Deep DC soundings in southwestern Finland using the Fenno-Skan HVDC Link as a source

!

¢, J'" . I I

ELSEVIER Physics of the Earth and Planetary Interiors 94 (1996) 275-290

PHYSICS O F T H E EARTH

AND PLANETARY INTERIORS

Deep DC soundings in southwestern Finland using the Fenno-Skan HVDC Link as a source

P. Kaikkonen a, *, T. Pernu a,1, J. Tiikkainen a, A.A. Nozdrina b, N.A. Palshin h, L.L. Vanyan h, I.V. Yegorov b

a Department of Geophysics, University of Oulu, FIN-90570 Oulu, Finland b Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow 117218, Russia

Received 4 January 1995; revision accepted 3 August 1995

Abstract

The Fenno-Skan high voltage direct current (HVDC) power line links Finland and Sweden across the Bothnian Sea. A 180-kin long cable has been installed at the sea bottom. A return current up to 1280 A flows through water and sea-bottom rocks. This source gives a unique possibility for large-scale DC sounding. First test measurements were done by the Department of Geophysics, University of Oulu in May, 1991. In June, 1992 a bilateral experiment was carried out by Finnish and Russian geophysicists. On the base of 42 sites measured during two field seasons apparent resistivity values were calculated. For the interpretation of the measured data a thin-sheet approximation was used. The model consisted of two thin sheets. The uppermost sheet had a conductance S due to the sea water and sediments according to a priori information, while the second one had a depth-integrated resistivity R due to the Earth's upper crust. A good conductor was located beneath. Comparisons of the experimental and theoretical apparent resistivities show that the R value for the Earth's upper crust in southwestern Finland is about 3 × 10 s ~ m 2, which is in good agreement with the value calculated from the magnetotelluric (MT) data in the Proterozoic Central Finland Granitoid Complex. However, it is more than a factor of 10 lower than the value calculated similarly from the MT data in the Kuhmo region in the Archaean part of the Fennoscandian Shield in eastern Finland and the reported value based on the magnetohydrodynamic (MHD) data in the Kola Peninsula. The results do not reveal any distinct crustal conductors within the research area. This experimental indicates that deep geoelectrical resistivity studies can be carried out successfully at rather large distances, at least 50 km, in the surroundings of a high voltage, DC power link.

1. I n t r o d u c t i o n

The F e n n o - S k a n Link (FSL) is an electrical power transmission line which opera tes with high

* Corresponding author. i Present address: Geological Survey of Finland, L~hteentie

2, FIN-96101 Rovaniemi, Finland.

voltage direct current ( H V D C ) via a monopo la r submarine cable, about 180 km in length, and via the g round in the Bothnian Sea approximately be tween Rauma , Finland and G~ivle, Sweden (Fig. 1). This cable, which lies on the bo t tom of the Bothnian Sea, is ra ted at 500 M W and it is the highest power and longest submarine H V D C cable in the world.

0031-9201/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0031-9201(95)03099-9

276 P. Kaikkonen et al. / Physics of the Earth and Planetary Interiors 94 (1996) 275-290

0 150 I = I I

Sweden Finland

/

I )

Bothnian Sea

CFGC

uma

Gi ~ 6 0 O N

20°E ckholm Stockh '¢~ ES nia i

,/

Fig. 1. Location of the Fcnno-Skan HVDC Link (FSL). The Central Finland Granitoid Complex (CFGC) and the Kuhmo region (KR) are also shown schematically in the map. RA stands for the research area of this paper.

The link was designed and built in 1987-1989 by the Swedish State Power Board (SSPB, Swe- den) and Imatran Voima OY (IVO, Finland). Interconnected with the AC network in Finland and Sweden, this link transfers electric power between Sweden and Finland in both directions. The link allows the sharing of current load in the main networks of Sweden and Finland, and as a whole reduces energy transfer costs and losses, as much as 40 MW during peak demand conditions (IVO International Ltd, 1986; Asea Brown Bover ABB Power Systems, 1990).

One of the system's converter stations is located in Rauma, Finland, and the other one is in Dannebo, south of G~ivle in Sweden. Via the overhead lines to the shore of the Bothnian Sea, these stations have been connected with well-in- sulated submarine cable and with the grounded metal electrodes in the sea near the shore, a titanium anode in Dannebo in Sweden, and a copper cathode in Pyh~iranta, south of Rauma in

Finland. The current in the earth through these groundings creates a relatively strong electric field in a wide area around the entire circuit. Accord- ing to preliminary measurements, the field can be recorded up to distances of several tens of kilometres from the cathode (Kuussaari, 1989).

