Terrestrial heat flow and groundwater circulation in the bedrock in the central Baltic Shield

Tectonophysics,

59

156 (1988) 59-74

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Terrestrial heat flow and groundwater circulation in the bedrock in the central Baltic Shield ILMO T. KUKKONEN Geologrcal Suruey of Finland, SF-02150 Espoo (Finland) (Received

November

26, 1987; revised version accepted

April 6, 1988)

Abstract Kukkonen,

LT., 1988. Terrestrial

Tectonophysics,

Unpublished

heat flow and groundwater

heat flow measurements

range from 270 to 1080 m in depth. indicating order

groundwater

of 5-10

borehole

mW/m2.

In many

measurements

fracture rapidly

at as much

vertical

the existence

groundwater, bedrock

in contrast

to recent

(central

in the central

Baltic Shield.

indicate

flow concept

of fracture

studies

saline in composition. in heat flow induced been realized.

previously

above-average

by geological

as the flow. The flow zones

zones in the holes were detected

in the same holes,

by groundwater The apparent

of the

logs and other

the water

flowing

This can be taken as an indication

from the central

gradient

in heat flow were typically

was supported fracture

The holes

and in the temperature

zones at the same depths

groundwater

than has previously

regionally

Baltic Shield) are presented.

in the heat flow density

to the more saline (and more stagnant)

that disturbances

agree well with values published Finland

in Finland

at depths of less than 500 m, but flows between

as 930 m. According

in areas of crystalline mW/m’)

in the bedrock

in eight holes. The variations

the groundwater

zones seems to be fresh or only slightly circulating

changes

were detected

cases

which indicated

the holes. The results indicate

western

from 17 drillholes

Sharp

flow disturbances

were usually encountered deeper,

circulation

156: 59-74.

groundwater

even in the

of relatively

encountered

deeper in

flow may be much more common heat flow densities

(mean

35.0 f 3.0

Baltic Shield, but the results from southern

and

heat flow values (> 45 mW/m’).

Introduction

waste in the bedrock, as groundwater flow is the only plausible mechanism for transporting nuclear

Problems of groundwater flow in crystalline bedrock are of great importance in geothermics.

wastes to the biosphere. In the Canadian Shield,

Heat transfer due to water flowing in the fractures of the bedrock could seriously distort the assumed thermally conductive state of the bedrock. Further, it would be difficult to estimate heat flow

shown

0 1988 Elsevier Science Publishers

groundwater

it has

been

clearly

may flow in crystalline

bedrock at depths of less than 1 km (Lewis and Beck, 1977; Green and Mair, 1983; Drury and

densities and temperatures deep in the crust and at Moho with the aid of measurements made in the upper 1-2 km if there were any significant transfer of heat related to groundwater flow in the crust. Apart from this, understanding groundwater flow effects in the upper bedrock is important when planning the disposal of high-level nuclear 0040-1951/88/$03.50

that

B.V.

Lewis, 1983; Drury, 1984; Drury et al., 1984). In the Baltic Shield, the vertical heat flow variation in the 11.8 km Kola superdeep drill hole can be attributed, at least partly, to groundwater circulation (Kremenetsky and Ovchinnikov, 1986; Milanovsky et al., 1987). The present author has recently revised the heat flow data of JBrvimgki and Puranen (1979), paying particular attention to

60

the vertical matically (the

variation

in apparent

corrected

central

Shield).

It heat

was

geological many

that could

detected in (Kukkonen,

(1975) logged

ern Sweden

(western

Baltic

disturbances

ranging

17 drill holes in northShield)

in many

from the surface

(1983) studied

and detected

of the holes to about

at

data from both geothermal studies, we would better

heat flow and groundwater

and hydrounderstand

flow effects.

The aims of this work were to study possible groundwater flow effects with the aid of detailed heat flow measurements Shield),

1987). Parasnis

Poikonen

in Finland noted

transfer

easily explain the heat flow variations the uppermost parts of the bedrock

depths

to combine

heat flow densities

Baltic

groundwater-flow-induced

water-flow

palaeocli-

and

to study

occurrence

in Finland

of flow in different

and to present

(central

Baltic

flow rates and the frequency

of

types of formation,

new heat flow data on the Baltic

Shield.

500 m.

several shallow ( < 200

Methods

for identifying

groundwater

flow effects

m) holes in the environment of two Finnish nuclear power plants. At Hlstholmen, southern Finland,

in geothermal logs

where the bedrock

In crystalline rocks, groundwater flow creates characteristic anomalies on temperature-depth plots and Bullard plots. There are three basic anomaly types and their quantitative interpretation in detail can be found in the literature (Ramey,

is rapakivi

granite,

a heat flow

change of about 9 mW/m* was detected at a depth of 90 m; the heat flow change was attributed to groundwater flowing in a fracture zone at a velocity of 0.43 m/day. Studies of groundwater Canadian

chemistry

in

the

Shield (e.g., Frape and Fritz, 1982; Fritz

and Frape, 1982; Frape et al., 1984) and the Baltic Shield (Nurmi and Kukkonen, 1986; Blomqvist et al., 1987b; Nurmi et al., 1988) have shown that the composition of groundwater changes dramatically with depth. According to Nurmi et al. (1987), the Baltic Shield groundwaters in the bedrock have a layered mode of occurrence. The uppermost part of the bedrock to a depth of 300-900 m in inland areas and 50-200 m in coastal areas around the Baltic Sea is characterized by fresh groundwater with a low level of total dissolved solids (TDS < 1 g/l),

high tritium

contents

and

a stable

isotope

composition (ansO, 6*H) representing meteoric values. Below the fresh waters, there are layers of brackish (TDS l-10 g/l) and saline (lo-100 g/l) groundwaters and brines (> 100 g/l) which often have very low tritium contents and stable isotope compositions different from those of the meteoric fresh water. It is evident that these saline waters must have residence times longer by several orders of magnitude than the near-surface fresh waters. In the Kola superdeep hole, where the maximum TDS values of the deep brines were higher than 300 g/l, the vertical variation in groundwater composition and salinity was related to changes in heat flow density as well (Kremenetsky and Ovchinnikov, 1986). It is obvious that if we were

1962;

Jaeger,

1965;

Drury

and

Jessop,

1982;

Poikonen, 1983; Drury, 1984; Drury et al., 1984; Beck and Shen, 1985). The types are: (1) flow in a drill hole from one fracture zone to another; (2) temperature spikes on a temperature log indicating drilling fluid penetration to a hydraulically conducting fracture zone; and (3) a change in gradient on a temperature log or Bullard plot indicating groundwater flow in a fracture zone. Anomaly types 1 and 2 are due to the drill hole itself and the circulation of the drilling fluid and groundwater

in the hole. They cannot,

therefore,

be used for studying groundwater flow in the fractures of the surrounding bedrock, even though they are good indicators of hydraulically conducting and active zones. These anomalies should be avoided in heat flow determinations to prevent serious biases in results (Drury, 1984). The third anomaly type is the most important when studying flow in fractures. If the flow in a planar fracture is laminar and has persisted for thousands of years, and steady-state circumstances can be assumed, the mass flow rate up or down the dip can be estimated in a two-dimensional case as (Drury et al., 1984): m, = AQ/cg

tan CL

(1)

where m, is the mass flow rate (kg s-l m-i) across a column of rock 1 m wide, AQ is the heat

