Basic magnetic theory
ENVI-MAG Manual B1-7
Overview
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Figure B-7 Variation of profile according to depth and magnetization.
The variation with depth of the amplitude of the local disturbance vector is
a function of the distance between the sensor and the target. This variation
with depth is called the fall-off rate. The intensity of the local disturbance
varies from one model to another.
Variations in the Earth’ s magnetic field
The Earth’s magnetic field varies with time, i.e. it is not constant. As the Earth
rotates, the outer layers of the ionosphere interact with the solar wind to cause
minor fluctuations in the magnetic field. Depending upon the duration and
intensity of these fluctuations, they are given different names.
Diurnal variation
Fluctuations with a period lasting about one day are called diurnal
variations. These can be considered much like tides that ebb and flow during
the course of a day. However, they are not predictive and are usually not a
problem when conducting magnetic surveys. This diurnal drift can cause a
variation of the order of 50 nT/Hour. The following figure (Figure B-8)
illustrates a typical diurnal variation of the total field.
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