Magnetic surveys measure the magnitude and orientation of the Earth’s magnetic field. Magnetic field at Earth’s surface depends on field generated in Earth’s core, magnetic mineral content of surface materials and remnant magnetisation of surface rocks. Ground magnetic survey mainly used to investigate subsurface geology on the basis of anomalies in the Earth’s magnetic field resulting from the magnetic properties of the underlying rocks. Usually, ground magnetic survey is employed in locating subsurface magnetic materials for possible exploration.
Most common magnetic minerals found are pyrrhotite, magnetite and iron sulphide. Magnetite when found with sufficient purity and quantity may become an iron ore deposits.
How It Works?
Ground magnetic survey are usually performed over relatively small area on a previously defined or suspected target. The survey are conduct in parallel lines between 10-50m with regular stations interval between 5m – 100m depending on survey area. Data acquisition or data collection are made using proton magnetometer. These instruments (magnetometer) will provide absolute readings of the total magnetic field accurate to plus minus 0.1nT (nano Tesla).A base station is established to monitor diurnal variations every 0.5 – 1.0 hours.
Correction or reduction of magnetic data is necessary to remove all causes of magnetic variation from the observations other than those arising from the magnetic effects of the subsurface. There are three type of magnetic data correction or reduction:
- Diurnal variation correction
- Geomagnetic correction
- Elevation and terrain correction (optional)
On ground magnetic survey, diurnal variation correction can be made by taking (monitoring) magnetic data at fixed base station periodically throughout the day. These base readings are used to correct for temporal variation in the measured field. Meanwhile, geomagnetic correction employed to removes the variations the effect of a geomagnetic reference field from the survey data. This correction is equivalent to the latitude correction in gravity survey.
The vertical gradient of the magnetic field is only some 0.03nTm-1 at the poles and -0.015nTm-1 at the equator, so an elevation correction is not usually applied. The influence of topography can be significant in ground magnetic surveys but is not completely predictable as its depends upon the magnetic properties of the topographic features. Therefore, in magnetic surveying terrain corrections are rarely applied.
Specialised software is used to process the corrected magnetic data and present in colour contoured map or anomaly map.