Jökull - 01.12.1961, Page 39

discussion of similar topics in the case of the
Wairakei thermal area in New Zealand, the
reader is referred to the fine papper of Studt
(1958).
For the sake of completeness, a point of major
interest will be discussed briefly.
The drilling carried out in the thermal areas
of Iceland has revealed the important fact that
the heat output of the wells can surpass the
natural output of the area before drilling. An
increase by a factor as high as 20 has been ob-
tained in one case. This poses the important
problem as to the origin of the additional flow
of heat ancl water.
It is possible that the wells may induce a
considerable decrease of the natural impedance
of the flow and thus lead to an increased circul-
ation in the entire hydrothermal system.
On the other hand, a transitory increase of
the flow can also be obtained on the basis of the
large amount of heat accumulated in the up-
flow zones of the individual areas. The density
of water decreases with increasing temperature,
mainly above 100° C. The density of water at
100° C is 0.96 gr/ml, at 200° C 0.87 and at
250° C only 0.80. Therefore, cold water in the
formations surrounding the thermal areas has
a tendency of encroaching on the hot water
within the discharge zone to drive it out. How-
ever, the cold water entering the hot rock is
heated by the contact with the rock and new
hot water is formed. This transitory circulation
can, therefore, be maintained by the surplus
heat in the rock and will last as long as there
is surplus heat present..
Moreover, the steam flow from wells in high-
temperature areas may partially depend on the
boiling of pore-water in the hot rock. Porous
rock at temperatures above 100° C and satura-
ted with water can act as a lieat reservoir in a
somewhat different way. Wells drilled into the
rock may induce a decrease of pressure ancl a
subsequent boiling of the water in the pores.
At a not too great porosity the boiling will
largely depend on the heat content of the
rock. As a matter of course, the temperature
of the rock has to be near to the boiling tem-
perature of water at the depth of the rock
formations. Both types of heat reservoirs may
be encountered in the high-temberature areas.
The present author (Bodvarsson, 1956) has
estimated the total potentialities of thermal
areas in Iceland lor power production at
sóme 300 megawatts steady power and a reco-
verable lieat reservoir of some 15,000 mega-
wattyears.
At present natural heat is utilized in Ice-
lancl mainly for domestic and green-house
heating. The total amount of heat utilized corre-
sponds to a yearly saving of fuel oil of approxi-
mately 60.000 metric tons, that is, about 350
kilograms per vear per capita.
6. GEOPHYSICAL EXPLORATION OF
THE NATURAL HEAT RESOURCES.
Geophysical exploration has been of con-
siderable importance for the development of
the natural heat resources of Iceland. In some
cases the exploration work has been of a deci-
sive importance. A review of the methods in-
volved and a number of case histories have
been given elsewhere by the present writer
(Bodvarsson, 1950). For detailed information
the reader is referred to this paper.
The geophysical exploration is carried out
mainly for two purposes. Firstly, for the un-
covering of structural features. Secondly, for the
study of the subsurface temperature field. The
latter methods are generally referred to as the
direct methods, whereas the structural methods
represent the indirect methods.
The indirect methods involve the convent-
ional technique as the magnetic, gravitational
and the seismic methods. Their application to
the natural heat prospecting does generally not
differ much from the methods of prospecting
for oil and minerals. The main purpose is the
uncovering of intrusives, mainly dikes, and tec-
tonic structures.
The introduction of the seismic methods for
the study of the flood basalts represents probab-
ly the main advance during the past 10 years.
For further information the reader is referred
to the papers by Báth (1960) and by Báth and
Tryggvason (1961).
The direct methods involve a technique spe-
cially adapted to the natural heat prospecting.
The main purpose is the study of the subsurface
temperature field, mainly the base temperature
and the extension of the rock heated to this
temperature. The methods involve the thermal,
the electric resistivity and the geochemical met-
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