61

flow density

difference

(W/m2)

between

hole sec-

tions above and below the fracture zone, c is the specific heat of water (J kg-’ K’), g is the undisturbed

temperature

the dip of the fracture m, gives the vertical

gradient

(K/m)

upward, negative downward). The mass flow rate can be converted metres per second water.

by dividing

If the thickness,

porosity,

it is possible U (m/s). flow velocity, (Poikonen, 1983):

into cubic

it by the density

d (m), and

n, (dimensionless),

are known,

and LYis

plane (degrees). The sign of direction of flow (positive

of

the effective

of the fracture

zone

to calculate the average In a porous medium

ceeds

a critical

value.

However,

in shield

where the geothermal order of lo-20 mK/m

gradient is typically and the temperature

1000 m is lower

20°C

than

thermal

areas of the above

convection

takes place only if the hole diameter is greater than 75-100 mm (Drury et al., 1984). In Finland the hole diameter is usually 46 or 56 mm. It

has

recently

palaeoclimatic applied

to study

turbances

been

corrections indirectly

(Kukkonen,

proposed of heat

that flow

groundwater

the

can

be

flow dis-

1987). If the bedrock

tem-

perature around a drill hole is controlled only by conduction of heat, the vertical variation in the apparent heat flow densities should correlate with the thermal disturbances caused by variations in

q=.GAn e

(2)

the surface

temperatures.

If there

is significant

where q is the mass flow rate (m3 s-l m-‘) and A is the cross-sectional area of the fracture zone

groundwater flow (i.e. transfer of heat) in the bedrock, the measured vertical variation in heat

measured on a plane parallel to the strike and perpendicular to the dip (A = d. 1 m*). Thus the flow velocity can be calculated as average

flow density model.

(Poikonen,

1983):

u = q/n,d

= m,/h,d

(3)

where S is the density of water (kg/m3). The effective porosity of a fracture zone can be estimated by undertaking geophysical and hydraulic measurements in the hole (e.g., resistivity and neutron-neutron

measurements,

sonic

logs

deviates

from

the

palaeoclimatic

Valuable information can be gained not only from pure heat flow methods but also by logging the fluid resistivity of the drillhole. Zones of groundwater flow are often related to changes in fluid resistivity.

Measurements

and reduction of data

and

hydraulic pumping tests). In practice, a change in gradient on a temperature-depth log is not sufficient to establish a groundwater flow system in the surrounding bedrock. The change in gradient must also be related to a change in heat flow. However, the variation in apparent heat flow density can also be due to refraction if there are inclined interfaces of conductivity in the drill hole (Jaeger, 1965; Jones and Oxburgh, 1979). If refraction plays any role in the measured data, it can be qualitatively identified using conductivity-heat flow density plots, as heat flow maxima can be expected in rocks with higher conductivity and minima in rocks with lower conductivity (Jones and Oxburgh, 1979). Thermal convection in an open drill hole may disturb the temperatures if the hole diameter is sufficiently large and the geothermal gradient ex-

A set of 17 holes was chosen for this study (Fig. 1, Table 1). The main criterion on which the holes were chosen

was their depth,

but a few shallower

ones were also accepted if they represented areas with no or poor coverage by earlier heat flow measurements. Temperature was measured with equipment constructed at the Geological Survey of Finland. The measuring element is a temperature sensitive micro circuit (Analog Device 590 H) capable of an absolute accuracy of 0.1” C and a resolution of 0.01’ C. Temperature readings were taken at 2-5 m depth intervals to ensure that flows along the hole were not misinterpreted as true gradient changes (cf. Drury et al., 1984). For easy identification of gradient changes the original temperature-depth logs were first reduced by subtracting a graphically determined gradient from the measured temperatures.

mean

62 TABLE

1

Technical

information

site

of the drill holes studied

Code

Latitude

Longitude

(Nl

(El

Elevation

Vertical Local topodepthz) graphic variation')

(m.a.s.1.)

(ml

(ml

Hole inclination

Hole casing

Depths of cementation during drilling

I")

Drilling year

Measurement year

1986

1986

1983

1986

(m)

Cm)

Espoo

R301

6O"ll'

24"50'

7

20

254

61

Kangasnlemi

R301

62"02'

26"35'

100

50

(264)

45

70

Keltele

R494

63017'

26"22'

127

70

600

76

444

“0

1985

1986

Kerimakl

Krm/Hd-32

62"Ol'

28"57'

103

40

700

73

250-400

“0

1984

1985

Kisko

IL-61

60014'

23"31'

50

40

333

60

not

1984

1986

Kolar,

R162

67"33'

23"56'

215

100

548

74

265,

Kolarl

R197

67"33'

23"56'

205

100

558

74

not

“0

known

not

known

known

not

329

known “0

1977

1986

no

“0

1980

1986 1985

Lavla

TVO-testhole

61039'

22"41'

130

30

958

79

O-65

“0

1984

NummvPusula

R357

60"28'

26"46'

115

40

272

64

no

“0

1983

1985

Outokumpu

OKU-737

62"47'

29013'

90

15

650

84

no

no

1981

1985

Outokumpu

OKU-740

62"47'

29013'

90

15

(884)

76

“0

1981

1985

Outokumpu

OKU-741

62"47'

29013'

90

15

(1080)

76

“0

1981

1985

Paralnen

R311

64"ll'

22019'

13

40

1980

1985

PyhdJarvi

PYS-35

63"40'

26"05'

65

20

1980

1987

Ranua

R128

66006'

26"lO'

168

5

Sodankyla

Rl

67"38'

26"15'

205

Vihanti

VIH-1732

64"23'

25"OS

93

1) ')

Valid

to a distance

Values

of about

in parentheses

give

3 km from the

the

greatest

260-280 no

550

72

not

known

not

95-104,195-203

known

o-475

890

79

(567)

61

no

“0

1978

1986

80

495

47

“0

no

1984

1985

5

734

79

“0

1979

1986

o-135

hole.

attained

depth.