The converter stations are remotely controlled in the regional dispatch centres, one for southwestern Finland and one for eastcentral Sweden. In the dispatch centres the operators have access to most of the information in the converter stations and they can perform control operations, such as a start or a stop of the power transmission, setting of the current direction, change of the power level or the control mode. In the normal operational mode, the link is in the constant power control. The desired power level is achieved by setting the DC voltage at its maximum value and then controlling the current.

The voltage in the link is 400 kV and thus the nominal value of a current is 1250 A, corresponding to the power of 500 MW. In practise, the maximum current is a little higher, about 1280 A. When the amount of power transferred in the link is changed, the current changes accordingly. Normally the power in the link is cut off during the night-time, in the day-time the power transfer is determined by market demand. The power is normally changed at the constant rate of 50 MW min-1 (Pernu and Tiikkainen, 1992).

The electric field weakens up to distances of some tens of kilometres from the electrode like the electric field of a single pole current source, approximately inversely proportional to the square of the distance from the source (i.e. like l / r2) . At greater distances, a geometrical attenuation of this field is more complex, as influences of both electrodes are involved. Each grounded electrode can, therefore, be applied as the controlled and very powerful sources for a deep resistivity sounding, especially when the electric field caused by a current change in the link is measured.

Controlled source electrical methods have been used for deep research earlier, e.g. by Van Zijl (1969, 1977), Van Zijl et al. (1970), Van Zijl and Joubert (1975), Lienert and Bennett (1977), Sternberg (1979), Connerney et al. (1980) and

P. Kaikkonen et al. / Physics of the Earth and Planetary Interiors 94 (1996) 275-290 277

Duncan et al. (1980). Ward (1983) in his review dealt with the topic.

The aims of this research were to study the possibilities of using the Fenno-Skan Link as a source for deep geoelectric soundings in the surroundings of the link, to investigate possible crustal conductors, and finally, to try to estimate

the depth-integrated resistivity R of the upper part of the crust in the research area by combin- ing the experimental data and the results of numerical modelling. The estimated R value is then compared to values obtained from other investi- gations in other parts of the Fennoscandian Shield.

~J

~J

s

' s

' s

%

= = ~ ~ Mica schists and ~ Rapakivi granite 5 4 3 2 I 0 |Okra gneisses

Scale ~ Jotnian sandstone ~ Granodiorite and quartz diorite

• Sounding site with corresponding apparent resistivity value [Qm]

Fig. 2. Location of the cathode in Finland. The measured apparent resistivity values in ~ m stand adjacent to the measuring sites (e). Major bedrock units are presented schematically based on the map by Simonen (1980).

278 P. Kaikkonen et al. /Physics of the Earth and Planetary Interiors 94 (1996) 275-290

2. Electrical conductivity in the surroundings of the Rauma cathode

In the immediate surroundings of the Rauma cathode there is a conductive sea water layer, a few metres in thickness, and Quaternary deposits in the sea bottom. The resistivity of the sea water is mostly between 2 f~m and 5 f~m. The resistivity of the Quaternary deposits in the sea bottom has not been determined but it is assumed to be between 5 f~m and 50 f~m. The resistivity of the bedrock is of the order of 103 f~m or 104 f~m, but especially in fractured zones in the coastal area it can be of the order of 102 f~m or even lower. Resistivity in the coastal area is strongly influenced by the salinity of pore water. In a wide area around the cathode in Rauma, Pyh~iranta and Laitila the bedrock consists mostly of gran- ites and mica schists, which are covered by the thin Quaternary deposits (Fig. 2). Northeast of Rauma there is the Jotnian Satakunta sandstone formation, in which the resistivity is of the order of 102 f~m. According to audio-frequency magnetotelluric (AMT) and DC resistivity soundings

carried out in several separate points in this area, including islands near the coast, resistivities are mostly between 1000 f~m and 10000 f~m to the depths of a few kilometres (IVO, unpublished report, 1986).

