The general characteristics of the geophysical methods used for exploration of geothermal resources is given. Their application in the regional, intermediate (concessional) and local scales is considered and effectiveness of their usage depending on geological conditions is demonstrated.
Regional geophysical surveys are aimed at studying geological structures (inter-arc basins, subduction, spreading and rift zones, faults) that could host the geothermal reservoirs and contain appropriate ways of the heat transfer to the surface. The main methods used for regional studies are magnetotelluric (MT) and seismic surveys, while gravity and aeromagnetic ones are often applied for increasing the reliability of conclusions drawn basing on the former two methods. Seismic tomography of the earth’s crust (being quite expensive) reveals the geometrical boundaries of the lithological units and is especially good in detection of the regional stratigraphy. Magnetotelluric sounding (relatively cheap method) results in the resistivity model of the studied area that may be interpreted in lithological terms and guide the most favorable areas for future geothermal exploration.
None of the geophysical methods used without geological or other geophysical information can produce reliable results that can be used for decision making on the existence of the geothermal reservoir and its boundaries in the concessional scale. So, seismic and EM methods are often supplemented by gravity and/or aeromagnetic surveys of different scale that depends on the required resolution. Magnetic anomalies correlate quite well with horizontal zonation of the structures, while the gravity maps are used to locate deep faults and to delineate fracture zones characterized by low density.
The tasks of the geothermal studies at local scale are spatial delineating of the geothermal reservoir, indirect evaluation of the temperature and permeability of the geothermal zone and finding the best locations for drilling. Electromagnetic and, in particular, magnetotelluric sounding is a direct method for in situ studies of 3D structure and fluid circulation in the geothermal area. Due to theoretical relations between the electrical resistivity, on the one hand, and temperature, porosity and permeability formed by the presence of alteration minerals, on the other hand, it is often applied for the indirect estimation of these parameters.
Thus, geophysical sounding of the geothermal zones could be used
- to reveal stratigraphical layering;
- to produce a static image of the reservoir and surrounding structure;
- to locate fractures and faulted zones and determine the strike orientation;
- to detect the boundary between the cap formed by clay minerals and high temperature reservoirs;
- to estimate the permeability and porosity values;
- to monitor the phase change of pore fluid in fractured rocks and resident fluids resulting in resistivity changes in the host rocks;
- to improve the accuracy of reservoir temperature estimates;
- finally, to reduce exploration costs.