Geology Reference
In-Depth Information
Figure 3.8
Mid-latitude total field anomaly due to induced magnetisation.
(a) The induced field. (b) The anomaly profile, derived as described in the
text.
and E, where it is at right angles to the Earth's field. The anomaly will
be positive between these points and negative for considerable distances
beyond them. The anomaly maximum will be near D and there will be a
quite strong, but broader and negative beyond C, with a minimum near B
but detectable well beyond A. The peak on the profile is thus offset towards
the magnetic equator (Figure 3.8b). Applying this sketching technique at
the magnetic equator, where the inducing field is horizontal, shows that the
total-field anomaly there would be negative and centred over the body and
would have positive side-lobes to north and south.
Because each positive magnetic pole is somewhere balanced by a negative
pole, the net flux involved in any anomaly is zero. The fields from the positive
and negative poles cancel out above the central parts of a uniform and
horizontal magnetised sheet, and only the edges are magnetically detectable.
Strongly magnetised but flat-lying bodies may consequently produce little
or no anomaly.