The drill cores were sampled every 10 m, if possible; only in some cases 20-30-m intervals had to be accepted. The conductivity of the sam-

Figs. 2 and 3. The calculated apparent and palaeoclimatically corrected heat flow densities for each hole are presented in Table 2. Details of the

ples was measured at the Geological Survey of Finland with divided bar methods on apparatus described by J&vim&i and Puranen (1979).

interpreted groundwater flow in the fracture zones are given in Table 3. The variation in vertical heat flow in the holes

The heat flow densities determined

Bullard’s (1939) technique. chosen

of the drill holes were

at depth sections

following

of about 100 m using

The depth sections were

the gradient

changes

on the tem-

perature-depth plots and the Bullard plots. The calculated apparent heat flow densities were corrected for palaeoclimatic effects with analytical half-space models (Jessop, 1971; eermak, 1976). The palaeoclimatic temperature history for the past million years in the central Baltic Shield area was applied (Kukkonen, 1987). Topographic rections were not needed.

cor-

Results The reduced temperature-depth plots and the Bullard plots of the holes studied are given in

is considerable.

In the apparent

values,

the varia-

tion ranges from a few to several tens of mW/m’, with

the errors

smaller

than

+2

of determination mW/m2.

being

A similar

observed in the palaeoclimatically flow densities. The absolute values of the

typically

variation

corrected drill-hole

is heat

means

range from 12.7 to 68.0 mW/m’ (apparent values) and from 17.7 to 73.5 mW/m2 (palaeoclimatically corrected values). The standard errors of the drillhole mean heat flow values decreased in six cases and increased in ten cases after applying the palaeoclimatic corrections. Water flow in a drill hole between separate fracture zones was detected in nine holes (Fig. 2). The flow systems were identified on the tempera-

63

0

Archaean

complexes

Svecokarelian supracrustal

rocks

Syn-and

late-kinematic

granitoids

Rapakivi

granites

Jotnian

and

Caledonian

sedimentary

Fig. I. A simplified

geological

map of Finland,

central

rocks

Heat

flow

sites

f-

Espoo

Z-

Kangasniemi

3-

Keitele

4-

Kerimaki

5-

Kisko

6-

Kolari

R162

7-

Kolari

R197

8-

Lavia

9-

Nummi-Pusula

lo-

Outokumpu

OKU-737

1 l-

Outokumpu

OKU-740

12-

Outokumpu

OKU-741

13-

Parainen

14-

PyhPjsrvi

1%

FIanua

16-

Sodankyll

17-

Vihanti

Baltic Shield, and the locations

of the heat flow sites.

ture-depth plots on the basis of the characteristic anomalies indicating water flow in the hole (see Drury et al., 1984, for details). Because of the short depth interval of the temperature readings,

down

the

tation of the temperature profile. In most cases, the flow takes place at depths of less than 400 m. The exception was the Lavia hole, which showed

temperature

profiles

(Fig.

2) are practically

continuous. Therefore, it was rather easy to note the sharp temperature steps indicating water flowing out of the hole and the decreased gradients in the flowing sections of the holes (e.g. the Keitele, Kisko, Kolari, Lavia, Sodankyll and Vihanti holes, Fig. 2). In some cases the surface temperature extrapolated from the uppermost 100-200 m of the hole was 0.5-l’ C below the expected value of the area, indicating cold surface water flowing

the hole (e.g., Kolari

R162, and Vihanti

in

Fig. 2). Further, in one case (Kolari R197) water flowed out of the hole to the surface at a rate of 15-20

l/mm,

confirming

the qualitative

interpre-

water flowing out of the hole at a depth of 930 m (Fig. 2). The anomalies indicating fluid circulation in the drill holes were not interpreted quantitatively for two reasons. First, the drill hole itself is an unnatural hydraulic short-cut between separate fracture zones, and second, the possible casing of

REDUCED 2 Kanaasniemi

TEMPERATURE(“C)

4 Keitele

6

Kenmlki

8

ESClOO

Kisko

10 Kolari

12 I Kolari

Lavia

0 200 400

T

600

600

800

800

F tl P

Nummi-Pusula

0

Outokumpu

Parainen

Pyhljtirvi

Ranua SodankylB

Vihanti

0

200

200

400

400

600

600 800

800

/

\

12.7

11.8

1000

1000 10.9

1200

1200 2 Fig. 2. Reduced geothermal

temperature-depth

gradient

4

plots of the holes studied.

of the hole from the measured

are given at the end of each curve (mK/m). flow in fracture

6

The data were reduced

temperatures

The temperature

zones (Table 3) are indicated

for easy identification scale is only relative.

with an asterisk. lines and arrows.

the hole and the injection cement that was sometimes used during drilling (Table 1) have a marked effect on the flow rates in the drill hole. The identified

flow systems

are indicated

perature-depth profiles (Fig. 2). Gradient and heat flow density ing groundwater flow in fracture

in the tem-

changes indicatzones were ob-

served in eight holes at depths between 78 and 496 m (Table 3). The procedure for identifying the anomalies was the following. Gradient changes observed on the temperature logs were first checked to see if they were related to heat flow changes at the same depths (Table 2). In order not to misinterpret refraction anomalies as groundwater flow effects, it was next ascertained that there was not any considerable (> 0.5 W m-’

10

8

by subtracting of gradient The gradient

Flows in the hole between

fracture

12 the graphically changes. changes

determined

The subtracted attributed

mean

gradients

to groundwater

zones are indicated

with dashed

See text for details.

K-r) change in conductivity at the same depths. If the geological and other geophysical logs of the holes were available, changes were checked cations (e.g. strongly

the depth of the gradient for any fracture zone indifractured drill cores, pres-

ence of clay gouge, resistivity minima, etc.). It was only after all these criteria were satisfied that the groundwater flow disturbances were accepted as such (Table 3). The groundwater salinity data (Table 2) served only as supporting evidence. It must be noted that groundwater flow effects are of course also possible where there is a variation in thermal conductivity. Sites of conductivity contrast are usually lithological boundaries, and fracture zones often exist at or near these boundaries. However, it would be very difficult to

65

TABLE

2

Apparent Hole

and palaeoclimatically

corrected

AZ

k

105-302

3.lOf0.13

N

21

heat flow densities, s

9

1.3

48.0+0.5

rock types and groundwater :

12000

Q’

A9

9

8.P.

-0.62

49.9

Rock

types of the holes studied

Migmatltic granite, [Ref.

Keitele

182-256

2.27t0.04

11

25.4t4.4

-1.50

41.3

Mica (Ref.

I8

39.9*0.1

-1.26

43.2

17

42.4fO.l

-1.95

47.5

2.60+0.09 2.69+0.03

1.0

33.0+1.0

6,

57-95

2.97io.52

5

104-198

3.85?0.71

11

207-336

3.72t0.53

15

345-492

3.58tO.25

I8

500-597

3.78tO.26

13

605-700

5.27tO.74

13

mean

1.5

RI62

78

104-197

3.67fO.28

12

205-329

5.14tO.28

14

-3.87

39.8

Hornblende

31.9

mica

29.3t0.7

-0.76

32.2

hjemite

39.1kO.8

-1.60

44.6

41.4tO.8

-2.05

49.4

47.6t2.0

-2.14

55.9

10000 8.P.

trond-

Fresh (Ref.

(300 m)

mean

1.5

34.7t3.8

60.4tl.O

mean

11000 8.P.

