values were taken as equal to that of the surrounding seas (see Fig. 1). This means that the results calculated at the uppermost part of the mesh cannot be used for comparison with the experimental data. A circularly polarized, homogeneous field was taken as a primary field. Figure 2 presents calculated telluric ellipses on the background of the isolines of the logarithm of
227 70
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Fig. 1. The scheme of the conductance (S) for the Kamchatka Peninsula and the surrounding seas with the symbols used. The experimental profile used for the comparison is also shown. Note: the scale is not similar in both directions. S-symbols in 5: A=25,B=75,C=150,D=350,E=750,F=1500,G=3500, I = 7500, K 15000, L = 25000.
the near-surface conductance. One can see that the hodographs of the primary field are transformed to ellipses. As an example the results calculated for
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the line number 37 which corresponds to the most interesting structures are presented in Fig. 3. ~ A 1 A * 1 igure a escn es e og-con uc ance a ong this line and the calculated major (log A) and minor (!og B) axes of the te!luric ellipse. One can conclude from Fig. 3a that the major axis is more distorted in the resistive areas (e.g., in the moun— .
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tains. t e co umns from o an rom 0 29), while the minor axis is more distorted on the surrounding conducting seas and the low—resistive areas of the Karnchatka (e.g., the columns from 25 to 27). One can see in Fig. 3c that the parallel component (EY2) of the electric field is less distorted by the near-surface inhomogeneities (similarly to the E-polarization case for a two-dimensional model, see e.g., Berdichevskyand Dimitriev, .
.
1976). The ratio of the major to minor axis varies along the line number 37 from 2 to 14 (Fig. 3b), while for the whole Kamchatka region the varying range is from 1 to 60. For the mountains at the Kamchatka Peninsula this ratio has the value of 8. —
228 KAMCHATKA 25
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One can see in Fig. 3b that the main direction of the tellunc field in the sea areas is nearly parallel to the coast line, i.e., to the y-axis. Note that the angle PHI is the angle between the major axis A and the x-axis and it can vary from —90° to +90°.The peninsula (i.e., the columns 18—30) is characterized in a general sense by the values of the angle PHI being 00. The columns 25—27 which correspond to the low-resistive area of the peninsula show increasing values of PHI. This general feature of polarization can be seen also in Fig. 2. Another example from the profiles across the Kamchatka Peninsula is presented in Fig. 4. This profile is the line number 52 from the northern part of the region. General features along this profile are fairly similar to the ones along the line number 37. The most significant difference is con-
2S
COLUMN Xl ~
Fig. 3. The calculated results along the line number 37 (see Fig. 2) for the Kamchatka area. (a) semi-major (A) and semi-minor (B) axes on the background of the conductance (S) (b) ratio A/B and direction (PHI) of major axis measured from x-axis (—90°
20
PROFILE: LINE 52
COLUMN III
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COLUMN III
Fig. 4. The calculated results along the line number 52. See Fig. 3.
nected with the less-conducting area on the column number 28. Its character on the line 37 is clearly two-dimensional, while on the line 52 it is a three-dimensional isometric area. Owing to this different geometric character the behaviours of the angle PHI and ratio A/B differ along the lines 37 and 52 on and around the column number 28.
3. Comparison For comparison one experimental “profile” was used (see Fig. 1). The western end of the profile is near the Ohotsk Sea coast of the Kamchatka Peninsula. The profile crosses the Western Mountains (I) and the Central Valley (II) and ends in the Eastern Mountains (III) (Fig. 5a). The conductance of the sedimentary cover along this profile varies from 25 up to 1000 S (Fig. 5a). In the comparison four calculated parameters, i.e., the semi-major axis (A) of the telluric ellipse (Fig. Sb), the semi-minor axis (B) (Fig. 5c), the ratio A/B (Fig. Sd) and the direction (PHI) of the —
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major axis of the telluric ellipse (Fig. Se) were used. From the experimental data the maximum and minimum values of the apparent resistivity (p~~)( and p~m)and the direction (q) of the major axis of the impedance polar diagram were used. As an example apparent resistivity curves are presented for the sites 1 and 2 of Fig. 1 (Fig. 6). The site number 1 is situated at the western foot of the Western Mountains. The apparent resistivity cornponent perpendicular to the general direction of
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Fig. 6. Examples of the expenmental apparent resistivity curves at the points (a) 1 and (b) 2 (see Fig. 1). Curves are approximately parallel ((I) to the general direction of the Kamchatka Peninsula and perpendicular ( 1) to it.
