GEUS Bulletin 9, Scientific results from Lopra-1, pp. 91-107

Thermal structure of the deep Lopra-1/1A borehole in the
Faroe Islands

Niels Balling, Niels Breiner and Regin Waagstein

Information on temperature, temperature gradients, thermal conductivity and heat flow from the c.
3.5 km deep Lopra-1/1A borehole in the Faroe Islands is presented and analysed. The upper 2450 m
of the drilled sequence consists of thick tholeiitic basalt flows and the deeper parts of hyaloclastites
and thin beds of basalt. Temperature data originate from high precision temperature logging a long
time after drilling to a depth of 2175 m (the original Lopra-1 borehole) and from commercial tempe-
rature logs measured a short time after drilling to a depth of 3430 m (Lopra-1/1A). The high-preci-
sion temperature log determines accurately levels of inflow of groundwater to the borehole and signif-
icant thermal disturbances to a depth of c. 1250 m. Below 1300 m, no significant disturbances are
seen and interval temperature gradients for large depth intervals show only small variations between
28 and 33°C/km. The mean least-squares gradient for the depth interval of 1400-3430 m is 31.4°C/
km. In clear contrast to these overall very homogeneous, large-interval, mean temperature gradients,
great local variability, between gradients of 20-25°C/km and 45°C/km, was observed between about
1300 and 2175 m (maximum depth of the high-resolution temperature log). These gradient varia-
tions are interpreted to be due to thermal conductivity variations and to reflect varying secondary
mineralisation and mineral alterations.

A preliminary analysis of the Lopra-1/1A temperature-depth function in terms of long-term pal-

aeoclimatic signals indicates subsurface temperatures below about 1300 m to be in equilibrium with
mean surface temperatures significantly below zero during the last glacial period. A subsequent tem-
perature increase of 12-16°C occurred at around the termination of the last glaciation. The measured
temperatures (some after correction) and the thermal regime below 1300 m seem to represent con-
ductive equilibrium conditions without significant disturbances from the effect of drilling, ground-
water flow or long-term palaeoclimatic surface temperature variations.

Thermal conductivity measured on samples of basalt taken from drill cores and surface outcrops in

the area of the borehole shows values within a rather narrow range and a well-defined mean value for
low porosity basalts of about 1.8 W/m°C , while a few samples of lapilli-tuff/tuff from the borehole
gave values around 1.9 W/m°C . Lapilli-tuff and tuff seem to have higher matrix (grain) conductivity
than basalt. Heat flow is estimated at 60 ± 5 mW/m

. A heat flow of this magnitude is consistent with

the Faroe Islands being underlain by continental crust.

Keywords : Lopra-1/1A borehole, Faroe Islands, temperature gradients, thermal conductivity, heat flow

____________________________________________________________________________________________

N.B. & N.B., Department of Earth Sciences, University of Aarhus, Finlandsgade 8, DK-8200 Aarhus N, Denmark.
E-mail: niels.balling@geo.au.dk
R.W., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Deep boreholes generally provide the most reliable and
undisturbed direct information on temperature, tempe-
rature gradients, thermal conductivity and heat flow. Tem-
peratures and temperature gradients measured in shallow
boreholes may be perturbed for a variety of reasons in-
cluding local effects of groundwater movements, topo-
graphy and short- and long-term palaeoclimatic surface tem-

perature variations. The deep Lopra-1/1A borehole pro-
vides a unique opportunity for obtaining deep thermal
information from an area of the North Atlantic that is of
considerable interest to both the general geoscience com-
munity and to the hydrocarbon industry.

The Lopra-1/1A borehole is situated in the southern

island (Suðuroy) of the Faroe Islands (Fig. 1) (at 61°26

36"N, 6°4630"E). It was drilled in 1981 as a research

borehole to a depth of 2175 m below ground level (Ber-
thelsen et al. 1984). In 1996 the borehole was re-entered
by a consortium of exploration companies and deepened
to 3565 m measured depth corresponding to a vertical
depth below ground level of 3540 m. All depth values
given in this paper (if not stated otherwise) are vertical
depths measured from ground level 8.8 m above mean sea
level.

Thermal measuring results from the original Lopra-1

borehole were presented by Balling et al. (1984). The pur-

pose of the present paper is to integrate, analyse and dis-
cuss all available thermal information from the whole
depth range of the Lopra-1/1A borehole. It includes new
high-precision continuous temperature logging results
from the original hole measured a long time after drilling,
temperature measurements from the deepened part acquired

as part of the commercial logging runs during and shortly
after drilling. New thermal conductivity measurements
from core material from the deepened section and from
surface exposures in the Lopra-1/1A area have also been
made.

Temperatures and temperature gradient variations are

analysed in relation to disturbances from groundwater
flow, variations in rock thermal conductivity and infor-
mation on long-term palaeoclimatic surface temperature
variations. A new terrestrial heat-flow value for the Lo-
pra-1/1A site is presented.

Geological environment and lithology

The volcanic succession of the Faroe Islands

The Faroe Islands form part of the Palaeogene North Atlan-

tic Province of tholeiitic flood basalts. The Faroe volcanic
succession has been divided informally into the upper,

Fig. 1. a : Location map of the Faroe Islands in the northern North
Atlantic. b : The Lopra-1/1A borehole was drilled in the southern
Faroese island of Suðuroy. The total thickness of volcanic sequen-
ces in the Faroe Islands is at least 6.5 km. About 3 km is exposed,
and 3.54 km was drilled at Lopra-1/1A. Dashed lines A and B
mark boundaries between lower and middle and middle and up-
per basalt series respectively.

dle series

exposed

drilled

upper series

er series

3.5

FAROE
ISLANDS

Sandoy

Suðuroy

Lopra

Streymoy

FAROE

ISLANDS

1000

SCOTLAND

1000

60°

20°

60°

10°

0°

70°

20°

10°

0°

JAN MAYEN

GREENLAND

70°

ICELAND

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

middle and lower basalt series or formations with a total
thickness of more than 6.5 km (Rasmussen & Noe-Ny-
gaard 1970; Waagstein 1988). The lowermost 3.5 km,
which is entirely in the lower basalt formation, is known
only from the Lopra-1/1A borehole (Fig. 1).

Both seismic and other geophysical evidence (Bott et al.

1974; Richardson et al. 1998) and geochemical data (Ga-
rièpy et al. 1983; Hald & Waagstein 1983; Holm et al.
2001) indicate that the Faroe Islands are underlain by
continental crust. Pre-volcanic rocks have not been reached
by drilling. Linear magnetic anomalies associated with
oceanic crust occur 60-70 km north of the islands (Skog-
seid et al. 2000).

The flood basalts were formed by extensive volcanism

associated with the continental splitting between NW
Europe and East Greenland in upper Paleocene to lower-
most Eocene time (e.g. Skogseid et al. 2000). Larsen et al.
(1999)

used geochemical analyses and stratigraphic corre-

lations between the volcanic successions in the Faroe Is-
lands and East Greenland to interpret the Faroese lower
basalt formation as a pre-breakup sequence and the mid-
dle and upper basalt formations as syn-breakup sequen-
ces. Since deposition of the upper basalt formation, little
or no deposition has occurred in the Faroes. Volcanic ac-
tivity continued, however, on the Greenland side of the
rift with the eruption of an additional 3-3.5 km of ba-
salts in an area then located close to the centre of the Ice-
landic mantle plume.