75.652.6

42.3t3.9

-1.68

67.4

-0.96

79.6

Metavolcanite

mea"

38

68.Ok7.6

N

s

Q

1.0

35.6iO.7

mean

(Ref. 51

Not determined

t

9

73.526.1

A9

Q’

Rock

types

2.46t0.17

4

-0.40

36.7

Monronite,

2.16_tO.24

3

23.8t13.6

-1.51

28.0

skarn

3.44+0.98

5

43.1t5.9

-2.13

49.0

9000 B.P.

--.._____

2.75fO.36

13

mean

0.9

mean

2.49+0.07

14

-0.63

33.3

Monzonite,

2.55~i.08

9

22.7tO.6

-1.74

27.1

skarn

402-479

2.69t0.25

9

20.9*0.9

-2.19

26.4

516-554

2.09+0.10

4

30.9_+0.5

-2.24

9000 8.P.

43

mean

1.3

37.6iO.9

99-196

3.70f0.08

11

3.23?0.18

8

47.9?0.8

(Ref.15)

diotite,

iron ore

[Ref. 6)

Fresh

slightly

saline

(Ref.

15)

Fresh

(Ref.

________

26.6t2.8

205-273

saline

36.5

--._____

2.51~0.07

Slightly

(Ref. 61

37.9i6.1

172-298

---.___--._____

diorite,

____.___

34.2t5.6

31.7to.4

Groundwater types and depths of corresponding interfaces

iron ore

307-393

mean l

9500 8.P.

30.8i2.4

-2.0

44.5

Granodiorite,

-1.40

52.7

tonalite

283-399

3.55io.17

12

41.8iO.5

-1.99

48.7

409-495

3.41!0.19

10

42.520.8

-2.61

51.5

505-600

3.47_tO.O8

I,

49.2?0.4

-2.75

58.7

610-676

3.34t0.12

8

46.6?1.0

-2.69

55.7

690-798

3.28t0.08

13

45.3to.4

-2.48

53.9

808-920

3.41_to.o9

12

52.Otl.7

-2.12

59.3

938-956

3.42?0.21

3

54.2tO.5

-1.83

60.5

granite,

(Ref.

I71

7)

________ mean

Nummi-Pusula

3.46tO.05

96

mean

1.2

46.3kl.7

67.Ot1.7

mean

53.9il.8

22-75

2.97?0.16

7

-4.77

81.0

Granodiorite,

84-203

3.04?0.16

15

55.2+0.6

-1.07

58.3

gneiss

2.75:0.20

8

43.2tl.3

-0.66

45.1

211-268

_______________ mean

2.94?0.10

- saline

16)

(Ref. 4)

181-265

mean

gabbro,

gneiss,

307-394

mean

Lavi a

44.7t1.5

-1.48

24.8tl.0

-..____-.______

RI97

mean

____.___

4.17kO.19

k

437-546

Kolari

40.5t2.9

26.2?1.0

____.____

Kolari

3)

--..____

3.88+0.20

a2

9500 8.P.

________

______.._______

Hole

mean'93.5+13.6

46.9

264-441

mean

38.0?11.6

-3.15

450.595

mean

______________

-0.09

9 8

2.62?0.04

29.0

________

46.7tl.O

2.61?0.13 2.62+0.11

mean

K,sko

151

l&f.' 21

____.__._______

Kenmdki

(at least to 444 m)

(Ref.

Gabbro,

38-115

slightly

15)

Fresh

70.6 30.8

124-199

II50 ml (Ref.

gabbro

-4.0 -1.4

mean

-

gneiss,

Ci1.1t3.4 27.5i0.3

33

saline

Not determined

6 9

2.38t0.05

Fresh

gneiss

2.48+0.23 2.44?0.08

mean

gneiss,

mica

60-96

_______________

mica

amphibolite

1)

110-17s

0.8

Groundwater types and depths of corresponding interfaces

types

30

11000 8.P.

-_______ mea"

54.6t6.5

_.____.. mean

61.5i10.5

mica

(Ref. 8)

Not determined

66 TABLE

2 (continued) d2

k

Outokumpu

30- 79

OKU-737

89-197

HOlE

9

3.39?0.46

6

1.3

24.3il.8

-4.62

39.5

Mica

5.10?0.63

12

27.8k3.6

-1.53

32.8

Schist,

207-293

2.81+0.21

10

27.423.5

-0.74

29.8

wth

302-393

2.77~0.12

11

36.lTO.7

-1.41

40.8

and quartz

402-496

2.71tO.17

12

3EI.7to.7

-1.99

43.3

504-596

2.89~0.19

11

38.721.3

-2.22

46.0

604-645

2.89_tO.25

6

39.lil.3

-2.25

46.5

OKU-740

3.30t0.16

mean

70

58-133

2.98?0.31

9

142-327

3.84i0.36

21

1.3

a’

a9

9

10000 B.P.

32.9t2.3

gneiss,

10000 B.P.

-3.36

31.6

Mica

-0.69

31.1

schist,

8

41.6kl.7

-1.54

47.2

with

3.34tO.56

11

39.6+_1.2

-1.99

46.0

and quartz

504-617

2.68tO.13

14

35.3f0.6

-2.23

42.5

626-752

2.80*0.15

16

43.2tO.5

-2.19

51.1

761-881

6.00?0.61

15

41.2!0.5

-1.93

48.2

3.63tO.17

98

mean

35.6~3.3

29. 67

3.14tO.22

5

44.at3.1

10000 B.P -4.77

60.5

Mica

77-207

2.54?0.12

15

27.0t0.4

-1.53

32.0

schist,

216-251

2.43tO.25

5

13.3+o.e

-0.67

15.5

with

260-346

2.88~0.13

11

32.1?1.2

-1.05

35.6

and quartz

354-462

3.14to.40

14

30.2tO.6

-1.41

34.9

470-521

4.55_rO.98

7

36.6tl.2

-2.16

43.7

529-596

3.52tO.25

9

37.5i1.3

-2.23

44.9

605-698

2.94+0.51

13

36.1t0.9

-2.23

43.5

706-797

2.4720.22

12

32.0?0.4

-2.08

38.9

805-894

2.90to.27

10

35.9io.4

-1.86

42.0

902-1037

5.31+0.41

16

3l.lt2.4

-1.54

36.2

1045-1076

2.99!0.34

5

29.7+2.7

-1.29

34.0

Hole

Parainen

3.3010.14

121

a2

k

N

s

0.9

9

34.3t0.5

- (350-5501

(Ref.

18)

Fresh

- (350-4501

mean

tg

10000

B.P.