230
the Peninsula is 40 times greater than the one parallel (Fig. 6a). In contrast station number 2 is situated in a conductive area (the Central Valley). In this case the component of the apparent resistivity which is parallel to the axis of the valley is 4 times greater than the transversal component (Fig. 6b). In this paper we have used, for comparison, the longest available periods (— 1 h) of the experimental MT data. We assume that the change of the horizontal magnetic field intensity has a small contribution to the long period apparent resistivity. In this case the following relations are approximately valid —
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4. Conclusions A comparison between the results of the low frequency thin sheet modelling and experimental MT data for the Kamchatka area is presented. It suggests that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. These distortions are produced by the three dimensional conductivity inhomogeneities of the sedimentary cover. The less distorted component of the telluric field is parallel to the general direction of the Kamchatka Peninsula. This component is suggested to be used for deep investigations as was done by Moroz and Pospeev (1976).
Acknowledgements
(/prnax/prnin
and PHI p. One can see in Fig. Sb that the high resistive mountains yield high values of both theoretical and experimental major axis A. Similarly the minor axis B is compared in Fig. Sc. The ratio A/B (Fig. Sd) shows the best agreement between the calculated and experimental data. This ratio is 2 in the conductive areas and reaches the value of 6 for the high resistive parts of the profile. In these areas (mainly mountains) the telluric field is polarized approximately across the mountains (Fig. Se). In the Central Valley the telluric field changes its direction tending to be parallel to the valley. At the western part of the profile some small scale effects distort the data but the general (smoothed) behaviour of the calculated and experimental directions are practically the same. Thus there is a fairly good agreement between the calculated and experimental structure of the MT field. It means that inhomogeneities of the conductance of the sedimentary cover have the most important meaning in the MT field structure of the Kamchatka area. Since our calculations took into account only galvanic effects, it seems that the role of the induction anomalies is only secondary in this area, —
The authors thank Dr. Yu.F. Moroz for the field data and Dr. I.L. Osipova for help in the preparation of this paper.
References
—
—
Berdichevsky, M.N. and Dmitriev, V.1., 1976. Basic principles of interpretation of magnetotelluric sounding curves. In: A. Adam (Editor), Geoelectric and Geothermal Studies. Akad. Kiadó, Budapest, pp. 165—221. Debabov, AS., 1980. On the computer modelling of the EM fields in heterogeneous media. DokI. Acad. Sci. USSR, 250: 326—331. Dosso, H.W., 1966. A plane-wave analogue model for studying electromagnetic variations. Can. J. Phys., 44: 67—80. Dosso, H.W., 1973. A review of analogue model studies of the coast effect. Phys. Earth Planet. Inter., 7: 294—302. Fainberg, E.B. and Zinger, B.Sh., 1981. Electromagnetic induction in a spherical model of the Earth with a real distribudon of near-surface conductivity. Phys. Earth Planet. Inter., 25: 52—56. Haak, V., 1978. Interpretations-verfahren für die Magnetotellurik. Bayer. Acad. Wiss., Mtlnchen, 105 pp. Hermance, J.F., 1982. The asymptotic response of three-dimensional basin offsets to magnetotelluric fields at long pen. ods: the effects of Current channeling. Geophysics, 47: 1562—1573. Moroz, Yu.F. and Pospeev, V.1., 1976. Deep magnetotelluric surveys in Kamchatka. In: A. Adkm (Editor), Geoelectnc and Geothermal Studies. Akad. Kiadô, Budapest, pp. 708—711.
