What corrections does one need to make?

Diurnal variation correction

The Earth's magnetic field varies over it's surface from time to time and place to place. These changes are largely due to variations in solar flare activity and the effect that solar storms have on different locations on the Earth. There are several ways to remove the effects of diurnal variations. For smaller scale engineering-type applications, the use of a base (reference) station near the survey area is usually deployed. This base station can either be re-occupied at a fixed time interval throughout the survey or (if possible) an instrument can record the geomagnetic field continuously at this location. The diurnal variation of the geomagnetic field can thus be subtracted from the measurements obtained in the survey area.

This procedure is not pratical for marine or airborne surveys, however. These type of surveys are usually of regional extent and thus the records of the nearest magnetic observatory are often employed. Diurnal variations differ quite markedly from place to place and so, as a rule of thumb, the observatory used should not be more than about 100 km from the survey area. Diurnal variation during an aeromagnetic or marine survey can alternatively be assessed by arranging numerous cosssover points in the survey plan. The change in readings at each crossover point represents a change in the geomagnetic field over different time periods. This allows the entire survey to be corrected for diurnal variation, without the necessity of a base instrument.

Geomagnetic corrections

The geomagnetic correction is necessary because the Earth's magnetic field naturally varies as one moves from the poles to the equator. This variation is some degree predicable and known and therefore called the geomagnetic reference field. The International Geomagnetic Reference Field (IGRF) defines the theoretical undisturbed magnetic field at any point on the Earth's surface. About 90% of the Earth's field can be represented by the field of a theoretical magnetic dipole at the center of the Earth inclined at about 11.5 degrees to the axis of rotation. The magnetic moment of the geocentric dipole can be calculated from the observed field. If this dipole field is subtracted from the observed magnetic field, the residual field can then be approximateed by a second smaller dipole. This process can be iterated many times by fitting smaller and smaller dipoles to the ever decreasing moment until the observed geomagnetic field is simulated to the required degree of accuracy. The effect of each ficticious dipole contribution is know as a harmonic.

Elevation and Terrain Corrections

The vertical gradient of the geomagnetic field is only some 0.03 nT /m at the poles and -0.015 nT /m at the equator, so an elevation correction is rarely applied. The influence of topography can be significant in ground or near ocean bottom surveys, but it is not, in general, clear as to how to apply a terrain correction. Terrain correction depend upon the magnetic properties of the topographic features and it is not always clear what the appropriate properties are for nearby topographic features. Thus, in most magnetic surveys terrain corrections are rarely applied.

Having applied all of the forementioned correction, all remaining magnetic field variations should be caused solely by spatial variation in the magnetic properties of the subsurface and are referred to as magnetic anomalies .