Lithology

The upper 2450 m of the Lopra-1/1A borehole consists
of subaerial lava flows of tholeiitic basalt with an average
thickness of about 20 m. Most of the flows have a massive
core and a vesicular rubbly top. The lavas are commonly
separated by palaeosols made up of volcanic ash or mate-
rial eroded from the flow tops. The sediments range from
a few centimetres to more than 4 m in thickness. The deep-

er part of the well, from about 2450 m to total depth,
consists of hyaloclastites (lapilli-tuff and tuff ) and thin
beds of basalt.

Since deposition, secondary mineralisation and miner-

al alterations have occurred. The bulk thermal properties
of the basaltic sequences seem to be controlled mainly by
the two major minerals feldspar and pyroxene, which oc-
cur in roughly equal amounts in common basalts. How-
ever, it appears from our thermal gradient analysis that
within-flow variations of the degree of alteration of the
basalt is an important controlling factor for local varia-
tions in rock thermal properties and hence temperature

gradient variability. Some information on secondary min-
eral alterations and mineralisation is thus required for a
proper thermal analysis.

Olivine is a minor constituent that has been generally

replaced by clay. Haematite has formed from iron-rich
minerals under oxidising conditions, especially within flow
tops and interbasaltic sediments. The original plagioclase
feldspar is partly or completely replaced by albite in the
deepest part of the borehole due to very low-grade burial
metamorphism. Originally variable amounts of intersti-
tial glass representing frozen melt are completely altered
to clay and other secondary minerals. Most gas vesicles
and pores and fractures once filled with free water are now,
particularly at great depth, partly or completely filled with
low-temperature minerals deposited by flowing ground-
water. These mineralisations consist dominantly of clay
and zeolite minerals, silica minerals (chalcedony, agate,
quartz) or calcite. The vertical distribution of secondary
minerals and zeolite zones of the Lopra-1/1A borehole are
described by Jørgensen (1984, 2006, this volume). The
lithology and chemistry of the upper 2.2 km sequence is
described in detail by Hald & Waagstein (1984).

Temperature and temperature
gradients

Temperature logs and conditions of
measurements

Temperature information is available from several tem-
perature logs. Measurements have been carried out by sev-
eral techniques, either as dedicated temperature logging
or in combination with other log operations. Tempera-
tures were measured both during interruptions in the drill-
ing and after the drilling was completed. This applies both
to the original Lopra-1 borehole and to the deepened part
of Lopra-1/1A.

Temperature logs were run in the original Lopra-1 bore-

hole by The Icelandic Energy Authority, Orkustofnun,
the operator of the original hole. The last one was run in
1983, 17 months after drilling. These results are described
in detail in Balling et al. (1984). A more detailed contin-
uous temperature log has since been run in the original
Lopra-1 hole. It was carried out by the present research
group in 1994 to a depth of 2175 km, almost 13 years
after drilling, using the high-precision quartz-oscillator
system of the University of Aarhus. Measurements were
taken while running down-hole at a nominal speed of 10
cm/s with a sample rate of two seconds resulting in a sam-
ple interval of about 20 cm. Relative temperature resolu-

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

tion is better than 0.005°C and absolute accuracy is cali-
brated to about 0.05°C.

All temperature logs from the Lopra-1/1A deepened

section below 2175 m were acquired by the company
Schlumberger in combination with other logging opera-

tions relatively soon after drilling activities and circula-
tion of drilling fluid. The temperature data are available
as standard six-inch point measurements taken by ther-
mistor probes and are estimated to have an accuracy bet-
ter than 0.1°C.

Several temperature logs are thus available both for the

original Lopra-1 hole and the deepened Lopra-1/1A sec-
tion. Five of these have been selected as those giving the
most valuable information for interpretation (Table 1 and
Fig. 2). They cover depths from the surface to 3430 m.

Drilling

and

circulation

drilling

fluid

disturb

signifi-

cantly

the

temperature

structure

the

borehole

and

the

unperturbed

so-called

equilibrium

temperature-depth

distribution can be measured only a relatively long time af-

ter drilling. If sufficient time has not passed, corrections
to measurements must be applied (cf. Beck & Balling
1988).

In general, during circulation of drilling fluid, the up-

per part of the hole is heated and the lower part is cooled.
The time needed for a borehole to reach temperature equi-
librium depends on several factors including drilling his-
tory, temperature of the drilling mud and the required
accuracy of temperature and temperature gradients, but
may be relatively long compared to the duration of the
drilling. For deep boreholes like the Lopra-1/1A borehole,
at least one to two years may be needed to obtain both
accurate equilibrium temperatures over the whole section
and accurate local temperature gradients. Only for the
bottom part of the hole and the neutral zone of least dis-
turbances may near-equilibrium temperatures be meas-
ured much sooner after drilling and last drilling fluid cir-
culation.

As mentioned above, temperature measurements were

carried out in the original Lopra-1 borehole to a depth of
2175 m a long time (up to almost 13 years) after drilling.
The temperature logging results from the new section

Fig. 2. Selected measured temperature-depth profiles. Logs 1 and
2 are from the original borehole Lopra-1 and logs 3, 4 and 5 were
measured in the deepened part. Information on time of measure-
ments, details of depth intervals and time after drilling or last
drilling fluid circulation are given in Table 1. Log temperatures
presented here are uncorrected. Corrected temperatures are given
in Fig. 3.

Temp.
log

1
2
3
4
5

Date of

measurement

23.03.1983
04.08.1994
01.10.1996
02.10.1996
30.10.1996

Depth of

borehole (m)

2175
2175
3144
3144
3496

Measuring agency

Orkustofnun, Reykjavik

University of Aarhus

Schlumberger, Esbjerg
Schlumberger, Esbjerg
Schlumberger, Esbjerg

0-1974
0-2175

3020-3095
2170-3075
2990-3430

Time after

drilling/circulation

17 months

12 years 9 months

27 hours
50 hours
53 hours

Depth interval of

temperature data (m)

Table 1. Basic information about temperature logs from the Lopra-1/1A borehole

Five logs have been selected to give the most valuable temperature information.
The time elapsed between the last significant disturbance from drilling or last circulation of drilling fluid and temperature logging.

400

800

1200

1600

2000

2400

2800

3200

3600

Depth (

)

-10

100

Temperature (°C)

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

drilled in 1996 were, however, carried out no more than
27-53 hours after last drilling fluid circulation, cf. Table
1, and corrections must be considered.

Temperature disturbances are created not only by the

process of drilling. The upper part of the borehole is also
affected by upward water flow inside the borehole. After
drilling, the Lopra-1 hole (total depth 2175 m) started to
flow at a rate of about 10 l/min when the drilling mud
was replaced by fresh water of lower density. The well head
was closed between the end of drilling and the time of log
number 1 (17 months after drilling, cf. Table 1) prevent-
ing water from flowing freely to the surface. The hole was
opened on the day of temperature logging and after one
hour it began to flow at a rate of about 10 l/min. Tempe-
rature measurements were carried out from 3 to 6 hours
after opening. At the time of log number 2, measured in
1994, the hole was flowing freely at about the same rate
and had not been closed for several years.

