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 borehole geology data interpretation. Reykjakot is a part of the Hveragerdi high-temperature geothermal 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 farm. 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 survey, it lies well to the north of River Varmá. The thermal survey shows an anomalously hot ground with a N-S trend. It lines up with hot springs to the south and a fumarole 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 Varmá 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.