231 Price, A.T., 1949. The induction of electric currents in non-uniform thin sheets and shells. Q.J. Mech. AppI. Math., 2: 283—310. Vasseur, G. and Weidelt, P., 1977. Bimodal electromagnetic induction in non-uniform thin sheets with an application to the northern Pyrenian induction anomaly. Geophys. JR. Astron. Soc., 51: 669—690. Yegorov, IV., Chernyak, EL., Palshin, NA., Demidova, T.A. and Kaikkonen, P., 1983a. Numerical thin sheet modeling of the telluric field distortions by the hybrid technique. II. Theoretical background, an example, the computer pro-
gram, a test run and computer listings. In: SE. Hjelt and L.L. Vanyan (Editors), The Development of the Deep Geoelectric Model of the Baltic Shield, Part 1. Numerical Methods. Dept. of Geophysics, Univ. of Oulu, Rep. no. 7, pp. 39—78. Yegorov, I.V., Chernyak, EL., Palshin, NA., Demidova, T.A. and Kaikkonen, P., 1983b. Numerical thin sheet modelling of the telluric field distortions by the hybrid technique. I. Theory and an example for the Baltic Shield. Phys. Earth Planet. Inter., 33: 56—63.
Physics of the Earth and Planetary Interiors, 34 (1984) 226—231
Elsevier Science Publishers By., Amsterdam
—
Printed in The Netherlands
Letter Section The comparison of the experimental MT data with the results of numerical modelling for the Kamchatka Peninsula P. Kaikkonen 1, L.L. Vanyan2 B.A. Okulessky
2
and A.M. Poray-Koshitz 2
Department of Geophysics, University of Oulu, SF- 90570 Oulu 57 (Finland) 2 pp• Shirshov Institute of Oceanology, Moscow 117218 (U.S.S.R.)
(Received January 4, 1984; accepted March 20, 1984)
Kaikkonen, P., Vanyan, L.L., Okulessky, B.A. and Poray-Koshitz, A.M., 1984. The comparison of the experimental MT data with the results of numerical modelling for the Kamchatka Peninsula. Phys. Earth Planet. Inter., 34: 226—231. The experimental MT data and theoretical data calculated by the thin sheet technique for a low-frequency telluric field are analysed and compared for the Kamchatka Peninsula. The comparison shows a good agreement between the experimental and theoretical data and results in a conclusion that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. The component of the telluric field which is parallel to the general direction of the Kamchatka Peninsula is less distorted and should be used for the estimation of deep geoelectric structures.
1. Introduction
2. The structure of the MT field according to numerical modelling
Inhomogeneities of the near-surface part of the Earth’s crust can distort the deep geoelectrical information. This distortion effect is especially strong, if the conductivity contrasts are large. At the Kamchatka Peninsula and the surrounding seas conductances of the sediments and water layer vary from 25 to 25000 S. Numerical modelling was used to investigate distortions owing to this greatly varying near-surface conductance. For two-dimensional structures distortion effects were analysed in detail by Berdichevsky and Dmitriev (1976). For three-dimensional structures there are many papers analysing these distortions mainly by means of scale modelling (Dosso, 1966; 1973) or of the thin sheet technique (e.g., Vasseur and Weidelt, 1977; Haak, 1978; Debabov, 1980; Fainberg and Zinger, 1981; Hermance, 1982). As we are interested in the deep geoelectrical structure the low-frequency solution of the Price’s equation (Price, 1949) was used. The algorithm and cornputer program are described in detail by Yegorov et a!. (1983a). 0031-9201/84/$03.00
© 1984 Elsevier Science Publishers B.V.
