Geology Reference
In-Depth Information
and height is essential, and this is usually accomplished
by mounting the transmitter and receiver either on the
wings of an aircraft or on a beam carried beneath a heli-
copter. Compensating methods have to be employed to
correct for minute changes in the relative positions of
transmitter and receiver resulting from such factors as
flexure of the wings, vibration and temperature changes.
Since only a small transmitter-receiver separation is used
to generate and detect an electromagnetic field over a
relatively large distance, such minute changes in separa-
tion would cause significant distortion of the signal.
Fixed-wing systems are generally flown at a ground
clearance of 100-200 m, while helicopters can survey at
elevations as low as 20 m.
Greater depth of penetration can be achieved by the
use of two planes flying in tandem (Fig. 9.15), the rear
plane carrying the transmitter and the forward plane
towing the receiver mounted in a bird.Although the air-
craft have to fly at a strictly regulated speed, altitude and
separation, the use of a rotating primary field compen-
sates for relative rotation of the receiver and transmitter.
The rotating primary field is generated by a transmitter
consisting of two orthogonal coils in the plane perpen-
dicular to the flight direction. The coils are powered by
the same AC source with the current to one coil shifted
p
/2 (90°) out-of-phase with respect to the other. The
resulting field rotates about the flight line and is detected
by a receiver with a similar coil configuration which
passes the signals through a phase-shift network so that
the output over a barren area is zero. The presence of a
conductor is then indicated by non-zero output and the
measured secondary field decomposed into real and
imaginary components. Although penetration is
increased and orientation errors are minimized, the
method is relatively expensive and the interpretation of
data is complicated by the complex coil system. It is
possible to upward-continue airborne EM data. This
diminishes variations caused by height fluctuations and
anomalies of small, shallow sources.
Airborne TDEM methods, such as INPUT
®
(IN-
duced PUlse Transient) (Barringer 1962), may be used to
enhance the secondary field measurement. The discon-
tinuous primary field shown in Fig. 9.16 is generated
by passing pulses of current through a transmitter coil
strung about an aircraft. The transient primary field
induces currents within a subsurface conductor. These
currents persist during the period when the primary
field is shut off and the receiver becomes active.The ex-
ponential decay curve is sampled at several points and the
signals displayed on a strip chart.The signal amplitude in
successive sampling channels is, to a certain extent, diag-
nostic of the type of conductor present. Poor conductors
produce a rapidly decaying voltage and only register on
those channels sampling the voltage shortly after pri-
mary cut-off. Good conductors appear on all channels.
1.5msec
2msec
1.5msec
(a)
t
(b)
t
Receiver
in bird
Transmitter
20m
300m
(c)
100m
t
Flight
direction
Receiver
(d)
1
Channels
2
3
Transmitter
4
5
-
6
t
Output
Fig. 9.16
Principle of the INPUT
®
system. (a) Primary field.
(b) Receiver response to primary alone. (c) Receiver response in
the presence of a secondary field. (d) Enlargement of the receiver
signal during primary field cut-off.The amplitude of the decaying
induced voltage is here sampled on six channels.
-
Fig. 9.15
The two-plane, rotary field, EM system.