3. Experimental data

Field measurements were made during 4 days in May 1991 and 6 days in June 1992. The radial component of an electric field change was measured at a total of 42 sites which are located on five radial but partially overlapping profiles (Fig. 2). In each site only one measurement was taken. The measurement sites are located at distances of 6 km to 50 km from the cathode and the separation between the sites varied between 2 km and 10 km. The measured electric field component was produced by increases or decreases in the supplied power which occurred at the constant rate of 50 MW min -1 while the total power changed from 100 MW to 300 MW. The daily schedule for the power changes was provided by

6 t~

u~

3.5-

3.0-

2.5-

1 .5 -

1.0-

0.5-

0.0-

-0.5

Recording of the electric field. [ Power change in the link was from [ 100 MW to -80 MW (direction [ and amount of power was changed).[ Distance from the cathode: 17.5 km [ a transient from 30 MW

0 20 40 60 80 100 120 140 160 180 Time [s]

Fig. 3. An example of the recording showing the details of the link operations.

200


0 m

-200.~

" i ~ t .... I .... I .... I .... | .... l'' 0 50 100 150 200 250

Time [s] Fig. 4. An example of the electric field recording when high S/N. Measuring site is marked by • in Fig. 2.

2

ro -2

-4-

' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' 100 200 300 400 500 600

Time [s] Fig. 5. An example of the electric field recording when low S/N. Measuring site is marked by • in Fig. 2.

I000-

100-

I0-

(a) Profile WE

10000 (b)

l e O 0 ,

100

lO

Profile WE

J . . . . . I . . . . I . . . . I . . . . I . . . . I . . . .

I0 20 30 40 50 60 0 I0 20 30 40 50 60

Distance [km] Distance [km]

Fig. 6. (a) The measured electric field along the profile WE as a function of the distance from the cathode. The profile WE includes 10 measuring sites to the east of the cathode (see Fig. 2). (b) The measured apparent resistivity along the profile WE as a function of the distance from the cathode. The profile WE includes 10 measuring sites to the cast of the cathode (see Fig. 2).


t he I V O cen t r a l con t ro l s ta t ion . T h e n u m b e r

a d j a c e n t to e a c h p o i n t in Fig. 2 r e f e r s to t he

a p p a r e n t res is t iv i ty v a l u e s c a l c u l a t e d f r o m Eq . (1)

f r o m the m e a s u r e d e l ec t r i c f ie ld , as e x p l a i n e d in

t h e nex t sec t ion .

T h e p o t e n t i a l d i f f e r e n c e s w e r e m e a s u r e d rad i -

100000-.

E O 10000-

I 'E

1000-

100-

,,....:':

I

i

10 a)

bl~a~

• \ \ , , .. t t ~",1 i i~'~. 1 A'~II,

- . |

t t ~l "

i t . t. i l l t i l t

100

Range, km

R=108 T=0°C

R=108 T=15°C

= " " = R=108 10

- - - R=3.108 T_.=0°C

- - - R=3.108 T=15°C

" ' " = 9=3"100 1D

. . . . . R=10 9 T=0°C

. . . . . R=10 9 T=15°C

. . . . . R=109 1D

O

.6

E

i

10000

1000-

100-

- - R=10 s Az=60 °

R=10 e Az=90 °

R=10 s Az=120 °

- - - R=3.10 s Az=60 °

- - - R=3.10 e Az=90 °

- - - R=3.10 s Az_-120 °

. . . . . R=10 o Az=60 °

. . . . . R=10 o Az=90 °

. . . . . R=10 ~ Az=120 °

10 100

b ) Rar~a, km

Fig. 7. (a) Theoretical apparent resistivity as a function of the distance from the cathode and of the varying temperature of the sea water. The 1D curves depict the three-layer models without the sea. The R values (in f lm 2) are varied. (b) Theoretical apparent resistivity as a function of the distance from the cathode and of the azimuth of the measuring profile. The azimuth is clockwise from the north. The R values (in rim 2) are varied.


ally in one direction only with a dipole of 100 m in length, using non-polarizing Cu-CuSO4-electrodes and shielded cables. The measuring dipole was pointing towards the cathode. Due to the lack of a digital recording instrument with a high dynamic range, the signals were recorded both on paper recorder and instrumentation tape recorder in analogue form after amplification and filtering. In most cases the recording bandwidth was from 0 Hz to 10 Hz, at two sites it was from 0 Hz to 0.5 Hz due to high natural variations of the telluric field. In fact the lower bandwidth is sufficient for this kind of recording, but we were interested in seeing also the noise level in higher frequencies, and also in recording possible transient effects (e.g. residual load of about 30 MW in the link is always switched on or off instantaneously when

starting from or returning to zero power level) (Fig. 3).

