The electrical resistivity of the ground is derived from airborne electromagnetic measurements. The radio transmitters used operate in the frequency band 10–30 kHz (Very Low Frequency, VLF). These low frequency radio waves penetrate deep into both water and ground.
The Geological Survey of Sweden uses these low frequency radio waves to gather information about the subsurface. The analysis method is based on the fact that the radio waves change character depending on the electrical properties of the ground. The receiver is mounted in an aircraft with an antenna system measuring the signal in three orthogonal directions from two VLF transmitters.
The surveys are carried out along straight lines with 200 m separation. Survey height is 60 m and point distance 16 m. Collected data are then transformed to electrical resistivity, i.e. inability to conduct electric current.
The map shown is based on measurements until 2019. The resistivity data are used for identifying water bearing fracture zones and deformation zones in the bedrock, graphite and sulphide bearing horizons and clay occurrences. On the resistivity map it is also possible to distinguish highly resistive bedrock units from more conductive ones. In areas with large soil depths, the resistivity map also reflects variations in the quaternary layers.
SGU uses VLF data together with other information in the mapping of soil and bedrock and in mapping available water supplies in Sweden. Other examples of usage are mineral prospecting and environmental applications.
Today it is possible to obtain information on ground resistivity from approximately one third of the country.
Senast granskad: 2020-03-02