Jökull - 01.12.1979, Page 52

water from reservoirs at depth towards the surface.
In many areas the free flow of thermal water has
been increased significantly by deep drilling. A 1
km drillhole by a spring giving < 1 1/s commonly
yields several and sometimes tens of 1/s of water
that also is most often significantly warmer than
the natural spring. The free flow is, however, much
affected by the regional topography; high moun-
tains create a high hydrostatic head so that a spring
or a drillhole in a deep valley has a much higher
natural flow rate than a spring fed by a similar
aquifer in a lowland region. This difference is
overcome by the effective use of downhole pumps
in geothermal wells in the lowland regions. Typical
temperature profiles of drillholes are shown in
Fig. 4.
The flow channels from the recharge areas in the
highlands to the hot spring areas in the lowlands
are thought to vary from the Tertiary to the Plio-
Pleistocene provinces. In the subaerially erupted
Tertiary volcanics the flow channels appear to be
mainly dykes and faults but to a less extent thin
high porosity stratiform horizons. In the Plio-
Pleistocene strata, which are characterized by suc-
cessions of subaerial lavas intercalated with thick
piles of subglacially erupted pillow lavas, hyalo-
castites and detrital beds, potential flow channels
are much more abundant. There the most effective
large scale reservoirs and flow channels are thought
to be the pillow lava cores of hyaloclastite ridges
and high porosity stratiform horizons of fragmental
material which are likewise cut by dykes and faults.
There is a significant difference between the
aquifers encountered by drilling in the Tertiary
and the Plio-Pleistocene areas. In the Tertiary
strata the aquifers appear most often to be narrow
and connected with vertical structures (dykes and
faults). Data is available on the transmissivity in
drillholes in three thermal areas in Tertiary rocks
(Table 1). The transmissivity is of the order of
103m2/s, and thus an order of magnitude lower
than that of the most permeable Plio-Pleistocene
strata. The most intensely drilled thermal area in
Tertiary strata is at Laugaland near Akureyri in
N-Iceland. The strata is of basaltic lavas with
minor sedimentary interbeds. The hot springs are
associated with dykes. But at depth particulary one
dyke out of a whole dyke swarm acts as a main
aquifer. Small aquifers have been found connected
with both individual dykes and clastic interlayers,
but the best aquifers have apparently been en-
countered at the intersection of permeable dykes
and the interlayefs.
In Plio-Pleistocene strata the major aquifers tend
to be horizontal and occur most commonly at the
contacts of lithological units such as lavas and
hyaloclastites. The transmissivity is up to the order
of 10"2m2/s, and as the aquifers are more numerous
the intrinsic permeability tends to be one or two
orders of magnitude higher than that of Tertiary
TABLE 1.
Transmissivity (measured) and intrisic permeability (calculated) in selected hydrothermal syslems in Iceland.
(Compiled by Snorri Páll Kjaran, National Energy Authority, Reykjavík).
Transmissivity Instrinsic
LOW TEMPERATURE AREAS Tertiary: m2/s permeability millidarcy
Laugaland 2.6X103 50
Ytri-Tjarnir 2.1 X 10-3 50
Baer 1.1X10-3 30
Plio-Pleistocene:
Sydri-Reykir 2.5 X 10-2 13700
Ellidaár 3.5X10-3 6200
Laugarnes 6.0X10-3 4100
HIGH TEMPERATURE AREAS
Svartsengi 1.2X10-2 1000
Krafla 6X 10-5 to 6X10-3 1 to 100
50 JÖKULL 29. ÁR