Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 125

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Km, Kw = thermal conductivities of saturated medi-
um, and liquid, respectively.
Ti, T0 = temperatures of lower and upper boundary,
respectively.
The critical value of this parameter is 4it2 (^40). If it
is larger, a sustained convection is possible. The main pro-
blem in trying to apply this formula to actual conditions in
the upper part of the crust is the uncertainty in the permea-
bility and the thickness of the permeable layer. The normal
temperature gradient can be estimated from available data
from boreholes. An estimate of the thickness of the perme-
able layer can be obtained from the results of seismic refrac-
tion measurements in various parts of Iceland (Pálmason,
1963, 1967). These measurements usually indicate one or
more layers with P-wave velocities between 3 and 5 km/sec
and with an aggregate thickness of 2—4 km. These layers
are mainly flood basalts. They are underlain by a layer
with velocity between 6 and 7 km/sec, which is probably
less permeable than the flood basalts. A conservative esti-
mate of the thickness of a permeable layer would thus be
2 km, but it could very well be more. It should be noted
that the permeability of the flood basalts would be mainly
horizontal along the interfaces between the individual lava
flows, and furthermore along vertical fractures and dykes
where they are present.
Drilling and exploitation in the Reykjavík thermal area in
recent years provides an opportunity of obtaining a crude
estimate of the permeability of the flood basalts in one area.
Fig. 5 shows a model which can be used for this purpose.
Most of the productive holes have been drilled within a rough-
ly circular area of radius 400 meters. The producing aqui-
fers are confined to a roughly 1 km thick zone between 300
and 1300 meters depth. The deepest borehole, G-4 (cf. Fig. 4)
is 2200 meters deep and cuts an aquifer near the bottom.
For the past few years, the water discharge has been con-
trolled by pumping in accordance with seasonal changes in