The highest recorded temperature of 98.6°C was meas-

ured at a depth of 3430 m, 53 hours after circulation
when the hole was 3496 m deep (log 5 from 30 October
1996). The deepest point of temperature information is
3527 m where 92°C was measured on 2 November 1996,
17 hours after the latest drilling fluid circulation. Two
days later, when the drilling had reached its final vertical
depth of 3540 m (3565 m measured depth below rotary
table), a temperature of 91°C was measured at 3507 m,
22 hours after circulation of drilling fluid. These lower
temperatures measured later at slightly deeper levels demon-

strate the effect of cooling by drilling fluid circulation.

Correction of temperatures

A comparison of the raw temperature data of logs 1 (meas-
ured in 1983) and 2 (measured in 1994) shows a differ-
ence of 2-3°C at depths below 400 m and an almost con-
stant offset of 2°C between 1000 and 1600 m. Such an
almost constant difference is very unlikely to be caused by
water flow or any other effect associated with the bore-
hole and, from further data analysis, this difference is ascri-
bed to an instrumental calibration offset in log 1 by about
2.0°C. (Equipment used for log 2 measurements was care-
fully calibrated before and after logging.) After adding
2.0°C to the original log 1 values, log 1 and log 2 mea-
surements agree to within ± 0.2°C between 1000 and 1600

m, increasing to a maximum difference of 0.7°C at 1974
m, the maximum depth of log 1. In the topmost part of
the borehole, log 2 shows slightly higher temperature dif-
ferences (by up to 2-3°C) due to a longer time of tempe-
rature disturbance from up-hole water flow (cf. Fig. 2).

Measured log 1 and log 2 temperatures are both clearly

elevated in the upper part of the hole because of water
flow. At near-surface level, measured temperatures (Fig. 2)

are well above the mean ground temperature of about 7°C.
By temperature gradient analysis (se below), levels of wa-
ter inflow have been localised accurately. Below a depth
of about 1250 m, measured temperatures of logs 1 and 2
are unlikely to be disturbed significantly by flow of water
in the borehole, and log 2 temperatures are assumed to
represent undisturbed equilibrium values.

The temperatures on logs 3, 4 and 5, from depths be-

tween 2170 and 3430 m (Table 1 and Fig. 2), were meas-
ured between 27 hours (log 3) and 53 hours (log 5) after
drilling fluid circulation following drilling activities and
are thus disturbed. Measured temperatures on logs 3, 4
and 5 are, due to their deep position in the hole, lowered

400

800

1200

1600

2000

2400

2800

3200

3600

Depth (

)

-10

100

Temperature (°C)

Fig. 3. Measured temperatures on log 2 and corrected tempera-
tures of logs 4 and 5. Corrections were applied for the estimated
effect of drilling and drilling fluid circulation. The dashed line
has a constant gradient of 31.4°C/km calculated by least-squares
for the depth interval of 1400-3450 m and extrapolated to the
surface. The upper part of the borehole (above c. 1200 m) is dis-
turbed by upward flow of water inside the borehole. The negative
temperature at the surface intercept indicates that temperatures
below about 1200-1300 m are in equilibrium with a palaeosur-
face-temperature significantly below that of the present-day mean
surface temperature of about 7°C.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

by the circulation of drilling fluid to temperatures below
formation temperature. They are thus all lower than the
undisturbed formation equilibrium values.

The depth intervals over which measurements were

made at different times overlap partly. This makes it pos-
sible to estimate the size of temperature disturbances and
correct for them. The upper part of log 4 overlaps with
the lowest part of log 2 in the depth interval 2170-2175 m.

Log 4 temperatures were here 3.7°C below the tempera-
tures of log 2, which are assumed to be undisturbed. The
increase in temperature between logs 3, 4 and 5 (Fig. 2),
combined with additional log data not shown, has been
used to estimate the amount of disturbance by Horner-
type analysis. In the deepest part of log 5 a temperature of
98.6°C was measured at a depth of 3430 m, 53 hours
after drilling fluid circulation. Temperatures are estimat-
ed to have been reduced by 3-7%, so, applying a correc-
tion of 5%, the undisturbed value is about 103.5°C. In
the final selection of temperature data, only the almost

linear part of log 4 (2170-2775 m, Fig. 2) with correc-
tions between 3.7°C (top) and 4.0°C (bottom) was used.

After corrections of logs 4 and 5 for the estimated dis-

turbance due to drilling, the corrected temperatures fol-
low the same depth trend as that of the deeper part of log
2. Below 1100 m, measured temperatures on log 2 and
the corrected values on logs 4 and 5 fall within 1.2°C of a
constant gradient least-squares temperature line (Fig. 3).
As discussed above, the corrected temperatures on logs 4
and 5 are thought to represent equilibrium temperatures
to a good approximation (within 1-3°C) and to be suffi-
ciently accurate to calculate accurate mean temperature
gradients for the larger depth intervals. Measured tem-
peratures on log 2 (below 1200 m) and corrected values
on logs 4 and 5 are listed at 100 m depth intervals in
Table 2.

Equilibrium temperatures and temperature
gradients

Mean least-squares temperature gradients from selected
logs and depth intervals have been calculated and are list-
ed in Table 3. In order to ensure a homogeneous base for
the calculation of temperature gradients, all logs were re-
sampled to depth increments of 5 m. Mean gradients show
only small variations between 28 and 33°C/km. Log 2
yields a mean gradient of 32.9°C/km between 1400 and
2175 m and the combined data from logs 2, 4 and 5 for
the depth interval 1400 to 3430 m yield a temperature
gradient of 31.4°C/km. This demonstrates an overall very
homogeneous thermal gradient structure.

In clear contrast to the above overall small gradient

variations, significant local temperature gradient variabil-
ity is observed. Figure 4 shows running mean least-squares
interval temperature gradients (covering 5, 25 and 100 m
depth intervals) derived from the high-resolution log 2
run from surface to 2175 m.

Table 2. Listing of selected temperatures from

logs 2, 4 and 5

1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
3200
3300
3400
3430

66.2
69.1
72.0
74.7
77.8
81.5

n.d.

90.5
93.7
96.4
98.9

102.4
103.5

34.1
36.2
39.4
42.6
46.1
49.4
52.3
55.6
58.8
62.5

Temperatures

(

C)*

Depth (m)

Log 2

Log 4

Log 5

Measured temperatures above 1200 m are disturbed by water
flow inside the borehole (cf. Fig. 3) and are not listed.
Temperatures of log 2 are assumed to represent accurately the
undisturbed equilibrium temperatures.
Temperatures of logs 4 and 5 are corrected for the estimated
influence of drilling disturbances and may represent equilibrium
temperatures to within ± 1-3°C.

n.d.: no data.

