Geophysical exploration plays a very important role in geothermal exploration. Various physical parameters are measured, such as resistivity, density, sound velocity and magnetism of the subsurface rocks.
Resistivity surveys as carried out by Iceland GeoSurvey yield important information about the subsurface structure of geothermal systems. Temperature dependent assemblies of secondary minerals are formed by waterrock interaction (alteration) in the geothermal systems resulting in a very distinctive resistivity structure that can be used to map the size and shape of the reservoirs.
Monitoring and locating micro earthquakes gives information on fault movements and permeability as well as heat mining from heat sources and tomography inversion give information on sound velocity anomalies. Gravity surveys can give information on density variations such as dense intrusions. Magnetic measurements can be used to locate dikes and give information on demagnetization due to alteration.
A joint interpretation of complementary data (geophysical, geological and geochemical) is the crucial last stage in geothermal exploration.
Iceland GeoSurvey offers the following geophysical services for geothermal exploration and other applications:
- TEM-MT measurements, processing and interpretation. Transient electromagnetic (TEM) soundings are used to map temperature distribution in the uppermost 1 km of the crust. Magneto-telluric (MT) methods are applied to detect up-flow of thermal fluid into the reservoir.
- Seismic surveys, processing and interpretation.
- Seismic monitoring. Monitoring of seismic activity shows fracture zones at depth.
- Gravimetric measurements. Gravity surveys may reveal tectonic features in the reservoir.
- Surface GPS measurements.