Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 11

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a certain area, then information about the temperature can be
achieved bj measuring the electrical resistivity.
The National Energy Authority (N.E.A.) has made numerous
electrical soundings in Iceland in the last two decades, using in most
cases the Schlumberger configuration with spacing up to 1500 m.
These measurements have mainly been concentrated on geothermal
areas, which are local anomalies in the regional behaviour of the
electrical resistivity. In order to measure the resistivity of layer 3
it was necessary to extend these measurements to areas outside
thermal fields and to greater depths. In 1972 the N.E.A. obtained,
on loan from Brown University in Providence, resistivity instru-
ments capable of dipole-soundings with center spacing up to 10
km. Several dipole profiles were then measured in SW-Iceland.
In 1973 a number of dipole profiles and magnetotelluric measure-
ments were made on a profile across Northem Iceland. This was
a part of a joint research program of the N.E.A. and Brown Uni-
versity, to determine the regional variance of the electrical proper-
ties of the Icelandic cmst and upper mantle.
These measurements have shown a great scattering in the resis-
tivity of the surface layers. Typical values for the neovolcanic zone
(postglacial cold lava fields) are between several 1000 Qm and
several 10000 Qm but 100-1000 Qm in the Quaternary and Ter-
tiary flood basalt areas. The resistivity usually decreases with depth
and is often around 100 Qm at 1—2 km depth in geologically dif-
ferent regions.
The main purpose of the present paper is to determine the resis-
tivity and its lateral variation in the upper part of layer 3. In the
geologically older flood basalt zones the resistivity lies between 150
and 1000 Qm, but in the active zone it is around 10-20 Qm.
The higher resistivity in the older regions is probably caused by
much lower temperature and by lower porosity, caused by gradual
filling of pore spaces.
2. LOCATION OF FIELD SITES. FIELD MEASUREMENTS.
It is important to have in mind the regional geological structure
and the variation of depth to layer 3 in selecting field sites. Their
location is shown in Figure 1. At every site several Schlumberger
L