Table 3. Least-squares mean temperature gradients for

various depth intervals

Depth interval

(m)

1400-2175
2170-2770
2995-3430
1400-3430

Temp. gradient

(

C/km)

-6.7

0.0
6.2

-3.9

Log

2
4
5
2, 4, 5

32.9
30.1
28.2
31.4

* Intercept temperature value (linearly extrapolated temperature at

Intercept

(

zero depth) associated with each depth section.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

Fig. 4. Running mean least-squares tempera-
ture gradients for 5, 25 and 100 m depth
intervals as indicated. Temperature gradients
were taken from the high resolution log 2.
The dashed line shows the assumed
unperturbed mean background gradient of
32.9°C/km calculated for the depth interval
of 1400-2175 m (cf. Table 3). Levels of
significant inflow of groundwater into the
upper part of the borehole are clearly seen as
local gradient maxima. Below about 1250 m,
temperatures and temperature gradients are
thought to represent generally conductive
equilibrium conditions and gradient vari-
ability is mostly due to variations in rock
thermal conductivity (see also Fig. 5).

Depth (m)

200

400

600

800

1000

1200

Depth (

)

1400

1600

1800

2000

2200

5 m

25 m

100 m

Temperature gradient (°C/km)

The upper part of the log is disturbed by water flow.

The original hole was uncased below 180 m and water at
above hydrostatic pressure was able to enter the hole
through local fractures or permeable beds. The tempera-
ture and temperature gradient logs combined show clear-
ly levels of significant disturbance due to inflow of water
to the borehole. They are characterised by a locally high
temperature gradient. Above the level of inflow, both tem-
perature gradients and temperatures are reduced. At the
approximate depths of 292, 360, 444, 1111, 1132 and
2130 m, temperature drops of between 0.2 and 0.6°C are
observed, resulting in locally high temperature gradients.
The highest local temperature anomalies of 0.5-0.6°C
occur at 444 and 1132 m, where also maximal tempera-
ture gradient perturbations are observed (Fig. 4). A local
minor disturbance is seen around 1538 m (see also Fig.
5). Most of these thermal perturbations, in particular those
above 1100-1200 m, are easily interpreted in terms of
inflow of ground water at approximately local formation
temperature into a section of the borehole where tem-
peratures are artificially elevated due to upwards-flowing

water coming from deeper levels of higher formation tem-
perature. Local lowering of borehole temperature may also
occur if water flows downwards through inclined frac-
tures from levels of lower formation temperature to levels
of higher formation temperature. However, this does not
seem to have happened here.

Longer wavelength temperature-gradient minima are

observed at about 550-800 m and 1160-1250 m that are
not clearly associated with localised zones of inflow of water
(Fig. 4). These zones of low gradient may be due to broader
zones of water flow into the borehole, perhaps associated
with an almost steady migration of ground water within
porous or fractured parts of the formation that was initi-
ated long before drilling.

Below 1200-1300 m, the temperature-depth function

of log 2 follows the overall almost linear trend of tempe-
rature increase with depth (Fig. 3). This trend is over-
printed by significant local temperature gradient varia-
tions down to the maximum depth of log 2 of 2175m
(Fig. 4). The local gradient variations are of a different
character from those discussed above, but might at first

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

sight indicate similar disturbances from flow of water.
However, a detailed comparison between temperature gra-
dient variability and other petrophysical log characteristics,

including density and neutron porosity logs as well as cal-
liper log data, shows remarkable correlations beginning at
a depth of 1250-1300 m (Fig. 5). Mean least-squares tem-
perature gradients based on 5 m averaging intervals reveal

local maximum gradients of up to 40-45°C/km and local
minimum values down to 20-25°C/km. Local intervals
of high temperature gradient correlate with intervals of
high density, low neutron porosity and decreased calliper.
Intervals of low temperature gradient correlate with in-
tervals of low density, high neutron porosity and increased
calliper. The correlation between temperature gradient var-
iability and short-range variations in borehole calliper is
particularly remarkable. Maximum temperature gradient
variability and close correlations are most pronounced
around 1450-1550 m and 1750-1850 m, but a good gen-

eral correlation with the physical properties of the various

basaltic flow units is observed for most of the depth range
shown in Fig. 5.

A correlation of low calliper with increased tempera-

ture gradients cannot be explained by potential water flow
inside the borehole at these greater depths. Any flow of
water would have the opposite effect of lowering gradi-
ents in narrow parts of the borehole due to locally increased

flow rates. Above a depth of about 1200 m no correlation
is observed. Here temperature gradients seem to be con-
trolled by the flow of water. The temperature gradient
variations below about 1250 m consequently need to be
explained in terms of a linkage between resistance to the
drill bit, the mineralogical composition and structure and
bulk rock thermal properties of the formation. These ob-

served correlations and the inferred variations in rock ther-
mal conductivity are discussed in detail below.

Influence of palaeoclimate

Surface temperature variations penetrate into the subsur-
face, and temperature and temperature gradient measure-
ments from boreholes may be used to extract information
on short-term as well as long-term surface palaeoclimatic
temperature variations (e.g. Dahl-Jensen et al. 1998;
Huang et al. 2000; Kukkonen & Joeleht 2003). The Lo-
pra-1/1A borehole is not particularly well-suited for such
purpose due to disturbances of the upper part of the hole
by water flow. We shall not, therefore, go into detail, but
deal only with some main effects of long-term climatic
variations.

Simple linear extrapolation of the temperature-depth

function from the deeper parts of the borehole to the sur-
face yields a negative intercept temperature of -3.9°C for the

depth interval of 1400-3430 m and of -6.7°C for the
interval 1400-2175 m (Fig. 3 and Table 3). The low inter-
cept temperatures indicate that temperatures in the deeper

parts of the borehole are in equilibrium with a surface
temperature significantly below the present-day mean
ground temperature of about 7°C. This simple linear extra-
polation of deep mean temperature gradients seems justi-
fied in our preliminary study because of the apparent ho-
mogeneity of mean thermal conductivity and tempera-
ture gradients over large depth intervals (see also next sec-
tion). The shorter intervals of 2175-2770 m and 2995-
3430 m give higher intercept values, but these are consid-
ered uncertain because of the short length of the intervals
that make the extrapolation more sensitive to local varia-
tions in thermal conductivity.

Forward

thermal

modelling using a thermal diffusivity

of basalt of 0.7

-6

/s shows that long-term surface

temperature variations of the magnitude associated with
glaciation and deglaciation are reduced in amplitude to
less than 1-2°C (depending on surface temperature am-

plitudes) at depths below 1300-1500 m. The most accu-

rate temperature gradient and temperature intercept val-
ue is probably that from the depth interval 1400-2175
m. Since porosity is likely to decrease with depth, porosi-
ties in the rocks sampled by the upper part of the bore-
hole may on average be slightly greater than in those sam-
pled by the deeper parts. This could point towards a slightly
lower thermal conductivity and hence a slightly higher
temperature gradient at shallow depths. This effect may,
however, be more or less cancelled by secondary mineral-
isation, which tends to increase the conductivity of po-
rous sections. This means that the mean characteristic
conductivity of the upper 1400 m may not differ much
from that of the interval 1400-2175 m, justifying the
extrapolation with a constant gradient.

We may thus interpret the surface intercept tempera-

ture of -6.7°C as an estimate of the long-term character-
istic mean for the cold period of the last glaciation. We
estimate the increase in surface temperature associated with
the termination of the last glaciation to be of the order of
12-16°C. This is a preliminary estimate considering the
approximate nature of our procedure, uncertainties relat-
ed to the lack of good temperature data from the upper
part of the borehole(which prevents extraction of a detailed

past

temperature-time

function)

and

uncertainties related

to possible vertical variations in thermal conductivity.

