Geology Reference
In-Depth Information
and 60 m for horizontal coils, and 7.5, 15 and 30 m for the more convenient
but more surface-influenced vertical coils.
Both the EM31 and the EM34-3 are calibrated to read apparent con-
ductivity directly in mS m
−
1
, but the low-induction-number conditions on
which these values are based are not always achieved in practice, since rel-
atively high frequencies are used to ensure that there will be a measurable
signal in most ground conditions. Figure 8.10, which is based on Techni-
cal Notes available on the Geonics website, shows the ways in which the
indicated conductivity would deviate from the true conductivity of a homo-
geneous half-space as this decreases, for both the EM31 and the EM34-3,
and defines the conditions under which the deviations become appreciable.
The indicated conductivity is always lower than the true conductivity; i.e.
the indicated resistivity is always higher than the true resistivity. For the
EM34-3 in the vertical-dipole (horizontal-coil) configuration, the deviation
is large (i.e. a factor of 50%) at the high but perfectly possible conductivity
of 100 mS m
−
1
(resistivity 10
m) and has become catastrophic by about
500 mS m
−
1
(2
m). The situation is much better for the EM34-3 in the
horizontal-dipole (vertical-coil) configuration, and for the EM31 in both
configurations. These curves can be used to correct readings obtained over
homogeneous ground and, in principle, also those obtained over layered
ground. However, the concepts of apparent resistivity and apparent conduc-
tivity are themselves somewhat nebulous and in most cases survey results
are only used qualitatively (as, e.g., in Figure 8.11).
Selected induction number values are also indicated on Figure 8.10, for
both the EM31 and the EM34-3. These demonstrate the difficulty of for-
mulating a general rule governing the induction number value at which the
approximation breaks down.
8.3 Fixed-Source Methods
CWEM surveys can be carried out using long wires, set up in fixed locations,
instead of coils as sources. There are many possible system geometries but
the general principles remain the same.
8.3.1 The Biot
-
Savart Law
The fields produced by straight, current-carrying wires can be calculated by
repeated applications of the
Biot-Savart Law
(Figure 8.12). The application
of this relationship to four wires forming a rectangular loop is illustrated in
Figure 8.13. If the measurement point is outside the loop, vectors that do
not cut any side of the loop have negative signs.
The Slingram anomaly of Figure 8.4 was symmetrical because the receiver
and transmitter coil were moved over the body in turn. If the source, whether
a coil or a straight wire, were to be fixed and therefore not pass over the
