Geology Reference
In-Depth Information
file with equal gradients on either side of the body.As the
inclination of the conductor decreases, the gradients on
either side become progressively less similar. The asym-
metry of the tilt-angle profile can thus be used to obtain
an estimate of the dip of the conductor.
Tilt-angle methods employing fixed transmitters have
been largely superseded by survey arrangements in
which both transmitter and receiver are mobile and
which can provide much more quantitative information
on subsurface conductors. However, two tilt-angle
methods still in common use are the very low-frequency
(VLF) and audio-frequency magnetic field (AFMAG)
methods, neither of which requires the erection of a
special transmitter.
H
s
H
p
θ
Horizontal
Fig. 9.3
The polarization ellipse and tilt-angle
q
.
H
p
and
H
s
represent the primary and secondary electromagnetic fields.
9.4.2 TheVLF method
The source utilized by the VLF method is electromag-
netic radiation generated in the low-frequency band of
15-25 kHz by the powerful radio transmitters used in
long-range communications and navigational systems.
Several stations using this frequency range are available
around the world and transmit continuously either an
unmodulated carrier wave or a wave with superimposed
Morse code. Such signals may be used for surveying up
to distances of several thousand kilometres from the
transmitter.
At large distances from the source the electromag-
netic field is essentially planar and horizontal (Fig. 9.6).
The electric component
E
lies in a vertical plane and the
magnetic component
H
lies at right angles to the direc-
tion of propagation in a horizontal plane. A conductor
that strikes in the direction of the transmitter is cut by the
magnetic vector and the induced eddy currents produce
a secondary electromagnetic field. Conductors striking
at right angles to the direction of propagation are not cut
effectively by the magnetic vector.
The basic VLF receiver is a small hand-held device in-
corporating two orthogonal aerials which can be tuned
to the particular frequencies of the transmitters.The di-
rection of a transmitter is found by rotating the horizon-
tal coil around a vertical axis until a null position is found.
Traverses are then performed over the survey area at
right angles to this direction. The instrument is rotated
about a horizontal axis orthogonal to the traverse and the
tilt recorded at the null position. Profiles are similar in
form to Fig. 9.4(a), with the conductor lying beneath lo-
cations of zero tilt. See Hjelt
et al.
(1985) for a discussion
of the interpretation of VLF data and Beamish (1998) for
a means of three-dimensional modelling of VLF data.
search coil is rotated about three orthogonal axes until a
null signal is obtained so that the plane of the coil lies in
the plane of the polarization ellipse. The tilt-angle may
then be determined by rotating the coil about a horizon-
tal axis at right angles to this plane until a further mini-
mum is encountered.
9.4.1 Tilt-angle methods employing
local transmitters
In the case of a fixed, vertical transmitter coil, the pri-
mary field is horizontal. Eddy currents within a subsur-
face conductor then induce a magnetic field whose lines
of force describe concentric circles around the eddy cur-
rent source, which is assumed to lie along its upper edge
(Fig. 9.4(a)). On the side of the body nearest the trans-
mitter the resultant field dips upwards.The tilt decreases
towards the body and dips downwards on the side of the
body remote from the transmitter. The body is located
directly below the crossover point where the tilt-angle is
zero, as here both primary and secondary fields are hori-
zontal.When the fixed transmitter is horizontal the pri-
mary field is vertical (Fig. 9.4(b)) and the body is located
where the tilt is at a minimum. An example of the use of
tilt-angle methods (vertical transmitter) in the location
of a massive sulphide body is presented in Fig. 9.5.
If the conductor is near the surface both the amplitude
and gradients of the tilt-angle profile are large. These
quantities decrease as the depth to the conductor in-
creases and may consequently be used to derive semi-
quantitative estimates of the conductor depth. A vertical
conductor would provide a symmetrical tilt-angle pro-