Linear change of the electric field generated by a current change in the link could be detected from the recordings by visual inspection. In order to determine the DC level shift in some disturbed recordings, an averaging was performed for the data obtained both before and after a current change. In cases where the signal could not be identified clearly, a maximal value for actual signal could be estimated, however. When the signal-to-noise ratio (S/N) was high, i.e. when the generated electric field change was large compared with the telluric field variations, the deter- mination of the voltage amplitude was easy (Fig. 4). In an opposite case (Fig. 5), especially when telluric field variations occurred in the periods

0

r, IC3 Z O

--3

U3 03

CD 0"3

tf3 (30

C3 CO

LO

C3

L f3 (33

C3 CO

80 85 90

I I 0.0 IO.O KM

SCRLE

95 100 105 110 115 120 COLUMN CX)

• ABOVE 0.4 I-o.e TO 0.4 I-o.8 To-0.6 ~--1.0 TO MO.e [ ] - I . 0 & BELOW

125

Fig. 8. A part of the larger (157 × 140) conductance map used in modelling. The measured and theoretical results in this paper are presented for that area. The cathode is located in the node (78, 78). Isolines are presented in log S. A thick broken line depicts a coarse coastline.

from 10 s to 100 s, the amplitude could be determined correctly only when the time interval of current change was known well. Stacking of several measurements in the same site would help in detecting weak signals from natural telluric noise, but due to limited time and only a few available current changes per day, it was not done. The main goal was to study the behaviour of the electric field in various distances and directions from the cathode and to get the general idea of possibilities for deep geoelectric sounding using the link as a power source.

ED

bD Z

-J

4. Analysis of the field data

The electric field values were normalized to a power change of 100 MW in the link, corresponding to the current of 250 A. An apparent resistivity Pa at each site was calculated according to the known formula

2~-r2UMN

Pa IaMN , (1)

where the geometrical factor k = 2zrr2/aMrq is appropriate to the three-electrode gradient array

~T

282 P. Kaikkonen et aL / Physics of the Earth and Planetary Interiors 94 (1996) 275-290

80 85 90 95 I00 105 I I0 115 120 125 t I COLUMN (X] 0.0 I0.0 KM

5CRLE

ABOVE: 3.5 3.0 1'0 3.5 2.5 TO 3.0 2.0 TO 2.5 1.5 TO 2.0 1.0 TO 1.5 0.5 TO 1.0 o.o TO 0.5 0.0 g @CLOW

Fig. 9. Isolines of theoretical apparent resistivity (log pa ) with the inhomogeneous land and sea conductance (see Fig. 8) and with the transverse resistance R ffi 3 × 10 s r i m 2 for the bipole source. The electrodes are located in the node (29, 42), which is outside of this figure in Sweden and in the node (78, 78) in Finland. Compare to Fig. 11(b) with the results for the pole-dipole system. A thick line depicts a coarse coastline.

5. Numerical modelling (pole-dipole) and UMN ---- measured potential dif- ference, r = distance from the cathode, aMN = electrode separation and I = current change.

In general, the measured data, apparent resistivity and electric field, (Fig. 2, Figs. 6(a) and 6(b)) suggest a rather strong influence of small- scale near-surface anomalies, which tend to be superimposed on the regional geoelectric response. Fig. 6(a) shows that the electric field falls off s teeper up to distances of 20 km from the cathode than at greater distances. The more conducting area between 20 km and 30 km from the cathode (Figs. 6(a) and 6(b)) could be seen also in the shallow altitude (40 m) aero-EM results from the region, i.e. it has clearly a near-surface origin (Peltoniemi et al., 1992).

C3

b . J O Z O

.-3

LO O3

Numerical thin-sheet modelling of electric field due to a dipole source (actually a bipole) was carried out. Sea water and sediments were ap- proximated with a thin sheet with the conductance S and the resistive upper and middle crust with a thin sheet with the depth-integrated resistivity R, respectively. An underlying conductor in this three-layer model corresponds to the lower crustal conductive layer.

A numerical modelling technique was developed for DC soundings with an MHD-genera tor grounded in the Barents Sea (Vanyan et al., 1991). Recent modification of this approach allows the length of the source bipole to be taken

!


C3 03

LI3 CO

0 00

1./3 ix.

