- Geomagnetism surveying is to observe and analyze the geological distribution of geomagnetism and its year-to-year variation in a particular area based on the measurements of three geomagnetic constituents (deflection angle, magnetic dip, and electromagnetic force) at geomagnetic or given points. The collected data from the surveying can be used to research magnetic declination for national base maps, exploration of underground resources, internal structure of the earth´s crust, and geophysics.
- The NGII has measured electromagnetic force from 1975 to 1978 by using an M-50 Propton magnetometer in order to observe abnormal magnetism in local areas of Korea. In 1978, it adopted GSI-type magnetic equipment to measure three geomagnetic constituents, leading to the conclusion that the overall value of the deflection angle in the Korean Peninsula was within the range of 5° and 7°30´ west variation.
- Since then, the NGII has conducted the regular surveying project on a yearly basis targeting specific parts of the given points. In 2010, all 30 geomagnetic points nationwide were observed en bloc, resulting in the making of the geomagnetism map of Korea.
- The list of geomagnetic points administered by the NGII
- The earth is a giant magnet, and the magnetic field created by the "Earth magnet" is called geomagnetism. Deflection angle refers to the angular difference between magnetic north and true north. When the deflection angles of magnetic north were measured at each point, the exact direction of true north can be determined. The deflection angles are marked on 1/25000 and 1/50000 scale topographic maps published by the NGII.
- The SI unit of geomagnetism is tesla (T). The unit nanotesla (nT), or one-billionth of tesla, is more commonly used because the earth´s geomagnetism is very weak. (50,000nT = 0.5 Gauss)
- Geomagnetism is expressed in terms of vector quantity by strength and direction. The strength of a magnetic field can be measured by a Propton magnetometer. The absolute value of direction of a magnetic field can be gauged by using a DI Meter equipped with magnetic sensors for non-magnetic traffic, or magnetic equipment that spins coil.
Changes in geomagnetic direction over the course of time are measured by a fluxgate-type magnetometer.
- ① Use of geomagnetic data
- A. The data can be used to draw up the geomagnetism map of Korea to show the strength of geomagnetism at each regional point and the long-term changes of the geomagnetism, and to modify the deflection angles marked on topographic maps.
B. The comprehensive interpretation of the observation data regarding GPS, benchmarks and gravity can be used to explain the earth´s internal structure.
- ② Method of indicating geomagnetism
- B. Three separate constituents are required to indicate geomagnetism.
Geomagnetic surveying instrument (MAG-01H)
Gravity observatory (left) and geomagnetic observatory (right)