A magnetic survey is one of the exploration tools used for measuring localized changes in the Earth’s magnetic field, caused by sedimentary rocks or mineral ore bodies. The magnetic properties of mineral ore bodies and igneous rocks allow them to be easily identified and mapped, via magnetic surveys.
Buried steel materials also produce strong local magnetic fields. This survey involves mapping changes in the magnetic field to determine the shape, size, and location of such ore bodies.
The intensity of magnetic field induced in the rock is 'k' times the Earth’s magnetic field, where 'k' is the magnetic susceptibility of rock.
Equipment Used in Magnetic Survey
Magnetometers are commonly employed for performing magnetic surveys. They are highly precise instruments, which measure local magnetic fields. They operate by using proton rich fluids around an electric coil. The protons are polarized temporarily by the magnetic field, which is generated with the application of current.
When the current supply is stopped, the protons realign along the Earth’s magnetic field. This realignment produces a small, measurable amount of current in the coil at a frequency proportional to the magnetic field intensity.
Magnetic Survey Planning
In mineral exploration, a magnetic survey is carried out along with gravity surveys. It is performed in two different stages: by delineating the structural features of interest, and for the identification of mineralized zones related to iron minerals or altered zones. Magnetic surveys play a key role in detecting ferrous ore, such as hematite, ilmenite, and magnetite.
While searching for conductive base metals with traverse lines, which are perpendicular to the geological strike, a magnetic survey is usually carried out, along with self-potential and electromagnetic surveys.
Data Acquisition and Processing
During data acquisition, a series of readings are taken at regular intervals on a survey grid. The spacing between reading stations and grid lines is based on the application of a survey. In general, smaller targets require dense survey grids and a high resolution.
Modern cesium vapor magnetometers are very sophisticated, which allows the collection of data in either continuous mode or as a set of point readings. The data is digitally stored, and various interpretation methods are applied to interpret the targeted material.
Sources and Further Reading