Jökull - 01.12.1978, Síða 114

COOLING OF THE SELFOSS
GEOTHERMAL SYSTEM BY INFLOW
OF COLD GROUNDWATER
Jens Tómasson,
National Energy Authority
The stratigraphy of the Selfoss geothermal
area is as follows; an upper zone which consists
of a Recent lava flow underlain by glacial
sediments down to a depth of 50— 70 m, and a
lower zone which consists of Quaternary vol-
canics. Cold groundwater flows freely in the
Recent lava flow, but the lower zone contains
the thermal water. Prior to pumping the
thermal water issued from the lower zone up
into the Recent lava flow. At the beginning of
utilization of the geothermal area for domestic
heating (1948) several shallow wells were sunk
into the Recent lava and the thermal water
extracted by pumps. Pumping created a
pressure drop in the thermal water system in
the lava causing an inflow of cold ground-
water and a drastic cooling. In 1951 drilling
was moved about 800 m from the initial wells;
deeper drillholes sunk into the Quaternary
strata, and the glacial sediments of the upper
zone isolated by casing. Corrosion of the cas-
ing in the upper zone has repeatedly caused
inflow of cold groundwater into the geother-
mal reservoir and to avoid this problem most
of the earlier holes have been filled by cement
or been recased.
TILT MEASUREMENTS AND RATE OF
MAGMA INJECTION AT KRAFLA
Eysteinn Tryggvason,
Nordic Volcanological Institute
Tilt observations at several locations in the
Krafla region, North-Iceland since the sum-
mer of 1976 show, that during inflation of the
Krafla magma chamber, the rate of inflation
and the shape of the surface bulge is nearly
constant. The shape of the bulge is in good
agreement to that predicted if a mass is in-
jected into a small spherical “magma
chamber” lying inside a homogeneous elastic
half space. In such a model, the center of the
spherial magma chamber must lie at a depth
of 3.2 ± 0.3 km below a point near the south
end of Leirhnjúkur to be in the best agreement
with the tilt observations. The uncertainty of
this location is about 500 meters. The tilt rate
agrees with an inflow rate into the magma
chamber of 4.6 m3/sec. or 0.4 million cubic
meters per day. There is indication that this
rate of inflow has increased slightly during the
15 months of observations from July 1976 to
October 1977.
SIGALDA HYDRO-POWER SITE,
GEOLOGICAL STUDIES
Elsa G. Vilmundardóttir,
National Energy Authority
Geological investigations in connection
with the Sigalda Hydroelectric Project have
almost solely been conducted by the National
Energy Authority on behalf of the National
Power Company. At the site the bedrock is
composed of moberg and tillite from the last
glacial epoch on hand and postglacial lavas
and sedimentary deposits on the other. The
powerhouse, headrace and tailrace canals are
located in moberg and tillite, but the reservoir
is mainly on lavas (Tungnaá lavas), which are
overlain by sediments deposited in the
previously existing lake, Krókslón. Studies
during the last 2—3 years have led to a rev-
ision of some details in the previously esta-
blished history of lake Krókslón. It can now be
considered fairly certain that the lake was
emptied about 1100 years ago, at a consider-
ably later date than previously assumed.
Studies of faults and tectonic fractures have
revealed that fractures of an easterly orien-
tation are more frequent in the vicinity of
Sigalda than in the adjoining areas to the east
and west.
112 JÖKULL 28. ÁR