A more detailed analysis of this problem, including in-

verse modelling, must be based on thermal information
from other boreholes, in particular from near-surface in-

GEUS Bulletin no 9 - 7 juli.pmd

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100

tervals where the Lopra-1/1A borehole is disturbed ther-
mally. This is beyond the scope of this paper. From in-
verse analyses of the temperature-depth function from
the GRIP-borehole on the Greenland ice sheet, Dahl-Jen-
sen et al. (1998) calculated a surface-temperature increase
of 23°C following the last glacial maximum. Using a sim-
ilar procedure on many boreholes, Kukkonen & Joeleht
(2003) obtained an average warming of 8°C for NW Eu-
rope at the termination of the last glaciation.

The deviation in the upper part of the Lopra-1/1A bore-

hole from the general linear trend of temperatures deeper
down (Fig. 3) thus has two main causes. The increase at
surface level from about 7°C to the measured borehole
temperature at surface of about 12°C (log 2) is due to the
upward flow of warm water inside the borehole. The re-
maining part, the increase from negative intercept values
to about 7°C, is interpreted to reflect the increase of tem-
perature at around the termination of the last glacial peri-
od. The penetration of this heating effect to a depth of
about 1200 m (Fig. 3) is in agreement with model calcu-
lations. Unfortunately, this depth level is also the approxi-

mate level above which temperatures and temperature
gradients are significantly disturbed from the flow of water

inside the borehole.

Thermal conductivity

A limited amount of suitable sample material was avail-
able for thermal conductivity measurements. Only one con-

ventional core was drilled within the deepened section
below 2175 m. However, a few rotary sidewall cores were
also long enough to be measured. Including published
measurements on four cored sections from the original
borehole, a total of 11 samples have been measured cover-
ing the depth range of 337 to 3531 m (driller's depths).
Rock materials measured represent massive basalt (7 sam-
ples), lapilli-tuff (3 samples) and tuff (1 sample). In addi-
tion to thermal conductivity, rock density and porosity
were also measured. Measuring results on samples from
the Lopra-1/1A borehole are summarised in Table 4. Sup-
plementary preliminary conductivity measurements were
additionally carried out on 14 samples taken from surface
exposures of basalts near the borehole.

Measuring techniques

Both the needle-probe transient line source technique and
the steady-state divided bar technique were applied to meas-

ure thermal conductivity. These are standard techniques

Table 4. Measured thermal conductivity, porosity and density and calculated matrix (grain) thermal conductivity and density

C1
C2
C3
C4
C5
Swc 46
Swc 37
Swc 13
Swc 6
Swc 5
Swc 4

Mean values, all samples (11)
Standard deviation

Mean values, basalt (7)
Standard deviation

Mean values, lapilli-tuff/tuff (except outlier Swc 37) (3)
Standard deviation

Sample

basalt
basalt
basalt
basalt
basalt
basalt
lapilli-tuff
lapilli-tuff
tuff
lapilli-tuff
basalt

337.5
860.1

1218.1
2177.3
2380.0
2441.0
2562.0
3438.0
3512.5
3514.5
3531.0

Depth*

(m)

1.75
1.85
1.74
1.79
1.79
1.87
1.35
1.88
1.91
1.93
1.84

1.79
0.16

1.80
0.05

1.91
0.03

Thermal

conductivity

(W/moC)

1.81
1.95
1.80
1.82
1.85
1.96
1.60
2.29
2.17
2.27
1.90

1.95
0.21

1.87
0.07

2.24
0.06

Matrix

thermal

conductivity

(W/moC)

2.96
2.98
2.94
3.00
3.00
2.93
2.47
2.76
2.78
2.68
3.10

2.87
0.18

2.99
0.06

2.74
0.06

Bulk

density

(10

kg/m

)

3.02
3.07
3.00
3.03
3.06
3.01
2.78
3.07
2.98
2.91
3.16

3.01
0.10

3.05
0.05

2.99
0.08

Matrix

density

(10

kg/m

)

3.0
4.5
3.0
1.7
2.7
3.9

17.3
14.8

9.9

12.1

2.7

6.9
5.6

3.1
0.9

12.3

2.5

Porosity

(%)

Rock

type

* Driller's depth below rotary table (to top of core).

Samples C1 to C5 are from conventional cores taken during drilling.

The Swc samples are small rotary sidewall cores taken after drilling.

GEUS Bulletin no 9 - 7 juli.pmd

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100

101

(l-

)

for laboratory rock thermal conductivity measurements
(e.g. Beck 1988). All measurements were carried out in
the

geophysical

laboratories

the

University

Aarhus.

Equipment and measuring procedures were similar to
those described in Balling et al. (1981).

Most rock materials from the deepened section of the

borehole and from surface exposures were measured by
the needle probe technique. The needle probes used have
a nominal length of 50 mm and an outer diameter of 1.5
mm. The measured sidewall cores are cylindrical with a
diameter of about 24 mm and lengths ranging from 18 to
30 mm. The probe length cannot be reduced significant-
ly since interpretation is based on line source approxima-
tions. The probe length thus exceeds that of the samples
to be measured. This difficulty was largely overcome by
placing the rock sample of unknown conductivity along
the critical central position of the needle probe where the
temperature rise function is measured and extending the
sample by 'end materials' of known conductivity close to
that of the material to be measured. By iterative trial and
error procedure, the difference between conductivity of end

materials and conductivity of rock sample was reduced to
less than 0.3 W/m °C. Experience suggests that this is
sufficiently small for an accurate sample conductivity
measurement. Any further boundary effects due to the
small size of samples were minimised by immersing the
samples in water-saturated sand with conductivity close
to that of the rock samples being measured.

All samples were water-saturated under vacuum before

measurement, which was carried out at normal laborato-
ry temperature (about 20°C) and pressure (1 atm.) con-
ditions. All needle probes were calibrated and tested by
measurement of standard materials of known thermal con-
ductivity. The heating period for the probes was 40-60
seconds and the temperature rise at probe centre typically
2-4°C. Sample conductivity was determined as a mean
value of at least three individual measurements and indi-
vidual measurements on the same sample generally did
not differ by more than 3-5%. The unknown thermal
conductivity was calculated from the temperature rise data
using the iterative least-squares inversion technique of
Kristiansen (1991).

Samples c1 to c5 (Table 4) were originally measured by

the divided bar technique and the results were reported in
Balling et al. (1984). These older measurements seem to
be somewhat too low. A comparison of new needle probe
measurements with old divided bar measuring results on
material from the same basalt cores shows that the old
measurements are systematically about 15% too low. The
older low results seem due to the small dimensions of the

samples. Previous measurements are thus corrected by +

15%. Considering all sources of experimental uncertainty,

reported conductivity values are estimated to be accurate
to ± 0.1 W/m °C.

Rock bulk density of water saturated samples and poro-

sity were measured on all samples. Density was measured
by using the Archimedes principle of buoyancy. Weight
of samples in air and immersed in water, respectively, yields
sample weight and volume. Porosity was determined by
measuring loss of weight of the water-saturated samples
when drying them at about 110°C for 1-2 days. Repeat-
ed determinations of porosity on selected samples suggest
a precision of

1% for low porosity samples (2-5%) and

1-2% for samples of higher porosity (10-20%).

