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Integrated MT/Gravity geothermal exploration in Hungary: A success story

Hungary is promising for utilization of low temperature (< 150°C) geothermal energy because of its high thermal gradient, reaching almost 50° C/km over most of the country. This high gradient is mainly caused by a relatively thin layer of the Earth’s crust in that area and partly due to the non permeable lower Pannonian sediment layer that covers a large part of the country. Our geothermal evaluation project in Hungary has, since 2007, yielded over 30 possible well sites for geothermal energy production and utilization. The first drilled well was recently successful. The correlation between resistivity and temperature is associated with the local degree of hydrothermal alteration. Most high-temperature hydrothermal systems are indicated by a low resistivity layer over the geothermal reservoir which is caused by clay mineral alteration. Electrical methods provide information about rock properties, temperature, and the degree of hydrothermal alteration. This information can be used to determine the geometry of hydrothermal reservoirs, its depth, location of fracture zones, and the permeability distribution. To complement the electromagnetic method of choice (magnetotellurics or MT), gravity surveys were acquired along the MT survey lines with higher density spacing to assist in detecting fault systems. Gravity data may be used to interpret the subsurface and to aid in locating prospective heat sources. Integrating the MT and gravity data reduces the ambiguity of either dataset and produces a more robust interpretation. The distribution characteristics of the fault zones with relatively low resistivity and with boundaries outlined by cooperative constrained inversion of MT and gravity data indicate that the prospective zones for potential geothermal reservoirs in the survey area are along the midnorthern part of the AMT/MT survey line 1 and the middle part of AMT/MT survey line 2. Based on integrated processing and interpretation of electromagnetic, gravimetric and seismic combined with stratigraphic information; the position of the first geothermal well site was selected and drilled in the Szentlőrinc survey area. Hot water with temperatures in excess of 85°C, estimated to have a peak heating capacity of 4 MW, was found at depths of 1,620 to 1,790 meters. This discovery was possible due to the utilization of different geophysical and geological methods to determine the best well location. The integrated approach which uses different datasets has proven to be a very effective method for locating the most promising areas for geothermal exploration. Utilizing this method in Hungary, with the goal of supplying 700,000 homes with geothermal energy within the next decade, is readily possible.