Geophysical surveys involve the application of the principles of physics to the study of the Earth. The surveys operate based on the fact that the physical properties of the rock material that makes up the Earth and the distribution of those properties affect the nature of physical fields measured on or near the ground. Analysing the measured fields reveals how their physical properties vary both vertically and laterally inside the earth, depending on the geology.
Geophysical research methods are divided into those that employ the Earth’s natural physical fields and those that require the input of artificially produced energy. Methods based on natural fields make use of the Earth’s gravitational, magnetic, electric, and electromagnetic fields, looking for disturbances (anomalies) in them, the geological properties of which may also be of interest for reasons related to the economy and environmental protection. Artificial methods, meanwhile, involve the generation of electric or electromagnetic fields that are then used in a similar way as natural fields. The most widely used geophysical method in the world produces seismic waves, the speed and mapping of propagation of which provide information on the distribution of mineral resources in the Earth’s crust. While natural fields provide better information about the properties of the deeper layers of the earth, methods that rely on artificial fields offer a more detailed and easy-to-interpret picture of the geology of the studied region.
Geophysical surveys provide a relatively quick and cost-effective tool for obtaining information about the geology of the Earth.
Geophysical methods enable the possibility to identify and delineate potential mineral resources that would be too costly to explore by means of a drilling programme. While geophysical surveying does not eliminate the need for drilling, it does allow us to optimise the cost of exploration.
Magnetometry
Most of mainland Estonia has been mapped by aeromagnetic surveying at a scale of 1:25,000 (flight line spacing 250 m, flight altitude 100 m). The islands and Väinameri have been mapped at a scale of 1:50,000 (flight line spacing 500 m, flight altitude 300 m). Ida-Viru County has been covered by a network of aeromagnetic surveys at a scale of 1:200,000.
Gravimetry
Gravimetric measurements have been used in Estonia since 1968 for geological mapping and for improving the geoid model. Gravimetric data also allow us to assess the structure of and tectonic faults in the basement. Detailed investigations (at a scale of 1:50,000) have been performed in the regions of northern Estonia, western Estonia, Tartu, Viljandi, and Valga.
Geophysical borehole and well surveys
Geophysical sondes allow the possibility for detailed investigation of geological cross sections, lithologic boundaries, the locations of fractures, and hydrogeological parameters, as well as for assessing the technical state and conformity of a bored well.
