Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 342

of deploying a separate B- eld sensor, AEM systems obtain
both responses solely from dB/dt measurements made with
a coil sensor in one of the following three ways.
Firstly, recall from Section 5.7.1.7 that for systems trans-
mitting a step-change in primary field, the off-time dB/dt
measurement is the impulse response of the ground. As an
alternative to using a separate B- eld sensor, the B- field
measurement can be obtained by mathematically integrat-
ing the dB/dt measurements and adding the initial value of
the magnetic field at the beginning of the decay period.
Known as the recovered B-field, it requires that dB/dt
measurements be also made during the on-time of the
transmitted pulse. The measurement obtained with this
quasi B-field sensor has (depending on the shape of the
transmitted pulse) many attributes of the step response.
Secondly, and recalling Section 5.7.1.7 , for systems
transmitting a triangular pulse the dB/dt measurements
made during the pulse on-time are the step response. The
off-time measurements approximate the impulse response.
The third method involves making a very large number
(usually thousands) of equi-timed dB/dt measurements
continuously during the full period of the system wave-
form, known as full-waveform sampling. The measure-
ments include the strength of the primary field and the
decaying secondary field. The precisely known primary
field is mathematically removed from the measurements
and then any response of the ground can be computed
from the highly sampled time series, usually the step and
impulse responses (see Section 5.7.1.7 ).
Similarly to ground EM systems, AEM receiver meas-
urements are normalised for the transmitter loop moment,
the transmitter current at the time of the measurement (see
Section 5.7.4.1 ) and the receiver coil moment.

smaller number of channels representing the decay of the
secondary magnetic field.
The system self-response (the electrically conducting
airframe produces its own EM response) is also removed
from the data. It is measured during a calibration flight at
high altitude free from the ground response. Corrections
can also be applied for variations in receiver height above
the ground.

5.9.2 AEM systems

AEM systems carry both the transmitter and the receiver
on the moving platform. They also include a total field
magnetometer, radio altimeter to monitor the terrain clear-
ance, and GPS navigation and positioning. Some systems
also record the orientation of the transmitter loop and the
location of the receiver coils. Note that the magnetometer
is operated during an EM system off-time period to avoid
interference between the two systems. The larger along-
line sampling interval of AEM surveys, compared with
conventional aeromagnetic systems, means that the mag-
netic data are generally of lower resolution than data
acquired by dedicated aeromagnetic systems. Sometimes
radiometrics is included, provided the survey aircraft is
capable of carrying the additional weight of the equipment.
AEM systems are classi ed into two main categories:
towed-bird fixed-wing and rigid-frame helicopter systems.
Both categories have a number of system and response
characteristics in common. The operational characteristics
of each class of aircraft are the main factors determining
system selection: i.e. fixed-wing aircraft for wide-area
regional surveying and helicopters for local-area low-level
surveying. AEM systems are continually evolving, so we
provide only an overview of the main system features. Note
that most of these systems can be

5.9.1.3 Noise suppression
The receivers in TD-AEM systems make a large number,
usually thousands, of measurements during the measure-
ment period. A variety of noise suppression algorithms are
applied to these data post-survey to attenuate system noise,
powerline noise and sferics. Stacking is applied to improve
the signal-to-noise ratio (see Section 2.7.4.1 ) . However,
given that the survey platform is continually in motion
during the measurement period, a compromise is required
between achieving an acceptable signal-to-noise ratio and
the inevitable reduction in resolution and sensitivity due to
the motion. Too many stacks distort the data, reducing
both the temporal and spatial resolutions. Throughout the
process the large volume of measurements are reduced to a

c geo-
logical targets and environments by adjusting their system
base frequency (see Section 5.7.3.1 ) .

' tuned ' to speci

5.9.2.1 Towed-bird systems
Fixed-wing towed-bird systems have a large transmitter
loop surrounding the aircraft, suspended from the air-
craft

s wing-tips, nose and tail ( Figs. 5.98 and 5.99a ).
The receiver is mounted in a

'

and towed behind the
aircraft, a short distance from the trailing side of the
transmitter loop, in the separated-loop con guration (see
Moving-loop mode in Section 5.7.3.2 ) . Oscillatory motions
of the towed receiver bird, i.e. yaw, pitch and roll, produce
changes in the measured strength of the secondary field

' bird '