0 • RBOVE 4 .0 ~" • 3.5 To 4.0 tr~ • 3.0 TO 3.5 co • 2.5 TO 3.0

• 2.0 TO 2.5 0 • l .S TO 2.0 co • I.o TO 1.5

[] o.s ro 1.o • [ ] 0 . 5 g BELOW

80 85 90 95 I00 i05 II0 ]15 120 125 J i COLUMN CXI 0 . 0 10 .0 KM

SCRLE Fig. 10. Isolines of theoretical apparent resistivity (log Pa). Land area is homogeneous with conductance of 0.1 S. The transverse resistance is R = 3 × 10 s l )m 2. Conductance of the sea is varying. A thick line depicts a coarse coastline.


into account. The apparent resistivity is calculated according to:

2T/'F 2

P a = I VL'x "k- E ; ' (2)

where r is the distance between the electrode and the measurement point, I is the current and E x and Ey are the electric field components.

Modelling was done at two stages. First the mesh size was 105 × 101 nodes in the x- and y-directions, respectively, and covered the area from 55°N to 66°N and 17°E to 30°E with a node spacing of 1 km near the electrodes and of up to 100 km with increasing distance from the electrode.

Bathymetry, temperature and salinity data for the Gulf of Bothnia were used for constructing

the conductance map (Levitus, 1982). Depth- averaged water temperature and salinity for dif- ferent seasons were calculated using hydrological data. The average summer temperature varies from 6°C to 14°C and the averaged winter temperature from 1.4°C to 3.9°C. Salinity does not practically depend on temperature but depends on the location of a measuring site and varies between 0.5% and 0.7%.

Conductance maps for the Gulf of Bothnia for temperature values of 0°C (winter) and 15°C (summer) were constructed. Conductance for the land area was taken as a constant value of 0.1 S. Numerical modelling was carried out with the transverse resistance values of R equal to 108, 3 × 108 and 109 ~ m 2.

The results are shown in Fig. 7 as the profiles of the apparent resistivity vs. distance from the

o (a)

U Z

__1

80 85 90 95 100 I05 110 115 120 ~ _ _ ~ COLUMN (X ) o.o 10.o KM

SCRLC

I ABOVE q.O 3 . 5 TO 4 .0 3 . 0 TO 3 . 5 2.5 TO 3.0 2.0 TO 2.5 1.5 TO 2.0 1.0 r0 l.s

I o.s To l.o [ ] 0 . 5 & BELOW

125

Fig. 11. Isolines of theoretical apparent resistivity (log Pa) for the inhomogeneous land and sea conductance (see Fig. 8). A thick line depicts a coarse coastline. Transverse resistance R is varying as follows: (a) 10 a l%m 2, (b) 3 × 10 s t im 2, (c) 6 x 108 t im 2 and (d) 10 9 ~Qm 2.

P. Kaikkonen et al. /Physics of the Earth and Planetary Interiors 94 (1996) 275-290 285

electrode. Fig. 7(a) shows the influence of the Gulf of Bothnia on the apparent resistivity curves and their dependence on the temperature. The apparent resistivity curves have a minimum at a range from 6 km to 8 km due to distortion. The dependence on the tempera ture is not significant and will not be taken into account in the further calculations. The 1D curves are just for the corresponding homogeneous three-layer models without the sea. Fig. 7(b) shows that there is no dependence on the azimuth of the measuring profile in the range from 60 ° to 120 ° clockwise from the north.

The second mesh was denser than the first one. The minimum node spacing is the same, 1 km, but the coverage in area was only 477 × 380 km 2. The mesh size was 157 x 140 nodes (Kaik- konen, 1993). Fig. 8 shows a part of that mesh

with the conductance distribution for the uppermost thin sheet which is now inhomogeneous not only for the sea but also for +he land area. The area covered by the mesh in Fig. 8 is showed approximately by R A in Fig. 1. Determinat ion of the conductance values S (S = o" x h) was done using 200 m as the thickness (h) for the thin sheet and t h e electrical conductivities (o~) mentioned above in Section 2. Transverse resistance R was homogeneous for every model at a time, and was changed from 107 ~ m 2 to 10 l° ~ m 2. However, the results for these limiting values are not presented here as they differed so much from the measured apparent resistivities.

First we checked how justified it is to use the pole-d ipole system for calculating apparent resistivities from the measured data. The theoretical apparent resistivity distribution taking into ac-

0 O~

CO

0 CO

LF)

0 D-.

IF) ~0

0 M3

I

0

( . , 3 0 Z O

-2

80 05 90 95 100 105 110 115 120 I COLUMN IX)

0,0 10.0 KM

SCALE Fig. 11 (continued).