With known porosity, bulk density and bulk thermal

conductivity, the solid matrix (grain) density and thermal
conductivity may be estimated. The computation of ma-
trix density is straightforward assuming proportional con-
tribution of solid matrix and water. Matrix thermal con-
ductivity was computed from the geometric mean for-
mula relating bulk conductivity, k_b, matrix conductivity,

k_m, conductivity of water, k_w, and porosity (pore fraction),

¢, by k_b

= k

. Bulk conductivity and porosity are

measured. The conductivity of water at room tempera-
ture is 0.6 W/m °C.

Measuring results

Thermal conductivity measuring results are listed together

with the density and porosity determinations in Table 4.
Mean values and standard deviations were calculated for
all 11 samples, for the basalts alone (seven samples) and
for lapilli-tuff/tuff alone(three samples, excluding the sam-

ple swc 37). Variations between samples are small. All ba-
salts are of low porosity(< 5%) and the conductivity is with-

in the range of 1.75-1.87 W/m °C with a well defined mean

value

1.80

W/m °C. The lapilli-tuff and tuff samples

have higher porosity (10-17%) and show a wider range of

conductivity,

between

1.35

and

1.93

W/m °C. Except for

sample swc 37, the lapilli-tuff/tuff samples have a higher
solid matrix conductivity than the basalts. Omitting swc
37, mean matrix conductivity of lapilli-tuff and tuff is
2.24 W/m °C as compared to 1.87 W/m °C for basalt.
This

difference

matrix

conductivity

explains

why

lapilli-

tuff and tuff have slightly higher conductivity than basalt
despite a higher porosity and higher content of free water
of low conductivity. The anomalous sample swc 37 has
the highest porosity(17.3%) and lowest bulk and matrix

conductivity measured. This is possibly due to the pres-
ence of a significant amount of secondary analcite, a min-

eral of very low conductivity(1.3 W/m °C)(Horai 1971).

GEUS Bulletin no 9 - 7 juli.pmd

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101

102

The thermal conductivity measurements on core ma-

terials from the Lopra-1/1A borehole have been supple-
mented by preliminary measurements of samples of ba-
salts taken from surface exposures in the local area of the
borehole. A total of 14 samples were measured. Measure-
ments were again carried out on water-saturated samples
using the needle probe technique. Eight samples of low
porosity( 4%) have a mean conductivity of 1.77 W/m °C

(range 1.67-1.86 W/m °C) and six samples of higher poro-
sity (porosity range 5-26% and mean porosity 13%) have
a mean of 1.51 W/m °C (range 1.38-1.59 W/m °C). The
conductivity of low-porosity basalts is thus well defined,
having a value of about 1.8

0.1 W/m °C. Conductivity

decreases with increasing porosity due to the presence of
water of low conductivity. Details of these measurements
are not shown, but the results conform well to and sup-
plement those from the borehole samples.

Other studies

Our conductivity results on basalts agree well with results
obtained by others. Oxburgh & Agrell (1982) measured
more than one hundred samples of basaltic flows, intru-
sions and breccias covering the full depth range of the 2
km deep Reydarfjordur borehole in eastern Iceland. Their
measurements on water-saturated samples show increas-
ing thermal conductivity with decreasing porosity and
increasing sample depth. Single measurements from basal-

tic flows and intrusions range between 1.4 and 2.2 W/m
°C. Mean values over 500 m intervals increased with depth
from about 1.6 in the upper part of the borehole to 1.7-
1.9 W/m °C in the central and deeper part of the hole.
Measurements on 17 rock samples classified as breccias
showed the highest values of conductivity and the widest
spread, 1.6-2.8 W/m °C.

As part of heat-flow measurements in shallow boreholes

in the south-eastern part of the Deccan Volcanic Prov-
ince, central India, Roy & Rao (1999) measured thermal
conductivity on about 25 core samples of basalt and sev-
eral samples of fresh massive basalt from outcrops. They
obtained sample values within the narrow range of 1.6-
1.8 W/m °C

with a well-defined mean value of about 1.7

W/m °C.

The same range of measured thermal conductivity is

found in the large dataset of Robertson & Peck (1974) on
basalts from Hawaii for water-saturated samples of low poro-

sity

(2%

about

10%)

and

low

olivine

content

(0-5%).

With increasing porosity and pore-water content, conduc-
tivity decreased significantly and variations in mineral
content played an important role (see also Horai 1991).

Conductivity variations at Lopra-1/1A and
their causes

In a conductive steady-state geothermal regime, variations
of the temperature gradient are related to variations in
rock thermal conductivity. Intervals of high thermal con-
ductivity result in low temperature gradients and inter-
vals of low thermal conductivity result in high tempera-
ture gradients. This simple inverse relation follows from
Fourier's law of heat conduction, which relates heat flow
to the product of thermal conductivity and temperature
gradient. For depth sections of low heat production, con-
ductive heat flow is almost constant and temperature gra-
dient variations will reflect variations in rock thermal con-
ductivity.

Water has a thermal conductivity of 0.6 W/m °C, which

is much less than a rock matrix of overall basaltic compo-
sition with a mean conductivity around 2 W/m °C. Sig-
nificant variations in porosity of the basalt will therefore
result in major conductivity variations and associated var-
iations in conductive temperature gradient. Thermal con-
ductivity will decrease with increased content of pore wa-
ter and the temperature gradient will increase. We ob-
serve that sections of high neutron porosity are intervals
of local low temperature gradient (Fig. 5). This means
that a significant part of the water in rocks of apparent
high porosity must be bound in water-bearing secondary
minerals. Furthermore, some of the secondary minerals
(water-free or not) must have a thermal conductivity sig-
nificantly above that of the mean value of the minerals of
unaltered basalt.

Maximum temperature gradients are generally observed

within the massive (non-porous) cores of basalt flows,
which are characterised by high density and low neutron
porosity (Fig. 5). This mainly reflects the presence of feld-
spar, a primary igneous minerals of low conductivity.
However, secondary minerals of even lower conductivity
must be present as well.

The large local conductive temperature gradient varia-

tions observed between about 1250 and 2175 m(Figs 4, 5)

are thus interpreted to originate from significant vertical
variations in mean thermal conductivity. In order to main-
tain a constant heat flow of around 60 mW/m² (see next

section), local intervals of minimum temperature gradi-
ent of 20-25°C/km must indicate conductivities within
the approximate range of 2.5-3.0 W/m °C, and local inter-

vals of maximum temperature gradients of about 45°C/
km must indicate conductivities around 1.3-1.4 W/m °C.
The thickness of lithological units of maximum tempera-
ture gradient variations and inferred maximum conduc-
tivity

variations

typically

the

range

5-20

(Fig.

5).

GEUS Bulletin no 9 - 7 juli.pmd

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102

103

Such variations of rock thermal conductivity by a fac-

tor of about two are not directly represented in our set of
conductivity measurements (Table 4). A potential for varia-
tion is, however, indicated by the observation that the mean
solid matrix conductivity of lapilli-tuff and tuff is 20%
higher than that of basalt. Zones of inferred increase of
conductivity are observed to be closely related to zones of
reduced density in the originally porous part of the basalt
flows. This may be explained by the presence of secon-
dary minerals of high conductivity. The secondary filling
of pores, voids and cracks must include minerals of ther-
mal conductivity significantly above that of normal basalt

matrix with a conductivity around 2 W/m °C.

