Jökull


Jökull - 01.12.1984, Side 112

Jökull - 01.12.1984, Side 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
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