Geology Reference
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Figure 1.5 The dipole field. The plane through the dipole at right angles
to its axis is known as the equatorial plane, and the angle, L , between this
plane and the line joining the centre of the dipole to any point P is sometimes
referred to as the latitude of P. The fields shown, at distances r from the dipole
centre, are for a dipole with strength (moment) M (see Section 3.1.1). The
values for the radial and tangential fields at P follow from the fact that M is
a vector and can therefore be resolved according to the parallelogram law.
The symbol
is used for the proportionality constant where magnetic fields
are concerned (Chapter 3).
ยต
by the separation distance. Field strength decreases as the inverse cube of
distance, and both strength and direction change with 'latitude' (Figure 1.5).
The intensity of the field at a point on a dipole 'equator' is only half the
intensity at a point the same distance away on the dipole axis, and in the
opposite direction.
Magnetisation is fundamentally dipolar, and electric currents circulating
in small loops are dipolar sources of magnetic field. Many radar antennas
are dipolar, and in some electrical surveys the electrodes are set out in
approximately dipole pairs.
1.2.6 Exponential decay
Radioactive particle fluxes and seismic and electromagnetic waves are sub-
ject to absorption as well as geometrical attenuation, and the energy crossing
closed surfaces is less than the energy emitted by the sources they enclose.
In homogeneous media, the percentage loss experienced by a plane wave is
determined by the path length and the attenuation constant . The absolute
loss is proportional also to the signal strength. A similar exponential law
(Figure 1.6), governed by a decay constant , determines the rate of loss of
mass by a radioactive substance.
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