Jökull - 01.12.1961, Blaðsíða 49

integrated output now amounts to 500 liters per
second at 180 to 230 °C. Temperature
surveys have been carried out in most of the
boreholes by means of a vapour pressure thermo-
meter. The data have been obtained at equi-
librium conditions. Data from two boreholes
are given in FIGURE V.
The area is geologically complex ancl the
structural control has not been revealed neither
by the drilling nor by other investigations. The
most prominent feature of the temperature data
is the apparent horizontal stratification of the
flow. This is apparent frorn the temperature
data. The temperature drops below the perme-
able liorizons which are indicated by the tem-
perature maxima.
This decrease of the temperature with depth
is an interesting phenomenon which is at pre-
sent not wholly understood. However, the prin-
cipal practical implication appears to be that
the Hveragerdi area is not an area of major up-
flow and that the main structures of upflow
have to be found elsewltere in the main thermal
area.
6. REGIONAL T'EMPERA TURE
CONDITIONS IN ICELAND.
Iceland has been subjected to volcanism
throughout the Tertiary period. Several thous-
ands of meters of flood basalts have been built
up during this period. The volcanism is still
active in some parts of the country.
A studv of the thermal conditions within the
flood basalts is of scientific as well as practical
interest. The State Electricity Authority has
initiated a general survey of the temperature
and heat flow conditions. A number of relative-
ly shallow boreholes have been drilled in non-
thermal areas and further lioles are planned.
The location of present holes is given in
FIGURE I.
Most of the non-thermal boreholes have been
been successful and have revealed a fairly linear
increase of temperature with depth. The surface
temperature gradient is clearly revealed bv the
boreholes. The results are given in TABLE I.
The data were obtainecl at equilibrium condit-
ions with thermistors.
A thorough discussion of the data given in
TABLE I requires a study of the influence of
the various factors given at the beginning of
paragraph (2). The main factors of interest are
percolation of subsurface water, erosion and
topography. The present paper does not allow
space for a discussion of this lcind. Some perti-
nent questions have been reviewed in a paper
by Bodvarsson (1957).
The main implication of the data is that it
appears possible to estimate the temperature at
the bottom of the flood basalts. The results of
the seismic work of the Swedish-Icelandic expe-
dition in 1960 (see Báth and Tryggvason, 1961)
inclicate that the flood basalts in the South-
West of Iceland have a thickness of 2 km,
whereas in the North a figure of more than 3
km is probable.
An extrapolation of the data in TABLE I
combined with a correction for possible in-
fluences of erosion indicates a temperature of
the order of not less than 150 °C at the
bottom of the flood basalts in the South-West.
In the North a figure of the order of 100
°C is obtained. As a matter of course, the
average nature of these results is to be ernpha-
sized and further observational data are needed.
But the present results appear to be of general
interest for the study of the thermal activity in
Iceland as shown by Bodvarsson (1961).
TABLE I.
Surface temperalure gradient in non-thermal boreholes.
Location Elevation Depth Temperature gradient
meters meters degress C/meter
(1) Thykkvibær (5) 90 0.093
(2) Arnarholt (20) 240 0.165
(3) Holtavörduheidi 340 32 0.054
(4) Tindar 15 105 0.111
(5) Thorskafjardarheidi 460 48 0.034
(6) Vadlaheidi 570 40 0.033
See for References next page,
47