Geology Reference
In-Depth Information
Figure 8.1
Coil systems for electromagnetic surveys.
8.1.1 System descriptions
In both CW and TEM surveys, sources are (usually) and receivers are
(almost always) wire loops or coils. Small coil sources produce dipole
magnetic fields that vary in strength and direction as described in Sec-
tion 1.2.5. Anomaly amplitudes depend on the coil magnetic moments,
which are proportional to the number of turns in the coils, the coil areas
and the currents circulating. Anomaly shapes depend on system geometry
as well as on the nature of the conductor.
Coils are described as horizontal or vertical according to the plane in
which the windings lie. 'Horizontal' coils have vertical axes and are al-
ternatively described as
vertical dipoles
. Systems are also characterised by
whether the receiver and transmitter coils are
orthogonal
(at right angles to
each other),
co-planar
or
co-axial
, and by whether the coupling between
them is a maximum, a minimum or variable (Figure 8.1).
Co-planar and co-axial coils are maximum-coupled, because in both cases
the primary flux from the transmitter acts along the axis of the receiver coil.
Maximum-coupled systems are affected only slightly by small misalign-
ments but, because a strong in-phase field is detected even in the absence
of a conductor, they are very sensitive to changes in coil separation. Or-
thogonal coils are minimum-coupled. The primary field is not detected and
small changes in separation have little effect, but large errors are produced
by even slight misalignments. In the field it is easier to maintain a constant
coil separation than a constant relative orientation, and this is one reason for
favouring maximum coupling.
Dip-angle
systems, in which the receiver coil is rotated to determine the
dip of the resultant field, were once very popular but are now generally
limited to the
shoot-back
instruments used in rugged terrain. Shoot-back
receiver and transmitter coils are identical and are linked to electronic units
