Geophysical aeronautical measurements, apparent resistivity (view service)

The display service shows the apparent resistivity in a simplified break with 200 times 200 meters resolution. The underlying information can be ordered in the data set Geophysical Flight Measurements, Electromagnetic Fields, VLF (detailed), then with a higher resolution. The apparent resistivity does not represent absolute values of the resistivity of the soil, but shows only relative changes from an assumed normal value. The normal value is 5000 M. The map of the apparent resistivity reflects the composition of both the bedrock and the soil layers. Deep recognition depends on the conductivity of the ground. In rock areas and areas with thin soil cover, the map mainly shows the resistance of the bedrock. In areas with large clay strengths, depth recognition is limited due to the low resistance, and the map therefore mainly shows the resistance of the soil layers. The map is derived from data collected using aeronautical measurements where the so-called VLF (Very Low Frequency) method has been used. SGU’s VLF measurement system records both the amplitude and the phase of the magnetic field in three orthogonal directions. By recording signals from two different transmitters simultaneously, the apparent electrical resistance of the ground can be calculated using a method developed by Becken and Pedersen (2003).