This interpretation is consistent with observations of

Oxburgh & Agrell (1982) who found that thermal con-
ductivity in the Reydarfjordur borehole generally increased
with the degree of alteration, with the highest conductiv-
ity of up to 2.8 W/m °C

occurring in rock samples broad-

ly classified as breccias.

In general, the thin sediment intervals occur within

the

broader intervals of low temperature gradient (Fig. 5),

indicating a mean thermal conductivity of the sediments
very close to that of the adjacent basalt flows. Values of
thermal conductivity quoted below are mostly from the
comprehensive study and listing of conductivity of rock
forming minerals by Horai (1971). High-conductivity
secondary minerals present locally in variable amounts in
the Reydarfjordur borehole include calcite (3.4 W/m °C),
chlorite (4-6 W/m °C), quartz (7.7 W/m °C), epidote
(2.6-3.0 W/m °C) and haematite (about 11 W/m °C). In
the Lopra-1/1A borehole, zones of maximum tempera-
ture gradient and inferred minimum mean conductivity
are generally found within the massive cores of the basalt

flows characterised by high density and low porosity(Fig.

5). This clearly points to a local increase in low-conduc-
tivity secondary minerals such as clay minerals (about 1.5-
2 W/m °C), analcite (1.3 W/m °C) and hydrous zeolite
minerals like stilbite (1.2 W/m °C).

In order to take a step further into the analysis of the

temperature-gradient variability related to mineralogical
variations, Fig. 5 also presents a curve showing the rela-
tive intensity of red colour reflected from the formation.
The colour information is extracted from a digital colour
photograph with 24-bit resolution of a montage of cut-
ting samples from the borehole (using the public domain
program ImageJ). The relative intensity of red is comput-
ed from the values of the red, green and blue channels as
the function red²/(green × blue). The main idea is to test

without extensive mineralogical analysis whether the min-
eral haematite might play an important role. Haematite
(Fe

) has a bright reddish colour and very high thermal

conductivity. It is formed mainly by oxidation of primary
magnetite and secondary iron hydroxides. The colour curve

shows that many intervals of maximum reddish colour
more or less coincide with intervals of low temperature
gradient. A close correlation is seen particularly within
the depth interval 1550-1700 m. An average cuttings lag
time correction of 3 m is applied for the whole section.
However, the lag varies with drilling rate, which varies
with the hardness of the formation, and a locally better
correlation may be obtained by applying a slightly differ-
ent depth shift of the colour curve.

From the continuously cored Vestmanna-1 borehole, also

in the Faroe Islands, the content of haematite in highly
oxidised tuffaceous claystone may be as high as 25% esti-
mated from bulk rock chemistry (unpublished data, R.
Waagstein). A unit of highly altered basalt or tuff consist-
ing of silicate minerals like pyroxene, plagioclase, clay and
zeolites with an assumed average matrix thermal conduc-
tivity of about 2 W/m °C plus 25% of haematite (con-
ductivity about 11 W/m °C) will have a bulk conductivity

of about 3 W/m °C, as calculated using the geometric

mean formula, (see above). This is sufficiently high to pro-
duce the lowest temperature gradients of about 20°C/km.
Nonetheless, units of increased thermal conductivity gen-
erally also have a high (apparent) neutron porosity, which
requires minerals of high hydrogen content. This means
that, although haematite may play an important part,
other components are contributing and further studies
are needed for a better understanding of the relation be-
tween rock thermal properties and secondary mineralogi-
cal components.

Estimates of heat flow

Basalts have low concentrations of the heat producing iso-
topes U, Th and K, resulting in low heat production, gen-
erally

within

the

range

0.2-0.6

-6

W/m

(e.g.

Ver-

doya et al. 1998; Chiozzi et al. 2003). The contribution
to surface heat flow from a 3.5 km deep section is thus
very small, of the order of 1-2 mW/m

. If not significant-

ly perturbed by effects of topography, groundwater flow
or potential long-term palaeoclimatic surface-temperature
variations, heat flow should be almost constant along the
drilled section. For sections of the Lopra-1/1A borehole
where temperature measurements are assumed to repre-
sent conductive equilibrium values, heat flow may there-
fore be estimated from the product of mean temperature
gradient and mean characteristic thermal conductivity.

There seems to be no significant perturbing effects at

depths below 1300 m. The Lopra-1/1A borehole is in an

GEUS Bulletin no 9 - 7 juli.pmd

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103

104

area of small topographic height variations and the effect
of topography upon temperature and temperature gradient

was modelled to be insignificant. Temperature perturba-
tions are below 1°C. The artesian flow of water in the
borehole is localised to levels above 1150-1200 m. As dis-
cussed above, both observations and model calculations
show that the influence of palaeoclimatic surface tempera-

ture variations is insignificant at depths greater than 1200-
1300 m.

Mean temperature gradients from long depth intervals

vary within the narrow range of 28-33°C/km (Table 3)
and are thus well defined. The main source of error in
estimating heat flow is thus the choice of mean character-
istic thermal conductivity. The uncertainty arises from the
presence of local variations of the temperature gradient
interpreted in the previous section in terms of conductiv-
ity variations associated with mineralogical changes. These
changes are difficult to quantify in detail and thus not
fully understood.

By transferring thermal conductivity from laboratory

measurements to representative in situ values, tempera-
ture and pressure dependency needs consideration. How-
ever, for basalt this dependency is small compared to most
other crystalline rocks. The decrease of thermal conduc-
tivity of basalt with increasing temperature is of the order
of only 5-10% for a temperature increase from 20°C in
the laboratory to a temperature of 50-100°C in a bore-
hole (cf. compilations in Kappelmeyer & Haenel 1974).
A conductivity decrease of this magnitude is likely to be
almost compensated by an equivalent increase of conduc-
tivity with pressure. The slightly lower temperature gra-
dient in the deeper parts of the borehole (Table 3) may
indicate a slight general increase in average thermal con-
ductivity with depth. This increase may be explained by
decreasing porosity resulting from secondary mineralisation.

The most accurate large interval temperature gradient

is the assumed conductive mean equilibrium gradient of
32.9°C/km between 1400 and 2175 m. Using our rock
thermal conductivity measurements in the range of 1.7-
1.9 W/m °C with a mean of about 1.8 W/m °C, we ob-
tain a heat flow within the range of 56-63 mW/m². For

the deeper parts of the borehole between 2200 and 3430
m, the temperature gradient is between 30 and 31°C/km.
The lithology is here represented by roughly equal amounts
of basalt and lapilli-tuff. Low-porosity lapilli-tuff may have
a mean conductivity of about 2.0-2.2 W/m °C and ba-
salt of about 1.8 to 1.9 W/m °C. This yields a mean con-
ductivity of about 2.0 W/m °C and a heat-flow estimate

close to 60 mW/m².

Some local intervals of massive basalt at depths between

2000 and 2115 m have well-defined temperature gradients

between 35 and 38°C/km with a mean value of 36°C/km.

