Airborne geophysical measurements
Airborne geophysical surveys are performed with the aid of fixed-wing aircrafts or helicopters. These measurements can show different properties of the bedrock or soils, from ground level down to several kilometers depth, depending on the method used. The information obtained is an important foundation for bedrock mapping, mineral exploration and physical planning.
Large areas can be surveyed quickly with the use of fixed-wing aircrafts, including areas with difficult terrain or places which are inaccessible from the ground. SGUs airborne geophysical surveys measure Earth’s magnetic field, naturally occurring gamma radiation from soils and bedrock, as well as electrical conductivity in the ground. The measurements are usually done with 200 m separation between the survey lines at an altitude of 60 m above ground. This provides comprehensive and detailed information over the survey area.
By interpreting airborne geophysical measurement data we get an initial picture of the geology within a survey area. The interpretations can be followed up by on-site observations and/or geophysical surveys on the ground. Airborne geophysical surveys make mapping more efficient and increase the quality of geological models.
Airborne geophysical measurements have been carried out by SGU since the 1960s. Today, information can be found over large parts of the country.
Small variations in the strength of the geomagnetic field can be registered with airborne geophysical measurements. This can be used to obtain information on, for example, depth and surface distribution of different rock types, as well as their strike and dip. Faults and their relative movement can in some cases also be identified in the magnetic patterns.
Soils or water does not affect a magnetic field measurement, which make it an important method for mapping bedrock where it cannot be directly observed.
Gamma radiation is part of our natural environment. Parts of the radiation come from the soil and bedrock beneath us .Gamma spectrometry is used to quantify the presence of the naturally occurring radioactive isotopes; potassium (40K), uranium (238U) and thorium (232Th). The radiation that originate from these isotopes has its source in the top few decimeters in the soil or bedrock. It is possible to identify bedrock units with separate occurrences of these elements in areas where the bedrock is present at the surface or close to the surface.
The distribution of potassium, uranium and thorium can also provide information on the genesis of the rocks and how they have been affected by geological processes. The information can also be used to identify areas where there is a risk of radon exposure.
By measuring how the ground reacts to electromagnetic waves it is possible to get information on its ability to conduct electricity. Rock types that conduct electricity well are often of interest to mineral exploration. Soils rich in water, as well as rocks with a high amount of water filled cracks, are also good electrical conductors.
SGU uses the VLF-method (Very Low Frequency), which is a conceptually simple but effective method to get information on how well the ground conducts electricity. The method involves measuring the magnetic field induced by currents in the ground, which in turn are generated by interaction with radio waves from transmitters in the VLF-band. The method works well in Sweden, where the bedrock generally has a low electrical conductivity.
SGUs airborne geophysics team is, since the Chernobyl accident in 1986, a part of Sweden’s preparedness program for nuclear and radiological events as issued by the Swedish Radiation Safety Authority’s (Strålsäkerhetsmyndigheten, SSM) expert support organization. This means that SGU participate in regular exercises and is available as a measurement resource in emergency situations.
SGUs role is to measure gamma radiation from fixed wing aircrafts in order to identify individual sources of radiation or map radioactive fallout. With airborne measurements, it is possible to efficiently measure large areas and obtain a comprehensive picture of the occurrence and intensity of gamma radiation.