Geology Reference
In-Depth Information
Figure 1.4
Lines of force from a semi-infinite slab. The lines diverge ap-
preciably only near the edge of the slab, implying that elsewhere the field
strength will decrease negligibly with distance.
termed
two-dimensional (2D)
and are often used in computer modelling to
approximate bodies of large strike extent. If the source 'point' in Figure 1.1
represents an infinite line-source seen end-on rather than an actual point,
the area of the enclosing (cylindrical) surface is proportional to its radius.
The argument applied in the previous section to a point source then leads
to the conclusion that the field strength for a line-source will be inversely
proportional to distance and not to its square. It follows that, in 2D situations,
lines of force drawn on pieces of paper can indicate field intensity (by their
separation) as well as direction.
1.2.4 One-dimensional sources
The lines of force or radiation intensity from a source consisting of a homo-
geneous layer of constant thickness diverge only near its edges (Figure 1.4).
The
Bouguer plate
of gravity reductions (Section 2.5.1) and the radioactive
source with
2
π
geometry
(Section 4.3.4) are examples of infinitely extended
layer sources, for which field strengths are independent of distance. This
condition is approximately achieved if a detector is only a short distance
above an extended source and a long way from its edges.
1.2.5 Dipoles
A dipole consists of equal-strength positive and negative point sources a
very small distance apart. Its
moment
is equal to the pole strength multiplied
