Geology Reference
In-Depth Information
b
′
a
b
a
r
A
a
R
B
b
b
a
(a)
(b)
Figure 1.2
Vector addition by the parallelogram rule. Fields in (a) that are
represented in magnitude and direction by the vectors
A
and
B
combine to
give the resultant
R
. In (b), the resultant
r
of the large field
a
and the small
field
b
is approximately equal in length to the sum of
a
and the component
b
a
of
b
in the direction of
a
. The angular difference in direction between
a
and
r
is small and therefore the component
b
a
in the direction of
r
is almost
identical to
b
a
.
a target. The observer can control the level of energy input to the ground and
also measure variations in energy transmissibility over distance and time.
Interpretation of this type of data can be more quantitative. Depth discrim-
ination is often better than with passive methods, but ease of interpretation
is not guaranteed.
1.2.1 Vector addition
When combining fields from different sources, vector addition (Figure 1.2)
must be used. In passive methods, knowledge of the principles of vector
addition is needed to understand the ways in which measurements of local
anomalies are affected by regional backgrounds. In active methods, a local
anomaly (
secondary field
) is often superimposed on a
primary field
produced
by a transmitter. In either case, if the local field is much the weaker of the two
(in practice, less than one-tenth the strength of the primary or background
field), then the measurement will, to a first approximation, be made in the
direction of the stronger field and only the component of the anomaly in that
direction will be measured (Figure 1.2b). The slight difference in direction
between the resultant and the background or primary field is usually ignored
in such cases.
If the two fields are similar in strength, there will be no simple relation-
ship between the magnitude of the anomalous field and the magnitude of
