D56 Role of passive seismic and geodetic data in geothermal exploration – NEA

The high-temperature geothermal activity in Iceland is intimately connected to the volcanic activity in the crustal spreading zone. Magmatic intrusions are heat sources but the presence of fluid and permeability is necessary in order to have exploitable geothermal resource. Permeability can roughly be divided into matrix and fracture permeability. Experience has shown that the best feed-zones in geothermal wells are most commonly associated with fractures.

The crustal spreading in the volcanic zone opens up new fractures and/or widens existing fractures. These fractures are flow channels of cold water towards the heat sources and for hot water towards the surface. It is therefore of great importance to understand the tectonics and map the fracture pattern and individual fractures. The resistivity of the subsurface rocks is diagnostic of the temperature distribution and is used to map the extent of the geothermal systems. It lies, however, in the nature of the electromagnetic methods that their resolution decreases with depth. The best way to understand the tectonics and to map the fracture systems is, besides geological mapping, to use geodetic measurements and micro earthquakes. Experience from the Hengill area has shown that there are, in a way, two tectonic regimes present. One is the spreading fissure swarm and the graben through the central volcano, which is very obvious on the surface. Resent geodetic measurements and earthquake data show another tectonic pattern, reflecting transform tectonic extending from east and into the central volcano and the fissure swarm. This tectonics is much less clear on the surface.

Comparison of subsurface resistivity, surface manifestations and the transform fracture pattern shows that it controls, to a large extend, the fluid flow in the geothermal system. The results of WP 3200 have allowed much more accurate location of faults. The relative earthquake location algorithm has been applied on most of the huge dataset from the local network in the Hengill area. This has resulted in much more accurate definition of active faults. This is exactly what is needed for the geothermal exploration. By using together the blurred results of the electro-magnetic methods and the location of faults, from the passive seismic, much better conceptual models can be compiled and drilling targets can be defined with greater confidence. Improved conceptual models are important for numerical simulation for the geothermal systems and better definition of drilling targets can substantially reduce the economical risk of unsuccessful wells.

The deliverables of the RETINA project, and in particular WP 3200, have added an important tool to geothermal exploration and research. The electricity marked in Iceland has increased dramatically during the last years, due to increasing demand for power intensive industries. It is foreseen that geothermal power production will largely meet the rapidly increasing power demand.

It is a general consensus among geothermal scientists that further development in geothermal exploration will be through more integrated use of different exploration techniques and interpretation. The RETINA project is a good example of an integrated project where different techniques and disciplines are combined to broaden the perspective. Integrated approach increases the value of individual components.

The importance of geothermal energy is increasing in the Icelandic Economy. Geothermal exploration and research is also increasing. In the coming years, exploration will be conducted on several prospects. The techniques developed under the RETINA project will be applied in this work. Furthermore, a new EU funded project, Integrated Geophysical Exploration Technologies for deep fractured geothermal systems (I-GET), will be launched at the end of this year. The main idea of that project is to integrate electro-magnetic and seismic methods. The Hengill area will be one of the three principal test sites. Techniques and results provided by the RETINA project will be used extensively in the I-GET project.