Geology Reference
In-Depth Information
hope that the primary signals will be reasonably well coupled to the target
conductors for at least some of the time.
9.1.5 Elliptical polarisation
The vertical and horizontal fields in Figure 9.4 differ not only in amplitude
but also in phase. If there is a 90
◦
difference in phase between fields of
the same frequency that are at right angles to each other, the result will be
elliptical polarisation
(Figure 9.6b). However, the secondary field generally
has a horizontal component that will combine with the primary field to
produce a field that is still horizontal but differs from its components in
both phase and magnitude. Combining this with the vertical component of
the secondary field produces a resultant that is tilted as well as elliptically
polarised. Because the secondary field has a horizontal component, the
tangent of the tilt angle is not identical to the ratio of the vertical secondary
field to the primary and, because of the tilt, the quadrature component of
the vertical secondary field does not define the length of the minor axis
of the ellipse. This seems complicated, but dip-angle data are usually only
interpreted qualitatively. Where quantitative interpretation is attempted, it is
usually based on physical or computer model studies, the results of which
can be expressed in terms of any of the quantities measured in the field.
Figure 9.6
Combination of vertical and horizontal magnetic field vectors.
(a) Horizontal and vertical fields in-phase; the vertical vector has its max-
imum value OA when the horizontal vector has its maximum value OP and
the resultant has its maximum value OT. At any other time, as when the
vertical vector has value OB and the horizontal vector has value OQ, the
resultant OS is directed along OT but with lesser amplitude. All three are
zero at the same time. (b) Phase-quadrature: the vertical vector is zero when
the horizontal vector has its maximum value OP, and has its maximum value
OA when the horizontal value is zero. At other times, represented by OB,
OQ and OS, the tip of the resultant describes an ellipse.
