Title: Geothermal mapping at Reykjakot in Ölfus, SW-Iceland

Abstract

Geothermal mapping has been done in the Reykjakot area by carrying out geological and
hydrogeological mapping, detailed soil temperature and magnetic surveys and barehole geology
data interpretation. Reykjakot is a part of the Hveragerdi high. temperature geotbermal field
which lies at the eastern border of an active spreading zone. Surface geology to the north of
the mapping area and borehole data show an eroded and faulted basement composed of a
sequence of subglacially extruded hyaloclastites, interbedded with subaerial basalts and minor
sediments. Late glacial sediments, thought to be of glaciofluvial and marine origin, overlie the
basement in the flat ground south of Reykjakot fann. These were covered by postglacial lavas,
which cover the southern part of the mapping area. The edge of the lavas is buried but,
according to the results of the magnetic SUlVey, it lies well to the north of River Varma.
The thermal survey shows an anomalously hot ground with a N-S trend. It lines up with hot
springs to the south and a fumarote field to the north, strongly suggesting a tectonic control.
Details in the thermal anomaly pattern suggest NE-SW en echelon fractures superimposed on
the main trend. Most of the geothermal manifestations along the zone became active in 1915
or 1916. A deflection of the thermal anomaly into a NW-SE direction near river Varma is
related to the general groundwater flow within the permeable postglacial lavas.
A geological model has been developed for the area which shows that the area is supplied by
NE-SW flowing hot geothermal water, heated by a source northeast of the area. These waters
move along the NNE-SSW and NE-SW fractures.
The siting of some boreholes in the area did not take advantage of the fracture permeability
which seems to control the upflow of hot waters there.