Geology Reference
In-Depth Information
Antenna
Electric
vector
Fig. 9.6
Principle of VLF method.
Dashed lines show a tabular conductor
striking towards the antenna which is cut
by the magnetic vector of the
electromagnetic field.
VLF field
Magnetic
vector
Direction
of
propagation
9.4.3 The AFMAG method
The AFMAG method (Labson
et al.
1985) can similarly
be used on land or in the air.The source in this case is the
natural electromagnetic fields generated by thunder-
storms and known as
sferics
. Sferics propagate around the
Earth between the ground surface and the ionosphere.
This space constitutes an efficient electromagnetic
waveguide and the low attenuation means that thunder-
storms anywhere in the world make an effective contri-
bution to the field at any given point. The field also
penetrates the subsurface where, in the absence of
electrically-conducting bodies, it is practically horizon-
tal. The sferic sources are random so that the signal is
generally quite broad-band between 1 and 1000 Hz.
The AFMAG receiver differs from conventional tilt-
angle coils since random variations in the direction and
intensity of the primary field make the identification of
minima impossible with a single coil. The receiver con-
sists of two orthogonal coils each inclined at 45° to the
horizontal (Fig. 9.7). In the absence of a secondary field
the components of the horizontal primary field perpen-
dicular to the coils are equal and their subtracted output
is zero (Fig. 9.7(a)). The presence of a conductor gives
rise to a secondary field which causes deflection of the
resultant field from the horizontal (Fig. 9.7(b)).The field
components orthogonal to the two coils are then un-
equal, so that the combined output is no longer zero and
the presence of a conductor is indicated. The output
provides a measure of the tilt.
On land both the azimuths and tilts of the resultant
electromagnetic field can be determined by rotating the
coils about a vertical axis until a maximum signal is ob-
tained. These are conventionally plotted as dip vectors.
In the air, azimuths cannot be determined as the coils are
(a)
(b)
45
0
= 0
/
45
Fig. 9.7
Principle of AFMAG receiver: (a) conductor absent,
(b) conductor present.
attached to the aircraft so that their orientation is con-
trolled by the flight direction. Consequently, only per-
turbations from the horizontal are monitored along
the flight lines.The output signal is normally fed into an
amplifier tuned to two frequencies of about 140 and
500 Hz. Comparison of the amplitudes of the signals at
the two frequencies provides an indication of the con-
ductivity of the anomalous structure as it can be shown
that the ratio of low-frequency response to high-
frequency response is greater than unity for a good con-
ductor and less than unity for a poor conductor.
The AFMAG method has the advantage that the fre-
quency range of the natural electromagnetic fields used
extends to an order of magnitude lower than can be pro-
duced artificially so that depths of investigation of sev-
eral hundred metres are feasible.
9.5 Phase measuring systems
Tilt-angle methods such as VLF and AFMAG are
widely used since the equipment is simple, relatively