ABOVE 4.0

3.5 TO 4.O :3.0 TO 3.5

[] 2 5 ,o , .o m 2 . o To 2 . 5 [] ,.s To 2.o

Imm o.5 TO 1.0 [ ] 0.5 : BELOW

125


count the whole bipole source (the electrodes in the nodes (29, 42) in Sweden and (78, 78) in Finland) is presented in Fig. 9. The results in the surroundings of the cathode differ from the pole-dipole calculated results (see Fig. l l(b)), but at greater distances, further than about 20 km from the cathode, i.e. the x-axis node value greater than 100, they are very much the same. So all further calculations were done and are presented for the pole-d ipole system.

The next model was run to see only the effects of the sea and the complicated shape of the coastline on the response, i.e. the land conductance was homogeneous with 0.1 S. In the results presented here (Fig. 10) R was equal to 3 x 108 r i m 2. As expected the behaviour of the response changes rapidly with location close to the coast. The inland behaviour is smooth and more or less

normal attenuation can be seen with increasing distance from the cathode.

Comparison of the measured apparent resistivity values (in Fig. 2) with the theoretical ones (Fig. 11) shows that R = 108 ~ m 2 mainly gives Pa values too small with increasing distance. On the other hand, R = 6 × 108 or 109 f~m 2 give much too large values of Pa" When R = 3 × 108 12m 2 the thin-sheet model used in the modelling gen- erates rather good agreement between the measured and theoretical apparent resistivity values as can be seen in Fig. 12, in which there are some measured values presented together with the theoretical ones. This means that if the S values used for the uppermost layer in our thin-sheet modelling are more or less real, then the depth- integrated upper crustal resistivity is close to R = 3 × 108 ~ m 2.

o (c)

Ld z _.3

80 85 90 95 I00 I05 II0 115 120 l J COLUHN (X) 0.0 I0.0 KN

SCRLE

Fig. 11 ( c o n t i n u e d ) .

125

~ ~VE 4.0 TO 4.0 TO 3.5 TO 3.0 TO 2.5 TO 2.0 TO 1.5 TO 1.0 & BELOW

P. Kaikkonen et aL / Physics of the Earth and Planetary Interiors 94 (1996) 275-290 287

In order to compare the obtained R value to the values from some other regions in the Fennoscandian Shield we used averaged MT data from the Central Finland Granitoid Complex (CFGC) and the Kuhmo region (KR) (see Fig. 1) (Korja, 1990) and calculated the best fitting layered models for the averaged curves presented in Fig. 13. Table 1 shows the obtained models from which it is possible to calculate the R values (R = p x h) for the upper parts of the crust in the CFGC and KR regions. The Proterozoic CFGC crust seems to have a very similar R value (= 2.1 x 108 t im 2) compared to the present research while in the Archaean KR region the depth-integrated resistivity is remarkably larger (R = 5.6 X 10 9 ~ m 2 ) . The R value for the Kola Peninsula has been reported (Kaikkonen et al., 1988, Vanyan et al., 1989) to reach also more than 10

times larger values, even R = 101° ~ m 2, than that obtained now in southwestern Finland.

The good agreement between the field measurements and the results from the rather simple models means that there is no need in the interpretation for any crustal conductors in the research area, e.g. for the continuation of the good crustal conductor discovered by Pajunp~i~i (1987) to run from the Tammisaari region (see Fig. 1), southeast in the coast of the Gulf of Finland, to northwest through the Satakunta rapakivi unit (see Fig. 2), i.e. the area of this research. Kaikko- nen et al. (1993) reported earlier, mainly based on some preliminary MT results but also on the FSL (Fenno-Skan Link) field data and their preliminary numerical modelling, and the interpretation that 'the FSL measurements seem to find that a crustal conducting zone would be located

o (d)

bJ Z

..2

80 85 90

I I 0.0 ]0.0 KN

$CRLE

95 lO0 105 110 115 120 COLUMN (X)

Fig. 11 (continued).

