Geology Reference
In-Depth Information
Figure 8.5
Field components due to a current-carrying loop (Tx) acting
as a magnetic dipole source. A is a constant dependent on coil moment
and current strength, and
F
(r) and
F
(t) are, respectively, the radial and
tangential components. The 'primary' field,
F
(p), measured by a horizontal
receiver coil (Rx) is obtained by adding the vertical components of each.
penetration. In hot, dry countries, salts in the overburden can produce surface
conductivities so high that CWEM is ineffective and has been superseded
by TEM.
8.1.5 Effects of coil separation
Changes in coupling between transmitter and receiver can produce spurious
in-phase anomalies. The field at a distance
r
from a coil can be described
in terms of radial and tangential components
F
(
r
)and
F
(
t
), as shown in
Figure 8.5. The amplitude factor
A
depends on coil dimensions and current
strength.
For co-planar coils,
F
(
r
) is zero because
ϕ
is zero and the measured field,
F
, is equal to
F
(
t
). The inverse-cube law for dipole sources then implies
that, if the separation is actually
r
(1
+
ε
), then:
F
=
F
O
/
(1
+
ε
)
3
where
F
o
is the field strength at the intended spacing. If
ε
is small, this can
be written as:
F
=
F
O
(1
−
3
ε
)
