Geology Reference
In-Depth Information
Figure 1.1
Lines of force from an infinite line source (viewed end on). The
distance between the lines increases linearly with distance from the source
so that an arc of the inner circle of length L is cut by four lines but an arc
of the same length on the outer circle, with double the radius, is cut by only
two.
magnetic and electromagnetic (EM) work, and even particle fluxes and seis-
mic wavefronts can be described in terms of radiation fields. Sometimes
ambiguity is unimportant, and sometimes both meanings are appropriate
(and intended), but there are occasions when it is necessary to make clear
distinctions. In particular, the term
field reading
is nearly always used to
identify readings made
in
the field, i.e. not at a base station.
Physical fields can be illustrated by lines of force that show the field
direction at any point (Figure 1.1). Intensity can also be indicated, by using
more closely spaced lines for strong fields, but it is difficult to do this quan-
titatively where three-dimensional situations are being illustrated on two-
dimensional media.
In Table 1.1 there is a broad division into
passive
and
active
methods.
Passive methods use naturally occurring fields (such as the Earth's magnetic
field), over which the observer has no control, and detect variations caused by
geology or man-made objects. Interpretation is usually non-unique, relying
a great deal on the experience of the interpreter. Active methods involve
generating signals in order to induce a measurable response associated with