- saline

dolomite rock

[Ref. 91

42.6~3.1

on-741

_____________ mean 32.2_+2.2

graphitic

skavn,

Outokumpu

mean

Fresh

(Ref. 9)

serpentinite

4.07?0.48

1.3

(Ref. 151

dolomite rock

gneiss,

336-398

mean

Saline

mean 39.a+2.4

408-495

mean

graphitic

serpentinite

skarn,

28.6_tO.9

19.4+0.6

Groundwater types and depths of corresponding interfaces

Rock types

s

mean

Outokump"

t

N

gneiss,

graphitic

serpentinite

skarn,

(950)

dolomite rock

very

- saline

saline

(Ref.

16)

(Ref.

9)

38.5k3.0

Rock

types

A9

Q'

-2.81

41.4

Amphibolite.

-1.78

39.9

sulphide

38-179

2.26t0.15

16

188-280

2.51tO.35

10

35.421.7

zag-340

3.25kO.17

7

44.ato.7

-2.54

51.3

rock,

467-500

2.71+0.16

5

39.1to.9

-3.24

47.3

10)

2.54tO.12

42

Groundwater types and depths of corresponding interfaces

sk?rn

bearing

mica

with

Fresh

- (350-450)

quartz

(Ref.

16)

gneiss

- saline

(Ref.

________ mean

PyhSjarvl

mean

45.0f2.6

59-137

3.13tQ.20

9

-2.84

38.1

Gneissose granite,

Fresh

- (600-800)

3.08t0.19

7

28.0f0.5

-0.75

30.6

leptite,

(Ref.

15)

215-293

3.5OtO.23

9

36.521.0

-0.53

38.3

antophyllite

302-398

3.57to.13

12

41.0*0.3

-1.13

44.9

amphibolite

408-493

3.68!0.15

10

42.7t0.6

-1.71

48.6

28.3t0.6

.4

9500 B.P.

503-595

3.79_to.10

11

44.2t0.4

-1.97

51.0

604-700

3.46?0.09

11

41.4to.3

-2.03

48.4

708-889

3.51~0.20

22

39.4t1.2

-1.84

45.8

mean

65-183

101

mean

0.8

mean

37.7t2.2

19.6t1.5

cordieriterock, (Ref.

11)

2.28tO.07

4

-0.87

21.8

Gabbro,

2.45_tO.14

4

29.9+2.7

-1.32

33.2

rocks

(Ref.

439-474

2.96

2

26.0

-2.41

32.00

and ultrabasic

mean

1.2

9000 S.P.

43.2t2.4

222-344

10

25.2t3.0

2.5t0.4

mean

-3.95

16.0

Basic

3.86_tO.16

16

11.9:0.5

-0.80

14.6

metavolcanites

206-293

3.71t0.24

11

17.9kO.7

-0.21

18.6

131

301-377

3.07t0.12

10

1r3.4to.3

-0.90

21.5

3.42t0.11

43

12.7t3.7

mean

(Ref.

15)

29.Ot_3.6

2.57iO.08

mean

8.P

Fresh.

12)

36- 79

mein

9000

ultramafic

86-198

7

- saline

________

________

3.46tO.06

mean 2.49_tO.l2 SodankylB:

mean

147-205

_________

R.XlUa

38.4t2.4

17.7fl.5

(Ref.

Fresh saline

(150 m) - slightly (Ref.

15)

67

TABLE

2 (continued)

Hole

A2

!rlhantl R1732

k

Hole

See

1.0

83-112

3.07+0.07

4

2.87+0.14

8

30.1!1.6

9000 B.P.

38.3

Granite,

37.4

mica

391-447

2.53t0.17

7

30.2t0.5

-2.02

35.9

2.81t0.16

11

33.120.5

-2.35

39.7

558-648

2.92_+0.12

11

32.710.7

-2.33

39.2

657-727

2.lS7+0.19

10

29.2?0.3

-2.15

35.2

2.81~0.05

78

Table

Depth

mean

31.710.9

mean

-6 2 Mean theriral diffusivlty of the hole (10 m Is), estimated density (Lahde, 1976) of the correspond,ng rock types.

a

Apparent

(Ref.

Fresh

(Ref.

151

14)

37.e0.7

mean

value

ford2

wth

standard

error

(WlmK).

The mean

for the

hole

Includes

also

samples

outsIde

the

listed

depth

intervals.

of samples.

heat of the

Palaeocllmatic

flow

dens,ty

latest

wth

disturbance

Palaeocl~mat~cally

corrected

history heat

c lOOmS/m;

error

on the

to geothermal

temperature

conductivity

standard

deglaclatlan

The palaeoclimatlc

Fresh,

granodiorlte,

gneiss

(ml.

5

References

types and depths of corresponding interfaces

1.

Interval

Number

Moment

Groundwater

Rock types

-2.91

N

Groundwater types

9’

-0.83

Conductlvlty

9’

ng

35.1+1.1

k

tg ng

9

456.E49

mean

02

s

132-200

t

Q

N

flow

slightly

hole

the drill-hole

(years (mK/m),

central

before

present)

calculated

Baltic

Shield

density

(mW/m2i,

Q'=Q-Ag.k.

saline

(100-500

mS/m);

for

(Dormer,

distinguish between refraction and groundwater flow effects in measured heat flow data, and so such cases were not included in Table 3. Heat flow changes related to conductivity contrasts were detected in many holes (e.g. the Kerimaki (at 600 m), R162 and Parainen

conductlv>ty

and the

speclflc

heat

(Schon,

19831

and

complied

saline

holes, Table

1978).

the median

depth

by Kukkonen

(500-5000

11 H. Appelqvlst, Geological Survey of Finland, written communication, 1986 2) A. Laitakarl, Geological Survey of Flnland, written comm., 1983 3 IJ. Nlkander, Geological Survey of Finland, written comm., 1985 4) J. Eeranhelmo, Outokumpu Oy, written comm., 1984 51 O-P. Isomakl, Outokumpu Oy, written comm.. 1986 6) Hlltunen, 1982 7) Saksa, 1985 81 E. Ralsanen, Geological Survey of Flnland, wr,tten comm., 1983 91 P. Hakaoen, Outokumpu Oy, wntten camm., 1981 IO) E. Lund&, Partek Oy, wr,tten ‘omm., 1985 11) R. Juhava, Outokumpu Oy, wrItten comm., 1980 12) Rekola, 1986 131 T. Mannlnen, Geological Survey of FInland, written comm., 1964 14) T. Makela, Outokumpu Oy, written comm., 1979 15) Blomqvlst et al., 1987a 16) Nurmi et al., 1988 171 Lahermo and Lampgn,

the Kisko, Kolari

mean

lmW/m21. site

dradient of the

from

mS/m);

1987

of AZ. (1987

very

I wasused.

saline

18) Biomqvlst

(> 5000

et al.,

See text

for details.

mS/m).