The map of the near-surface conductance (Fig. 1) was constructed using, among other things, the bathymetric data and MT results obtained by Moroz and Pospeev (1976) for the Kamchatka Peninsula. To reduce the computer time and storage in numerical modelling a hybrid technique was used combining the numerical and analytical solutions in an iterative sense (Yegorov et a!., 1983b). This calculation technique requires the constant conductance values on the boundaries of the mesh. Therefore at the most northern part of the peninsula the conductance values were taken as equal to that of the surrounding seas (see Fig. 1). This means that the results calculated at the uppermost part of the mesh cannot be used for comparison with the experimental data. A circularly polarized, homogeneous field was taken as a primary field. Figure 2 presents calculated telluric ellipses on the background of the isolines of the logarithm of
227 70
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Fig. 1. The scheme of the conductance (S) for the Kamchatka Peninsula and the surrounding seas with the symbols used. The experimental profile used for the comparison is also shown. Note: the scale is not similar in both directions. S-symbols in 5: A=25,B=75,C=150,D=350,E=750,F=1500,G=3500, I = 7500, K 15000, L = 25000.
the near-surface conductance. One can see that the hodographs of the primary field are transformed to ellipses. As an example the results calculated for
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Fig. 2. The tellunc ellipses on a background of the isohnes of the log S for the Kamchatka area.
the line number 37 which corresponds to the most interesting structures are presented in Fig. 3. ~ A 1 A * 1 igure a escn es e og-con uc ance a ong this line and the calculated major (log A) and minor (!og B) axes of the te!luric ellipse. One can conclude from Fig. 3a that the major axis is more distorted in the resistive areas (e.g., in the moun— .
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.
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tains. t e co umns from o an rom 0 29), while the minor axis is more distorted on the surrounding conducting seas and the low—resistive areas of the Karnchatka (e.g., the columns from 25 to 27). One can see in Fig. 3c that the parallel component (EY2) of the electric field is less distorted by the near-surface inhomogeneities (similarly to the E-polarization case for a two-dimensional model, see e.g., Berdichevskyand Dimitriev, .
.
1976). The ratio of the major to minor axis varies along the line number 37 from 2 to 14 (Fig. 3b), while for the whole Kamchatka region the varying range is from 1 to 60. For the mountains at the Kamchatka Peninsula this ratio has the value of 8. —
228 KAMCHATKA 25
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31
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One can see in Fig. 3b that the main direction of the tellunc field in the sea areas is nearly parallel to the coast line, i.e., to the y-axis. Note that the angle PHI is the angle between the major axis A and the x-axis and it can vary from —90° to +90°.The peninsula (i.e., the columns 18—30) is characterized in a general sense by the values of the angle PHI being 00. The columns 25—27 which correspond to the low-resistive area of the peninsula show increasing values of PHI. This general feature of polarization can be seen also in Fig. 2. Another example from the profiles across the Kamchatka Peninsula is presented in Fig. 4. This profile is the line number 52 from the northern part of the region. General features along this profile are fairly similar to the ones along the line number 37. The most significant difference is con-
2S
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Fig. 3. The calculated results along the line number 37 (see Fig. 2) for the Kamchatka area. (a) semi-major (A) and semi-minor (B) axes on the background of the conductance (S) (b) ratio A/B and direction (PHI) of major axis measured from x-axis (—90°
20
PROFILE: LINE 52
COLUMN III
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Fig. 4. The calculated results along the line number 52. See Fig. 3.
nected with the less-conducting area on the column number 28. Its character on the line 37 is clearly two-dimensional, while on the line 52 it is a three-dimensional isometric area. Owing to this different geometric character the behaviours of the angle PHI and ratio A/B differ along the lines 37 and 52 on and around the column number 28.
