Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 21

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3, which is defined with seismic measurements as a layer with
6.5 hm/s P-velocity, does not exist in this zone. The electrical resis-
tivity depends much more on temperature than does the P-velocity.
Pálmason (3) has shown that a variation of 200°C at the 2-3
layers boundary would give a negligible variation (ahout 0.065
km/s) in P-velocity. But such a variation in temperature could
cause a change in resistivity by a factor of 3-5.
According to Pálmason the temperature gradent may be of the
order of 180°C/km in the westem part of the neovolcanic zone in
SW Iceland where the site Thingvellir (DES) is located. He finds
much lower gradient on a regional scale near the eastern branch
of the active zone and also in Northern Iceland. But we believe
that, at least in a narrow zone in the middle of the northem active
zone, similar values are valid for the gradient as in SW-Iceland.
The high hydrothermal and volcanic activity in this area supports
this opinion. Using a 180°C/km temperature gradient we find
500-600°C at the 2-3 layer boundaries beneath Thingvellir and
Mývatnsöraefi. With this value and the observed resistivity of
10-20 fim we find, using Fig. 7, a porosity around 0.03 in the
upper part of layer 3 beneath the volcanic zones. If the tempera-
ture is much lower i.e. about 300°C at 2-3 km depth, then the
observed resistivity has to be caused by higher porosity or about
0.04 — 0.05 in the volcanic zone.
The low resistivity layer (10—20 fim) starting at about 2 km
depth reaching down to the top of layer 3 must have similar poro-
sity.
It should be emphasized that all above mentioned values for
porosity are obtained from the laboratory experiments of Parkho-
menko et al. (15), where conductivity in more saline water, even-
tually absorbed to the surface of pores, has been neglected. These
porosity numbers are therefore maximum values and the actual
porosity may be considerable lower.
Nevertheless we believe that the obtained difference in conduc-
tivity beneath the neovolcanic and the flood basalt zones must be
explained by difference in porosity, probably caused by gradually
filling of pore spaces approaching the geologically older areas.
The difference in conductivity could also be explained by about
ten times more saline ground water beneath the neovolcanic zone,