Jökull - 01.12.1961, Page 47

Depth in meters
Temperature°C
Moreover, gravity and magnetic surveys were
carried out in the area. The results revealed the
presence o£ a large basaltic intrusive in the east-
ern parts of the city. The combined evidence
furnished by the temperature, gravitational ancl
magnetic data indicated an upflow of thermal
water west of the intrusive.
Drilling was resumed on the basis o£ this set
of data. During the past 6 years a total of 25
boreholes have been drilled in the area. A bore-
hole of 2.200 meters was completecl in 1959.
This drilling has been successful and the inte-
grated output of the boreholes amounts to about
135 liters per second of thermal water. The
maximum production temperature is 138 °C.
The water is utilized for domestic heating in
the city of Reykjavik.
FIGURE II shows the lines of equal surface
temperature gradient in the city compiled on
the basis of all boreholes drilled sofar. FIGURE
III gives a temperature survey of the deepest
borehole. The data represent equilibrium tem-
peratures in the closed borehole. The same
figure also gives temperature data froni two
shallover boreholes. The borehole G-4 is located
close to the main zone of upflow of thermal
water, whereas boreholes G-10 and Sundlaugar
are located in the intrusive body mentionecl
above. The temperature measurements were car-
ried out with thermistors with the exceptions of
the deep clata in FIGURE III which were ob-
tained by a vapour pressure thermometer.
The data in FIGURE II show quite clearly
the area of maximum surface temperature gra-
dient. This maximum is due to the presence
of a thin permeable sill with a slight northward
slope. The depth to the southern edge is about
160 meters. The sill is permeated by water of
80 to 95 °C and is covered by empermeable
basalts. Some shallow wells produce from the
sill.
FIGURE III reveals quite clearly the base tem-
perature of the area which is about 146 °C.
This temperature prevails in the lowest 1000
meters of the borehole.
(b) Reykir thermal area.
The thermal area at Reykir was the first large
thermal area to be exploited in Iceland. Until
recently all thermal water distributed by the
Reykjavík Muncipal District Heating Service
was supplied by this area.
Drilling was initiated in 1933. In 1954 when
drilling was discontinued a total of 70 boreholes
had been completed in the area. The maximum
depth reached was621 meters. The outputofthe
boreholes is 370 liters per seconcl of water at an
average temperature of 87 °C.
The drilling, although (piite successful, had
not furnished any eviclence as to the general
structural control of the area. Then presence of
permeable basaltic layers indicated at 200 to 400
meters but the structure controlling the water
supply was unknown.
The area is located in a blockfaulted country
and there is topographical evidence that a ma-
jor SW—NE fault line of Tertiary age runs
through the area. The importance of this fault
appeared obvious.
In 1959 it was decided to clrill a new cleep
Itorehole in the area in order to cut this fault
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