3. Comparison For comparison one experimental “profile” was used (see Fig. 1). The western end of the profile is near the Ohotsk Sea coast of the Kamchatka Peninsula. The profile crosses the Western Mountains (I) and the Central Valley (II) and ends in the Eastern Mountains (III) (Fig. 5a). The conductance of the sedimentary cover along this profile varies from 25 up to 1000 S (Fig. 5a). In the comparison four calculated parameters, i.e., the semi-major axis (A) of the telluric ellipse (Fig. Sb), the semi-minor axis (B) (Fig. 5c), the ratio A/B (Fig. Sd) and the direction (PHI) of the —
229 ~
(ogS4~ 3
J~[c~n~
..‘
100
,-‘
A
____
I
I’
I
I
-
I
B~
I
-- -~
~I
I
-
~
d
A/B::
//
,~
2
io PHl8O~
e
ioo
~
/
4~
1516
—
‘iS!’
2.—4
26
~ioo
2
__
28
Fig. 5. Comparison between the experimental (solid line) and theoretical (dashed line) data along the profile presented in Fig. 1. (a) The conductance profile across the regions I, II and III (see the text), (b) The major axis of the telluric ellipse, (c) The minor axis of the telluric ellipse, (d) The ratio A /B, (e) The direction of the major axis of the telluric ellipse.
major axis of the telluric ellipse (Fig. Se) were used. From the experimental data the maximum and minimum values of the apparent resistivity (p~~)( and p~m)and the direction (q) of the major axis of the impedance polar diagram were used. As an example apparent resistivity curves are presented for the sites 1 and 2 of Fig. 1 (Fig. 6). The site number 1 is situated at the western foot of the Western Mountains. The apparent resistivity cornponent perpendicular to the general direction of
_______
io
—
—
/ I
~
-
___________
___________
1
10
2
f9’ .
.
.
.
[~½]
Fig. 6. Examples of the expenmental apparent resistivity curves at the points (a) 1 and (b) 2 (see Fig. 1). Curves are approximately parallel ((I) to the general direction of the Kamchatka Peninsula and perpendicular ( 1) to it.
230
the Peninsula is 40 times greater than the one parallel (Fig. 6a). In contrast station number 2 is situated in a conductive area (the Central Valley). In this case the component of the apparent resistivity which is parallel to the axis of the valley is 4 times greater than the transversal component (Fig. 6b). In this paper we have used, for comparison, the longest available periods (— 1 h) of the experimental MT data. We assume that the change of the horizontal magnetic field intensity has a small contribution to the long period apparent resistivity. In this case the following relations are approximately valid —
—
A
—
B
—
‘i/prna3/p~~~~ (ref) ~p~/p~°(ref)
A/B —
4. Conclusions A comparison between the results of the low frequency thin sheet modelling and experimental MT data for the Kamchatka area is presented. It suggests that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. These distortions are produced by the three dimensional conductivity inhomogeneities of the sedimentary cover. The less distorted component of the telluric field is parallel to the general direction of the Kamchatka Peninsula. This component is suggested to be used for deep investigations as was done by Moroz and Pospeev (1976).
Acknowledgements
(/prnax/prnin
and PHI p. One can see in Fig. Sb that the high resistive mountains yield high values of both theoretical and experimental major axis A. Similarly the minor axis B is compared in Fig. Sc. The ratio A/B (Fig. Sd) shows the best agreement between the calculated and experimental data. This ratio is 2 in the conductive areas and reaches the value of 6 for the high resistive parts of the profile. In these areas (mainly mountains) the telluric field is polarized approximately across the mountains (Fig. Se). In the Central Valley the telluric field changes its direction tending to be parallel to the valley. At the western part of the profile some small scale effects distort the data but the general (smoothed) behaviour of the calculated and experimental directions are practically the same. Thus there is a fairly good agreement between the calculated and experimental structure of the MT field. It means that inhomogeneities of the conductance of the sedimentary cover have the most important meaning in the MT field structure of the Kamchatka area. Since our calculations took into account only galvanic effects, it seems that the role of the induction anomalies is only secondary in this area, —
The authors thank Dr. Yu.F. Moroz for the field data and Dr. I.L. Osipova for help in the preparation of this paper.
References
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