1987b

than of a few mW/m2 (e.g. Lavia and Vihanti, Table 3). The highest value of AQ = 34 mW/m2 in the Kangasniemi hole is an exception, and the typical

values

range from 5 to 12 mW/m*

2).

termined

above and below the fracture

In the Espoo hole the negative gradient in the uppermost 100 m (Fig. 2) can be attributed to an increase in the ground temperature about 30 years ago, when a large building was erected a mere 15 m away from the hole site, and the ground was covered with asphalt.

in Table

2. The interpreted

The short depth interval of the temperature readings made it possible to detect gradient changes even smaller than 1 mK/m in depth ranges of 100-200 m. Therefore the smallest heat flow changes that could be attributed to groundwater flow effects were of the order of no more

(Table

3). The AQ values (Table 3) were calculated differences between the heat flow values

as de-

zones listed

flow zones

are indi-

cated with an asterisk in Figs. 2 and 3. Because of the applied scale of presentation the smallest heat flow changes are not necessarily obvious on the Bullard plots in Fig. 3. The estimated mass flow rates were of the order of lop3 to lo-’ kg s-t m-‘. The gradient value determined below the flow zone was used as an estimate of the undisturbed gradient in eqn. (1). Estimation of the fracture zone dip (eqn. 1, Table 3) was based either on geological logs of the measured and nearby holes (Kangasniemi,

68 TEMPERATURE

0

( C)

Figs. 2 and 3). Further, the standard errors of the mean values for the drill hole heat flow calculated from

-*

values

a “E

: 2 2 ul v,

tions

lc

result

200 -

300

explain

from

1987).

deficiencies

in the half-space

palaeoclimatic have

heat flow varia-

The

non-conductive

discussed

variation

may

transfer

(i.e.

heat

below) but also from model

temperature

been

corrected disturbances

the observed

(cf. Kukkonen,

(Kukkonen, here.

;: looH CL 200

palaeoclimatic

flow, discussed

issues

-’

ii

ii +

palaeoclimatically

that

groundwater plied

I 0

and

indicate

only partly

100 s

apparent

and

history.

in

1987) and therefore

detail

the apThese

elsewhere

are not repeated

Refraction anomalies seem to be present in some of the holes: Kisko, Kerimaki (below 600

-

m), Kolari (R162, below 400 m), Parainen and Pyhajarvi (Table 2 and Fig. 4). In these holes, the 300

-

heat flow density increases with conductivity. But there are also holes where the opposite occurs,

400 4

12

20

Fig. 3. Bullard plots of the drill holes studied. scale is relative.

The heat flow changes

water flow in fracture

36

28

The temperature

attributed

to ground-

zones (Table 3, Fig. 2) are indicated

an asterisk.

with

See text for details.

that is, the heat flow density decreases with increasing conductivity (Kolari R197 and Lavia). In other

cases,

the

heat

flow

density-conductivity

plots have a rather scattered appearance (Fig. 4). Radiogenic heat production can give rise to heat

flow

variations

(Jaeger,

1965;

1984) but the heat production have to be very high (> lo-50 Kerimaki, Nummi-Pusula, Vihanti and Pyhhalmi), on the rock type contacts (Kolari) or on VSPseismic studies (Lavia). Where no dip data were available a reference value of 45 o was used in the calculations (Keitele).

Vertical variations in temperature gradient and heat flow density can generally be attributed to

would to pro-

duce heat flow changes of 10 mW/m’ at depth intervals of a few hundred metres. The heat production values in the holes studied in this work were measured (Kukkonen, 1988) but no such anomalous values were encountered. Detailed

Discussion

Buntebarth,

contrasts pW/m3)

examination

of the heat flow densities

(Table 2) temperature-depth plots and Bullard plots (Figs. 2 and 3) together with the groundwater chemistry data from the same holes (Nurmi

four main factors: (1) palaeoclimatic effects; (2) structural effects (refraction); (3) radiogenic heat

and Kukkonen, 1986; Blomqvist et al., 1987b; Nurmi et al., 1988) showed that, in most of the cases, heat flow changes could be attributed to

production; and (4) groundwater flow. Palaeoclimatic effects due to Quaternary glaciations and later Holocene and recent climatic changes create

heat flow changes, the groundwater was fresh or slightly saline in composition or the change was

a complicated apparent heat flow profile in a purely conductive regime (Cermak, 1976; Beck, 1977; Nielsen and Balling, 1985; Kukkonen, 1987). However, the gradient and heat flow changes should be fairly smooth rather than sharp as detected in many cases in this study (Tables 2 and 3,

near a transition zone from fresh to saline waters. This is not an unexpected finding, as fresh groundwaters in the Baltic Shield are usually chemically and isotopically meteoric in composition (Nurmi et al., 1988) and must therefore circulate rather rapidly. Flow of saline groundwater

groundwater

flow in the bedrock.

At the depths

of

69

TABLE Details

3 of the interpreted

Hole

2

groundwater

flow in the fracture

AQ

zones

0

d

60

20

mf

uw

Rock type

Arguments for flow

References

Gabbro

1, 2, 4, 6

1

+34

+4.3

10-4

-11 +7

-1.6 +l.O

1o-4 10-4

(45)

6

10

Hornblende gabbro

1, 2, 5

2,lO

(45)

8

10

Gabbro, norite

1, 2, 4, 5

2,lO

445

-3

-4.5

1o-5

(45)

2

10

Hornblende gneiss

1, 2, 4, 5

2,lO

Kerimaki

340

-10

-7.5

1o-5

70

6

500

Graphitic mica gneiss

1, 2, 3, 4, 5

3, 11

Kolari CR1971

309

+9

t1.0

10-3

15

100

Monzonite

1, 2, 5

4, 10

Lavia

496

-7

-3.4

1o-4

20

6

Granodiorite

1, 2, 4, 5

5, 6

+12

+4.3

10 -5

75

Garnet-mica gneiss

1, 2, 4

7

207

+12

t4.6

1O-5

75

Garnet-mica gneiss

1, 2, 4

7

210

-9

-6.4

1O-5

70

Gneissose granite,

1, 2, 4

8, 10

1, 2, 4

9, 10

Kangasniemi

90

Keitele

120 260

Nummi-Pusula

Pyhljlrvi

78

15

amphibolite Vihanti

120

-5

-2.5

1O-5

75

Hole

See Tables 1 and 2.

2

Vertical depth to the flow (fracture) zone.

4Q

Heat flow density difference

30

Granodiorite, granite

(mW/m2) between above and below the flow zone. Negative and positive values

of AQ indicate flow downward and upward, respectively. mf (1

Mass flow rate (kg/sm) (equation 1). Dip of the fracture zone, taken from geological and geophysical logs of the hole. Values in parentheses are arbitrary and they where used only for reference in the estimation of mf.

d aW

Thickness of the fracture zone cm). Drill hole fluid electrical conductivity at the depth of the flow zone (mS/m)

Rock type

The wall rock type of the fracture zone.