[] ABOVE 4.0 [] a.s To 4.0 [ ] ~.o To 3.s [ ] 2 . s To a.o [] 2.0 TO 2.5 [] ~.5 ro 2.0 I l.o To 1.5 [ ] 0.5 TO ].0 [ ] 0.5 & BELOkl

125


C2

~ 2 C 9 Z C])

LD O3

m [] l l

l

L

L m [] i l l i

i

0 (33

03

0 (30

LF)

0

Lr) ~0 ¸

(Z3

80 8% 90 95 I00 105 I i 0 115 120 125 r ~ COLUMN (X) 0.0 I0.0 KM

5CRLE

I ABOVE 4.0 1 3.5 TO 4.C • 3.0 TO 3.5 • 2.s TO 3.0 • 2.0 TO 2.s 1 I.S TO 2.o

1.0 TO 1.5 [ ] o.s TO ~.0 [ ] 0.5 /~ BELOW

Fig. 12. Some measured apparent resistivity values (log Pa, e.g. 2.4) depicted on the presentation of the isolines of theoretical apparent resistivity (log Pa) with R = 3 × 10 8 f /m 2 (see Fig. 11(b)). A thick line depicts a coarse coastline.

at the distance of about 40 km east of the cathode'. But the more extensive thin-sheet modelling repeated here shows that the FSL data can be

modelled rather well (see e.g. Fig. 12) without any crustal conductors. However, this result based on simple thin-sheet modelling and in places a

Table 1 The best-fitting layered models for the averaged MT curves (see Fig. 13) from the Central Finland Granitoid Complex (CFGC) and the Kuhmo region (KR) (see Fig. I). The program AUTOMOD written by Prof. John Weaver has been used

Layer Best fitting models

CFGC KR

p ( f /m) h (km) R ( f / m 2) p ( f /m) h (km) R ( f /m 2)

1 9500 22.3 2.1 × 108 7440 7.6 2 310 10.9 249000 22.4 3 50 24.0 1910 41.0 4 230 340 93.0 5 2410

5.6 X 10 9

P. Kaikkonen et al. / Physics of the Earth and Planetary Interiors 94 (1996) 275-290 2 8 9

10 5

-o-CFGC

c. ~ 103 " ' " i ~ ..........................

1 0 2 . . . . . . . . I . . . . . . . . I . . . . . . . . i . . . . . . . . I . . . . . . . . I . . . . . . . .

10-2 10"1 10 o 101 10 2 10 3 10 4

Period (s) Fig. 13. The averaged MT curves for the CFGC region (31 soundings) and KR region (13 soundings) (redrawn from Korja, 1990).

rather sparse net of field measurements does not exclude entirely the possible existence of a crustal conducting zone east of the cathode. The use of more developed 3D modelling and more field measurements would perhaps enlighten this ques- tion.

conductors within the area investigated. In sum- mary, this experiment indicates that deep geoelectrical resistivity studies can be carded out successfully at rather large distances, at least 50 km, in the surroundings of a high voltage, DC power link.

6. Conclusions

Comparisons of the experimental data to the results from theoretical thin-sheet modelling show that the depth-integrated resistivity value for the resistive part of the upper crust in southwestern Finland is about 3 × 108 f~m 2. It is approximately the same as that in the Proterozoic Central Fin- land Granitoid Complex but more than a factor of ten smaller than that in the Archaean Kuhmo region in eastern Finland and in the Kola Penin- sula. The results do not reveal any distinct crustal

Acknowledgements

We wish to thank the IVO's staff for the good co-operation during the field work and two anonymous referees for their helpful comments and suggestions. This research is a part of Project 13 between the Academy of Finland and the Russian Academy of Sciences. The Russian au- thors were supported in part by grant MQF000 from the International Science Foundation.

290 P. Kaikkonen et al. /Physics of the Earth and Planetary Interiors 94 (1996) 275-290

R e f e r e n c e s

Asea Brown Boveri (ABB) Power Systems, 1990. Fenno-Skan HVDC Project (information booklet).

Connerney, J.E.P., Nekut, A. and Kuckes, A.F., 1980. Deep crustal electrical conductivity in the Adirondacks. J. Geo- phys. Res., 85: 2603-2614.

Duncan, P.M., Hwang, A., Edwards, R.N., Bailey, R.C. and Garland, G.D., 1980. The development and applications of a wide band electromagnetic sounding system using a pseudo-noise source. Geophysics, 45: 1276-1296.

IVO International Ltd, 1986. Fenno-Skan HVDC link (information booklet).

Kaikkonen, P., 1993. Results from numerical modelling for the measurements using the Fenno-Skan DC link as a source. In: The abstract book of the 19th meeting of the Nordic Association for Applied Geophysics (NOFTIG) Meeting, 25-27 January 1993 Oulu.

