Jökull


Jökull - 01.12.1984, Side 115

Jökull - 01.12.1984, Side 115
Figure 6. The Borgarfjördur thermal systems. 6. mynd. Jarðhitakerfi í Borgarfirði, myndin sýnir líklegar rennslisleiðir jarðhitavatnsins og skilin milli jarðhitakerfa. Not only do the northeasterly faults serve as the main flow channels for the system and the frac- tures open the way for the water to the surface but the associated seismicity of the area keeps them open, as it counteracts low temperature zeolitization or sealing which otherwise would gradually fill the channels. The flow pattern of the thermal waters of the five thermal systems of the Upper Borgarfjördur region and their recharge areas (Fig. 6) can be inferred from the combined results of the geologi- cal studies and the regional resistivity survey, in addition to the deuterium measurements by Árnason (1976). The Arnarvatnsheidi region is the probable recharge area for the Reykholt ther- mal system. There numerous active northeasterly and easterly faults (Saemundsson 1967) form favourable geological conditions for percolation of meteoric water. This is best demonstrated in the easterly trending Urdhaedir-fault, where open fissures are observed for several km. The water reaches 1-3 km depth and driven by the hydrostatic gradient, it flows laterally for about 50 km towards the lowlands in southwest. By using natural discharge and a mean base temperature of 125°C, the thermal power of the Reykholt system can be estimated about 210 MW. No thermal gradient holes have been dril- led in the region, but according to Pálmason et al. (1979) the regional thermal gradient can be expected to be 100-150°C/km. Using 125°C/km as an estimate and a thermal conductivity of 1.7 W/ m°C (Pálmason et al. 1979) the energy dissipation corresponds to a total conduction heat flow of an area of 1000 km2. However, total thermal drain- age is impossible to accomplish. Thus it can be assumed, that the Reykholt thermal system needs a recharge area that is at least twice that size. It is of interest to compare these results with calculations based on time-dependent heat sources. Bödvarsson (1982) calculated the energy balance of the Reykholt thermal system based on JÖKULL 34. ÁR 113
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