Arguments for flow

1) Change in temperature gradient 2) Change in heat flow density 3) Change in groundwater salinity and composition (Oata taken from Nurmi et. al. (1988) and Blomqvist et. al. (1987b) 4) Fracture zone indicated in the geological log or observed in the core samples by the author 5) Fracture zone indicated in the electrical resistivity logs

References The data of o, d, (I,., and rock type were based on the following sources: 1) 2) 3) 4) 7) 8) 9) 10)

A. Laitakari, Geological Survey of Finland, written comm., 1983 J. Nikander, Geological Survey of Finland, written comm., 1985 J. Eeronheimo, Outokumpu Oy, written comm., 1984 Hiltunen, 1982 5) Okko, 1986 6) Saksa, 1985 E. Rlislnen, Geological Survey of Finland, written comm., 1983 R. Juhava, Outokumpu Oy, written comm., 1980 T. MSkell, Outokumpu Oy, written comm., 1979 Blomqvist et al., 1987b 11) Nurmi et al., 1988

70

CONDUCTIVITY 3 s

4

parent

(Wm-‘K-l)

5 -1

80

80

: 0 2

H

I?’

flow

40

even

conductive

Kukkonen,

1987).

flow density the bottom

80

to a major

I- 60 z w 40 cc

metres intense

palaeoclimatically

2) have

error than the apparent

a thermally

low, 12.7

as a true conductive

if the

(Table

regime

However,

could be the reason

20

is abnormally

be taken

value, values

standard

l; I

heat

It cannot

corrected

4 3 E 6o

heat flow density

mW/m’.

a much

in the bedrock groundwater

for the anomalously

low heat

long and fracturing

lineament

several

4

5

drill holes studied.

heat flow density

Each point

corresponds

plots

to data

for the

from one

2). The data

strongly

drill hole between

depth section in Table 2.

in the bedrock cannot be excluded, however. the Kola superdeep drill hole, strong circulation brines was observed metres (Kremenetsky

the gabbroic

only a very small variation

Fig. 4. Conductivity-apparent

In of

at depths of several kiloand Ovchinnikov, 1986).

next kilo-

hundreds of metres wide, and of bedrock in the environment

below 150 m although 3

below

of the hole. The hole was drilled topographic

(cf. flow

if the flow zone were situated

is very probable. The temperature-depth of the Ranua hole (Fig. 2) was very

: 20 0. a

smaller

values indicating

point

several

profile complex

rocks showed

in conductivity to water fracture

(Table

flow in the

zones.

Unfor-

tunately, the number of conductivity samples available was too small (Table 2) for positive identification of flow zones in the bedrock. In the Vihanti hole, strong circulation of water seems to take place between the surficial parts of the hole and 350 m. Groundwater was interpreted as flow-

A fracture zone was very often detected in the geological logging of cores or in other geophysical

ing in a fracture

borehole

too, because temperature changes at 230 and 310-350 m coincide with fracture zones reported

measurements

at the same depths

as the

heat flow changes (Table 3). This is surely not a coincidence but a manifestation of active groundwater flow paths in the bedrock. The criteria on which heat flow changes were accepted as being caused by groundwater flow were applied with caution, and some cases were excluded from Table 3. For example, the three holes in the Outokumpu area (Table 2, Fig. 3), which are less than 500 m from each other, showed heat flow changes that were related to changes in groundwater composition. The variation in heat flow density is much larger in the upper parts of the holes (in fresh water) than in the lower parts of the holes (in saline water). This could be taken as an indication of rapid groundwater circulation in the fresh-water layer. Unfortunately, there are no results of resistivity logging available, and the existence of fracture zones at the depths of the heat flow changes is a matter of conjecture only. In the Sodankyhi hole (Table 2, Fig. 3) the ap-

possible

zone at 120 m (Table

3) and it is

that there are active flow systems

deeper,

on the geological log of he core (T. MBkel$ ten cormnun., 1979). The discovery

of several

writ-

flow zones in granitic

and other acid or intermediate rock types (Table 3) is somewhat surprising. These rock types have usually been considered to be hydraulically tight, especially in studies related to the disposal nuclear waste in the bedrock. This concept

of is

based mainly on the generally low fracture and lineament density in these rock types (Vuorela and Hakkarainen, 1982). But fracture aperture, too, is a major factor controlling the hydraulic conductivity of a rock type (Fyfe et al., 1984) and in granitic rocks fracture apertures can be much larger than in, say, metasedimentary schists. Further, the fractures in igneous rocks are typically long, and therefore extensive networks of waterconducting fractures may exist. Riinka (1983) studied pumping test results from 700 shallow

(< 100 m) wells in Finland well yields

were higher

and observed

in plutonic

rapakivi

yield values granite.

as a measure bedrock

were measured

depths

different

exchange

in the

can also be studied

hundreds

of metres

of indi-

and salinity waters

at

are strong

indicators of relatively rapid (meteoric) circulation in the bedrock. Depending on bedrock porosity and the circulation time, considerable heat flow disturbances may result (Kukkonen, 1987). It is interesting to note that only fresh groundwaters (electrical conductivity < 120 mS/m) were encountered in the Swedish test-site studies for nuclear waste disposal. The sites were in granitic

flow velocities

in Table

cases. Moreover,

in the drill hole. Fresh

of several

included

from wells in

rectly on the basis of the composition of groundwater

for the average porosity

the well. The efficiency

exchange

Estimates 2) were not

were no fracture

The yield of a well can be taken of groundwater

surrounding

groundwater

mafic

rocks. The

and felsic) rocks than in metamorphic highest

that the

(both

values available methods

ferent porosity

values for fracture

(1983) showed

that fracture

with

resistivity

galvanic

greater

than

methods. values expected

those

However,

for crystalline

there in most

zones. Poikonen

loggings

determined

are 2-3

determined rocks

(eqn.

yield very dif-

porosities

reasonable

to be somewhere

For example,

3, because

with

times

hydraulic

fracture

porosity

could

in any case be

between

0.001 and 0.10.

if the water were flowing

in a frac-

ture zone 10 m thick, and the mass flow rate were 5.10-’ kg m-i s-i, the average flow velocities would range from 5 . lop6 (porosity 0.001) to 5 . lop8 m/s (porosity 0.1) i.e. 158 to 1.6 m/y. The result suggests that groundwater flow can actually

or intermediate rocks and the sampling depths ranged from 100 to 720 m (Smellie et al., 1985).

be very rapid in Precambrian bedrock. In principle, the driving force of groundwater flow can be either (1) lateral height differences in groundwater level (topography); (2) variations in

Similar results were gained from the Swiss test-sites in deep (up to 2.5 km) boreholes in granitic and gneissic rocks. Only moderately mineralized

groundwater density (salinity variations); or (3) thermal convection due to the contrasts in radiogenie heat production in bedrock. Topographical

groundwaters encountered.

variation in the environment (Table 1) is small (usually generally be the only cause several hundreds of metres. water density variations can

(TDS -C 9 g/l; Kanz, 1987) were In the Lavia hole (Table 2) which

intersects mainly granodiorite and granite, the groundwater was very diluted from the surface to the hole bottom at 950 m (Lahermo and Lampen, 1987), and heat flow data gathered in the present study indicated groundwater flow in a fracture

the salinity

of groundwater

of the heat flow sites < 50 m) and cannot of flow at depths of Flow due to groundbe ruled out because in the Baltic

zone (Table 3) and at least two flow systems in the

generally increases downwards 1987) and the system is already

hole (Fig. 2). It seems that more attention

ble.

should

be paid to the hydraulic properties of granitoids before any final recommendations of their suitability for nuclear waste disposal can be given.