Although massive basalt units are inferred to have a slightly
reduced conductivity judged by their temperature gradi-
ents, average conductivity (inferred from our measure-
ments) seems unlikely to be lower than 1.6 and not above
1.8 W/m °C, resulting in heat flow in the range 57-65
mW/m².

Despite some uncertainty about details of the conduc-

tivity variations and their causes, we therefore estimate
terrestrial heat flow for the Lopra-1/1A borehole to be of
the order 60

5 mW/m². This value is about 15 mW/m²

higher than a previous estimate of Balling et al. (1984).
The main reason is new measurements showing higher
values of thermal conductivity and also the recognition
that the neutron log data cannot be interpreted in terms
of intervals of real high porosity and water-filled pores
resulting in reduced thermal conductivity. On the con-
trary, local intervals of apparent high porosity are gener-
ally observed as having low temperature gradients and thus
increased thermal conductivity.

Nearby areas of continental crust in the Faroe-Shet-

land Basin south-east of the Faroe Islands have present-
day heat flows between 45 and 65 mW/m² (Iliffe et al.

1999). In continental areas off the Norwegian coast, mean
heat flow is between 50 and 65 mW/m²

(Sundvor et al.

2000). A heat flow value of around 60 mW/m²is thus

consistent with continental crust underlying the Faroe
Islands. Heat flow from the mantle in Scandinavia is esti-
mated to be around 25-35 mW/m² (Balling 1995). If

similar values apply here, about 30 mW/m² must origi-

nate from heat produced by decay of radiogenic isotopes
in the crust, which requires a crust of continental compo-
sition. Otherwise, a significant cooling component and/
or significantly increased mantle heat flow must be as-
sumed, for which there is no other evidence. However,
this must be the case for areas of oceanic crust north of
the Faroe Islands, where heat flow generally between 60
and 75 mW/m² is observed (Sundvor et al. 2000).

Summary and conclusions

The Lopra-1/1A borehole drilled to a depth of 3.5 km
offers a unique opportunity of obtaining accurate infor-
mation on the thermal structure to a great depth in the
Faroe Islands. High-precision temperature logging was
carried out to a depth of 2175 m almost 13 years after
drilling. Temperatures in the upper 1200-1300 m are sig-
nificantly disturbed by upward flow of ground water in-
side the borehole. For deeper levels, between 2175 m and
total depth, only temperature logs from the commercial log-

GEUS Bulletin no 9 - 7 juli.pmd

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104

105

ging runs measured a relatively short time (up to 53 hours)
after drilling fluid circulation are available. These tem-
peratures have been corrected for the estimated effect of
disturbances. The deepest point of accurate temperature
information is 3430 m with a measured temperature of
98.6°C and a corrected, estimated equilibrium tempera-
ture of 103.5°C.

Temperature gradients calculated for depth intervals

greater than 500-1000 m show only small variations be-
tween 28 and 33°C/km. The least-squares mean gradient
for the undisturbed part of the borehole (1400-3430 m)
is 31.4°C/km. Levels of inflow of water to the upper part
of the borehole are seen as major peaks on the 5 m mean
interval

temperature

gradient.

addition,

significant

lo-

cal temperature gradient variability is observed in the high-
precision log between about 1250 m with minimum val-
ues down to 20-25°C/km and maximum values up to 45°C/

km. The latter variations correlate closely with variations
in other logging parameters and inferred lithological var-
iations within the lava succession and cannot be explained
by ground-water flow. Intervals of low temperature gradi-
ent generally match intervals of low density, high neutron
porosity and increased borehole calliper and intervals of
high temperature gradients match intervals of high densi-
ty, low neutron porosity and decreased calliper.

The observed correlation with neutron porosity is sur-

prising. In a conductive regime, the temperature gradient
should increase in lithological units of high porosity and
pores filled with free water of low thermal conductivity.
Here, we observe that units of apparent high porosity have
high gradients. This leads us to conclude that logged high
neutron porosity does not represent real high porosity units
with pores filled with free water. Instead, materials of rel-
atively high thermal conductivity compared to normal
basaltic material must be present in significant amounts.

The local temperature gradient variations are thus in-

ferred to originate from variations in thermal conductivi-
ty. The latter variation is ascribed to secondary minerali-
sation and mineral alterations. This may produce both
high conductivity minerals such as calcite, chlorite, quartz,
epidote and haematite and low conductivity hydrous min-
erals such as clay and zeolite minerals.

Such inferred local variations in rock thermal proper-

ties are only partly reflected in our thermal conductivity
measurements on core materials from the Lopra-1/1A
borehole and samples from surface outcrops in the Lo-
pra-1/1A area. These results, mostly on basalts and some
on lapilli-tuff and tuff, show homogeneous conductivity
with only small variations and mean values at about 1.8
(basalt) and 1.9 W/m °C (lapilli-tuff/tuff ). Our meas-
ured lapilli-tuffs and tuffs generally show matrix (grain)

conductivity about 20% higher than the basalts. The ele-
vated conductivity of the former rocks may be explained
by the abundance of secondary minerals with higher bulk
conductivity than basalt. The increased conductivity in
the originally porous part of flow units may be explained
in a similar way by secondary mineralisation, as mentioned
above. In some distinctly reddish intervals, haematite seems
to contribute significantly to the increase in conductivity.
However, further studies are needed in order to obtain a
better understanding of the correlation between rock ther-
mal properties and mineralogical alterations.

Because of the overall homogeneous mean tempera-

ture gradient structure, it is possible to obtain some infor-
mation on palaeo-surface temperatures during the last
glaciation by extrapolation of the temperature-depth func-
tion below 1200-1400 m to the ground surface. Extra-
polating the depth interval 1400-2175 m, a surface in-
tercept of -6.7°C is obtained. This is 13-14°C below pre-
sent-day surface temperature. Although the extrapolation
must be considered preliminary and approximate by na-
ture, it suggests a temperature increase of the order of
12-16°C at the termination of the last glacial period.

From well-defined temperature gradients and informa-

tion on mean characteristic thermal conductivity of the
drilled basaltic sequences, we estimate a conductive heat
flow at the Lopra-1/1A drill site of about 60 ± 5 mW/m².

This is about 15 mW/m² higher than the previous esti-

mate from the original borehole. The revised estimate is
due mainly to new, higher thermal conductivity measure-
ments and higher estimates of the conductivity of the
porous parts of the basalt flows by taking secondary min-
eralisations into account. A heat flow value of about 60
mW/m² is consistent with the Faroe Islands being under-

lain by continental crust.

From our analysis we may conclude that the thermal

regime and our reported temperatures, temperature gra-
dients and heat-flow value from below a depth of 1200-
1400 m represent conductive equilibrium conditions with-
out significant disturbances from the effect of drilling,
ground-water flow or palaeoclimatic surface temperature
variations. Temperature structure, temperature gradients
and heat flow may thus be taken as representative of a
larger area around the drill site with similar basaltic li-
thology. With respect to heat flow, an assumption of sim-
ilar lithology may not be necessary.

GEUS Bulletin no 9 - 7 juli.pmd

07-07-2006, 14:19

105

106

Acknowledgements

Valuable comments from two referees, Andrea Förster,
GFZ, Potsdam and Torben Bidstrup, GEUS, Copenha-
gen are gratefully acknowledged. This study has been sup-
ported by funds from the Danish Natural Science Research
Council.

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