Jökull - 01.12.1984, Page 112

THE RESISTIVITY SURVEY
The bulk resistivity of Quaternary and Tertiary
rocks in Iceland is practically independent of the
salinity and the temperature of the pore fluid as
long as the conductivity of the fluid does not
exceed approx. 0.2 1/Í2m (Flóvenz 1980).
Resistivity variations in the uppermost kilometre
of the crust do therefore primarily reflect varia-
tions in the amount of interconnected pores and
fractures of the rock (Flóvenz and Georgsson
1982, 1983). Consequently a regional resistivity
survey can be used to estimate the size of geo-
thermal systems and to map the flow pattern of
thermal waters from its origin in the highlands to
the geothermal areas in the lowlands. The regio-
nal resistivity survey in the Borgarfjördur region
was initiated on this basis.
The survey consisted of Schlumberger sound-
ings with a maximum current arm (AB/2) of 1500
m, except in the highlands, where most of the
soundings were extended to about 2200 m to get
information about resistivity at deeper levels.
About 100 soundings have been carried out so
far. These soundings cover all thermal systems in
the the valleys of Borgarfjördur and extend also
into the highlands. The soundings were inter-
preted one-dimensionally with a computer pro-
gram of Johansen (1977), based on an inverse
interpretation of the data curves.
Fig. 4 shows the resistivity at 600 m depth
below sea level for the Upper Borgarfjördur
region, but the picture is similar down to at least
1000 m, except for the Baer thermal system. The
largest Iow-resistivity anomaly coincides with the
Reykholt thermal system. The 30 Qm resistivity
contour seems to define its boundaries. It
includes all major thermal fields in Reykholtsdal-
ur and the neighbourhood except the Stóriás
field, covering an area of 250-300 km2. The area
of most intense geothermal activity is within the
20 Qm contour. The shape of the anomaly indi-
cates. two centres of thermal activity in
Reykholtsdalur, the Reykholt-Kópareykir ther-
mal field whose main feature is the high geo-
thermometer temperatures, and the Deildartun-
ga-Kleppjárnsreykir thermal field, which is
characterized by enormous natural discharge.
Even though it must be assumed that the main
flow of thermal waters from the highlands
towards Reykholtsdalur is at deeper levels, the
elongated shape of the anomaly suggests that it is
from northeast, thus indicating a recharge area in
the Arnarvatnsheidi region.
The large low-resistivity anomaly which
stretches from the Upper Nordurárdalur valley
south towards Varmaland with resistivity values
below 20 Qm, is associated with the Baer thermal
system. The shape points towards Snjófjöll as a
probable recharge area. The boundaries between
the Reykholt and Baer thermal fields are diffuse,
but soundings within the Baer thermal system
differ by showing higher resistivity at deeper
levels, indicating decreasing fracture porosity
with depth.
No boundaries are observed between the
Reykholt and Brautartunga thermal systems in
the outer part of Lundarreykjadalur. The low-
resistivity anomaly in the inner part of Lundar-
reykjadalur is associated with the England ther-
mal system.
The boundaries between the Reykholt and
Húsafell thermal systems are well defined but no
low-resistivity anomaly is associated with the lat-
ter. The high resistivity values in the Húsafell
region relate to intrusive rocks associated with
the extinct Húsafell central volcano.
DISCUSSION
The geothermal activity in the Reykholt system
is confined to a Late Tertiary lava pile of low
primary porosity which is to be contrasted with
the other major low temperature systems in
South and Southwest Iceland, which are in
Quaternary rocks with far higher primary poros-
ity.
A detailed study has been carried out at many
of the major thermal fields of the Reykholt ther-
mal system. These include the Deildartunga-
Kleppjárnsreykir and the Klettur-Runnar fields
(Georgsson et al. 1978), the Vellir thermal field
(Georgsson et al. in prep.), and the Hurdarbak
field (Georgsson and Haraldsson in prep.). Simi-
lar study has also been carried out at thermal
fields of the Baer system (Georgsson et al. 1981a
and b, Jóhannesson et al. 1979) and the Brautar-
tunga system (Flores 1981). The hot springs of
the major thermal fields in Borgarfjördur are
aligned along young fractures with northwesterly
or northerly trend (Fig. 5). Most of the major hot
springs are found at the intersection between
these fractures and northeasterly trending faults
and dykes. The resistivity survey suggests that the
110 JÖKULL 34. ÁR