Shield

(Nurmi et al., gravitatively sta-

Thermal convection attributed to radiogenic heat production contrasts in the bedrock has been shown to be a realistic alternative mechanism for regional flow in crystalline bedrock. Fehn et al.

Are the calculated mass flow rates (Table 3) realistic in terms of hydrogeology? The annual precipitation in Finland is about 600 mm (Kolkki,

(1978) and Fehn (1985) modelled high heat production granitoids

1969). If all the annual rainfall from a distance of 50 m were to flow into a hypothetical fracture zone dipping 45 O, the resulting heat flow difference in a steady-state flow would be about 100 mW/m* (eqn. 1, assumed gradient 12.5 mK/m). The calculated flow rates in Table 3 might easily be explained by the very low proportion of annual rainfall that migrates to the fracture zone at the surface.

England and showed that convective transport of groundwater is not only possible in batholiths and wall rocks but also explains the measured heat flow configuration of the area. The theoretical calculations of Fehn et al. (1978) were further supported experimentally by Durrance et al. (1982) with resistivity soundings and groundwater chemistry and isotope (U, 222Rn) data. Recently, Kukkonen and Pingoud (1987) have shown, with the

numerically the in southwestern

72

aid of finite element vection

rock in Finland flow density.

thermal

with its lower heat production

formation-wide kilometres.

and

extend

convection, Unfortunately

con-

study

heat

flow patterns,

differences to depths

must be of several

flow

(Table

effects

de-

3) are due to

they represent

only the surfi-

systems

the number

bed-

background

If the groundwater

in the present

cial parts of deep circulation crust.

that thermal

in the Precambrian

To create considerable

the horizontal

tected

modelling,

is also possible

of the upper

of holes

is too

times not (e.g., Sass et al., 1971; Kukkonen, The results the crystalline medium

indicate

that in geothermal

bedrock

should

consisting

but bounded transported

be approached

of blocks

by fracture

internally

zones

by groundwater

the depth

values

and

of metres

shal-

However,

a boundary

“shallow”

on the local geology

few dozens

heat is

flow. Therefore,

is not very easy to establish

as a

conductive

in which

low hole data can be very misleading.

pends

1987). studies

it

between

“deep”.

It de-

and can range from a

to kilometres;

is no

rule of thumb

of such large-scale systems. The results from the Kola superdeep hole (Kremenetsky and Ovchin-

taken into account, especially when extrapolating geothermal data to lower crust and upper mantle depths. The surface heat flow signal, which is of

nikov, 1986; Milanovsky et al., 1987) indicate, however, the groundwater may indeed flow at depths of several kilometres in the northern part of the Baltic Shield. One very important detail in the above results is that the Bullard plots and temperature gradients are very straight both above and below the flow

that can be applied.

there

small and the holes are too shallow for recognition

This must be

the order of 25-50 mW/m’ in the central Baltic Shield, may have a noise component of several tens of per cent due to groundwater flow. The 17 new heat flow values

(mean

37.5 f 3.0

zones (Table 2, Figs. 2 and 3). If the holes had not intersected the flow zones there would not have

mW/m’) presented are in good agreement with previous Finnish data by Jarvimaki and Puranen (1979) (mean 35.0 + 1.6 mW/m’), but they seem to indicate values higher than average in the

been any way of detecting the disturbances caused by groundwater flow in the fractures. This is due

southern and western parts Espoo, Kisko, Nummi-Pusula

to the hydraulic

Table

properties

of crystalline

bedrock,

2). It is possible

that

of the country (the and Lavia holes, the

southern

and

which is practically impermeable (hydraulic conductivity < lo-” m/s, porosity < 0.01) outside fractures. On the other hand, in a few hundreds or thousands of years after the onset of flow the temperature gradient is again very straight and stable within the errors of measurement on both

western parts of Finland are geothermically different from the rest of the country. This view is supported by the radiogenic heat production data of the holes and the heat production map of Finland (Kukkonen, 1988).

sides of the fracture

Conclusions

zone (Lewis and Beck, 1977;

Drury, 1984). These periods are geologically very short. The aperture and hydraulic properties of fractures can be changed by tectonic events, e.g. earthquakes, but the most plausible factors in the Baltic Shield are the removal of the continental ice load about 10,000 years B.P. and the subsequent and still continuing isostatic rebound of the lithosphere. The implications for palaeoclimatic corrections of heat flow data are obvious. Detection and identification of the palaeoclimatic (conductive) disturbances in a drill hole depend very much on the intensity of groundwater flow in the surrounding bedrock. This may be the reason why palaeoclimatic disturbances are sometimes reported (e.g., Crain, 1968; Cull, 1979) and some-

The

present

Shield indicate by groundwater

results

from

the

central

Baltic

that heat flow disturbance induced flow may be much more common

in the crystalline bedrock than previously realized. The gradient and heat flow density changes (typically 5-10 mW/m2), which were attributed to groundwater flow in the fracture zones, were encountered at depths of 78-496 m, but anomalies revealing flow in the holes between separate fracture zones were encountered even deeper, down to 930 m. The presence of fresh or slightly saline groundwater in the holes at the depths of the flow zones, together with the estimated values of mass flow rates (lop5 . ..10p3 kg s-i m-i), and the

73

corresponding indicate

average

flow velocities

that the (meteoric)

depths

of several

circulation

hundreds

Buntebarth,

( - 10 m/y) extends

of metres

to

in the bed-

rock.

G.,

Springer, Cermak,

new heat flow values presented further that the southern and western parts of

Finland

form

a geothermal

province

with

flow densities

higher than in the surrounding

of the central

Baltic Shield.

heat parts

KAPG

research

project

financed

by the Finnish

support and criticism; to the many colin exploration companies and other for allowing

me to log the holes and

take conductivity samples; to L. Rybach Zurich, Switzerland) and an anonymous

(ETH, referee

for their critical reviews of the manuscript; G. Hakli for revising the English.

and to

Crain,

I.K.,

Planet.

Beck, A.E., 1977. Climatically

perturbed

on regional

means. Tectonophysics,

and

Blomqvist,

temperature

1987a.

liquid wells. Geophysics,

okumpu

Rep. YST-56, Blomqvist, cambrian Toronto,

Geol.

Surv.

Geochemical bedrock

distribution

Bur.

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the significance

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Geophys.,

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M.J. and Jessop, A.M., 1982. The effect of a fluid-filled

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Geother-

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M.J. and Lewis, T.J., 1983. Water movement

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