JOURNAL OF THE BALKAN GEOPHYSICAL SOCIETY, Vol. 1, No 4, November 1998, p. 60-74, 10 figs.

Geoelectric structure of Sousaki geothermal Area (Greece)

deduced from two-dimensional magnetotelluric studies

D. Galanopoulos*, E. Lagios*, G.J.K. Dawes**, B.A. Hobbs**

^* Department of Geophysics and Geothermy, University of Athens, Panepistimiopolis-Ilissia, 157 84 Athens, Greece

^** Department of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK

(Received 7 April 1998; accepted 12 October 1998 )

Abstract: A short period (0.075-42 s) Magnetotelluric (MT) survey was carried to investigate the geoelectric crustal structure of Sousaki geothermal area in Greece. The MT data were initially processed and analysed by Lagios (1992) and Tzanis and Lagios (1993). They suggested that the crustal structure below Sousaki, comprises a complex geoelectric domain of intersecting conductive fault zones and resistive blocks which may include igneous intrusions. Their results were based on one dimensional (1-D) MT models, and thus to determine such a complex structure at least two dimensional (2-D) modelling should be undertaken. This was the main task of the present study. An attempt to construct a three dimensional (3-D) model for this area was proved unsuccessful due to difficulties in the design of the actual model grid. The MT data were finally modelled using a 2-D forward modelling technique. The adopted 2-D MT model concerns the top 2 km of upper crust and provides a different structure with that derived by the 1-D models. This was attributed to the presence of regional 2-D or local 3-D structures. The 2-D model is consistent with the geology and tectonics of Sousaki. The low resistivities (2.5-5 ohm-m) observed within the uppermost 100 m seem to be compatible with the Plio-Pleistocene marls and volcanics. The very low resistivities (0.5-1 ohm-m) observed at greater depths (0.5-1.5 km) are probably related to hydrothermally altered ophiolites. The low resistivities observed in the area combined with the various superficial thermal manifestations, make up a significant evidence for the existence of a geothermal field. The three major fractured zones identified in this area are probably those which should allow the rainwater and the geothermal fluids to flow and ensure the hydraulic continuation of the Earth’s surface with the top weathered zone of the limestone or ophiolitic basement.

Key words: Geothermal Exploration, 2-D Magnetotellurics, Sousaki, Hellenic Volcanic Arc.