Map viewers – geophysics
Here you will find information on ground geophysics and the magnetic field, gravity and gamma ratiation.
The “Ground geophysics” map viewer contains information on survey areas explored for prospecting purposes using geophysical methods of various kinds. The database contains information on both analogue and digital data at SGU.
Beside the geographical definitions, the meta database contains the following information: The name of the exploration area, the total number of measuring points in the area, exploration method, year of survey, name of the data file, report number (if available) and comments.
The measurements have been made using the following methods: magnetometer, electrical depth sounding, gravimeter, slingram, induced polarisation sounding, and electromagnetic measurements (VLF).
Magnetic field, gravity and gamma radiation
The “Magnetic field, gravity and gamma radiation” map viewer shows examples of some of the types of geophysical information possessed by SGU.
Geophysical measurements are used, among other things, to examine changes in the earth’s magnetic field and variations in gravity from place to place. They are also used to gain an idea of natural ground radiation, and to gain an impression of the bedrock where it is covered by soil or water.
When you use a compass, you also use the earth’s magnetic field, which causes the needle of the compass to point to magnetic north. The magnetic north pole differs from the geographical one, however. Moreover, the earth’s magnetic field is not static; its strength and direction fluctuates. The isogon map generated by this map viewer shows the magnetic declination for Sweden, and is updated each year.
The magnetic properties of the earth’s crust are mainly determined by the presence of the magnetic mineral magnetite in the various types of rock. The concentration can vary from almost zero to 10 per cent or more in gabbros, for instance, and up to 100 per cent in iron ores. Data on variations in the magnetic field therefore also provide information about the bedrock. This map viewer shows recorded variations in the total magnetic field (after subtracting the geomagnetic reference field DGRF 1965.0). By interpreting the geophysical information, it is possible to obtain information such as the depth distribution and surface distribution of various rock types, as well as their spatial distribution, strike and direction of dip. Faults and their relative movements can be seen as displacements in the magnetic pattern.
Gravity varies – it is strongest at the poles and weakest at the equator. Moreover, there are local variations, due, among other things, to the distribution of rock types of differing densities. Uses of gravity data therefore include mapping ore-determining structures, rock type distribution and groundwater reservoirs. The gravity map generated by the map viewer is a “Bouguer anomaly map” (ISGN71), the unit being mGal. It is based on gravity measurements performed by SGU, Lantmäteriet and other organisations.
By measuring natural ground gamma radiation, it is possible to estimate the concentration in the ground of the naturally occurring radioactive isotopes potassium, uranium and thorium. The map viewer shows the distribution of these elements in the uppermost layer of the bedrock or soil type.