Kaikkonen, P., Vanyan, L.L, Demidova, T.A., Yegorov, I.V. and Palshin, N.A., 1988. Numerical modelling of deep dipole-dipole MHD sounding. Phys. Earth Planet. Inter., 50: 266-269.

Kaikkonen, P., Korja, T., Pernu, T. and Tiikkainen, J., 1993. Crustal conductivity variations in southwestern Finland. In: S. Mertanen, A. Laiho and S. Tattari (Editors), XVI Geofysiikan P~iiv~it - Kaukokartoituksen Piiiviit, Espoo 11.-12.5. 1993, pp. 41-47 in Finnish, abstract in English.

Korja, T., 1990. Electrical conductivity of the lithosphere: Magnetotelluric studies in the Fennoscandian Shield, Fin- land. Ph.D. thesis, Department of Geophysics, University of Oulu, Finland, Acta Univ. Oul. A215.

Kuussaari, M., 1989. The measured Earth potential of the Fenno-Skan cathode. Imatran Voima Oy, Internal work- ing report 8911029 (in Finnish).

Levitus, S., 1982. Climatic Atlas of the World Ocean. NOAA Prof. Paper, No. 13, pp. 1-13.

Lienert, B.R. and Bennett, D.J., 1977. High electrical conductivities in the lower crust of the Northwestern Basin and Range: An application of inverse theory to a controlled- source deep-magnetic-sounding experiment. In: J.G. Hea- cock (Editor), The Earth's Crust, AGU Mono., 20: 531- 552.

Pajunp~i~i, K., 1987. Conductivity anomalies in the Baltic Shield in Finland. Geophys. J.R. Astron. S0c., 91: 657-666.

Peltoniemi, M., Korhonen, J. and Hjelt, S.-E. (Compilers), 1992. Electrical conductance of the surficial parts of the crust (0-150 m) as interpreted from airborne survey data. Map 29a. In: P. Alalammi (Editor), The Atlas of Finland, 5th edn, National Board of Survey, Helsinki, Finland, f. 123-126.

Pernu, T. and Tiikkainen, J., 1992. Measurements of the electrical field caused by the grounding cathode of the (high voltage) Fenno-Skan Link near the town Rauma, southwestern Finland. In: P. Kaikkonen (Editor), Proceed- ings of the Jubilee Symposium of the 10 years Finnish- Soviet co-work in geoelectrics, 18-19 December 1991, Oulu, Department of Geophysics, University of Oulu, Re- port No. 18, pp. 67-77.

Simonen, A., 1980. Precambrian in Finland. Geological Sur- vey of Finland, Bull. 304, 58 pp.

Sternberg, B.K., 1979. Electrical resistivity structure of the crust in the southern extension of the Canadian Shield - layered earth models. J. Geophys. Res., 84: 212-228.

Vanyan, L.L., Demidova, T.A., Palshin, N.A., Zhamaletdinov, A.A., Kuksa, Yu.I., Kaikkonen, P. and Korja, T., 1989. Interpretation of deep DC soundings in the Baltic Shield. Phys. Earth Planet. Inter., 54: 149-155.

Vanyan, L.L, Demidova, T.A., Yegorov, I.V., Palshin, N.A. and Medvedev, F.A., 1991. Interpretation of DC-sounding data from the Barents Sea shelf. Phys. Earth Planet. Inter., 69: 4-7.

Van Zijl, J.S.V., 1969. A deep Schlumberger sounding to investigate the electrical structure of the crust and upper mantle in South Africa. Geophysics, 34: 450-462.

Van Zijl, J.S.V., 1977. Electrical studies of the deep crust in various tectonic provinces of Southern Africa. In: J.G. Heacock (Editor), The Earth's Crust, AGU Mono., 20: 470-500.

Van Zijl, J.S.V. and Joubert, S.J., 1975. A crustal geoelectrical model for South African Precambrian granitic terrains based on deep Schlumberger soundings. Geophysics, 40: 657-663.

Van Zijl, J.S.V., Hugo, P.L.V. and de Bellocxl, J.H., 1970. Ultra deep Schlumberger sounding and crustal conductivity structure in South Africa. Geophys. Prosp., 18: 615-634.

Ward, S.H., 1983. Controlled source electrical methods for deep exploration. Geophys. Surv., 6: 137-152.

Documents

Deep DC soundings in southwestern Finland using the Fenno-Skan HVDC Link as a source