Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 344

5.9.2.2 Rigid-frame systems
Rigid-frame systems have the horizontal transmitter loop
and the receiver coils attached to a single rigid structure
made of lightweight non-conducting material. This main-
tains the separation and orientation of the transmitter loop
and receiver coils ensuring constant coupling between
them, which is essential when making measurements
during the transmitter on-time (see Section 5.9.1.2 ). Some
systems make use of a collapsible lightweight suspension
system. We also classify these as rigid-frame systems. The
whole structure is suspended below the aircraft ( Figs. 5.98b
and 5.99b ) . The receiver coils are located either on the
vertical axis of the loop or in the near-null position of the
primary field near the edge of the transmitter loop
( Fig. 5.99b ). Any offset between the receiver coils and the
centre of the loop is small compared with the survey height
so they can be considered coincident. The responses are
the same as those from the in-loop configuration used for
ground surveys (see Moving-loop mode in Section 5.7.3.2 ).
The in-loop configuration offers the distinct advantage,
over the separated-loop configuration of towed-bird
systems, of producing sharper anomalies with simpler
shapes that are independent of survey line direction, i.e.
there is no directional asymmetry (see Section 5.9.2.1 ) . It
also has less sensitivity to conductive overburden than the
towed-bird con guration. These features are demonstrated
by the model responses in Figs. 5.84 when compared with
those in Figs. 5.85 and 5.86 . The in-loop con guration also
has less sensitivity to conductive overburden than the
towed-bird con guration.
As with all helicopter systems, surveying is possible in
rugged terrains and at lower survey heights and speeds
than is achievable with fixed-wing aircraft, which leads to
increased signal strength and higher spatial resolution.
Helicopter systems

helicopter systems known as XTEM, SkyTEM, HELITEM
and AeroTEM. Their system waveforms, including the
time-window for their response measurements, are shown
in Fig. 5.97 . The waveform duty cycle is the proportion of
the cycle occupied by the transmitter on-time. Increasing
the base frequency increases the system

'

s response to shal-
low features and increases resolution of smaller contrasts
in conductivity (see Section 5.7.3.1 ). Readers requiring
further information about individual systems are referred
to the various system providers

5.9.3 AEM survey practice

There are two main considerations in the selection of an
AEM system. Firstly, non-target parameters such as the
extent of the survey area, i.e. whether it is a regional or
local area, and the nature of the topography in the area,
determine whether a

fixed-wing or a helicopter system is
appropriate. Secondly, and like ground EM surveying, the
main parameters that determine the target response are
conductor size, orientation and conductivity; depth to top;
conductivity of the host rocks; and whether conductive
overburden is present. As stated previously, it is dif cult
to build an AEM system that is fully multi-purpose, with
systems being optimised for a selected range of target
parameters.
Computer modelling the response of a range of target
parameters for the different AEM systems available, and,
importantly, for a range of their base frequencies, assists in
selecting the appropriate system. The ability to adjust the
system base frequency (see Section 5.7.3.1 ) allows the
system to be to a particular geological application;

for example see Fig. 5.100 . Furthermore, measurements
made over a longer decay period provide more information
for discriminating between poor and good conductors (see
Conductor quality in Section 5.7.2.3 ) . Modelling can set the
maximum depth of detection of a body of given size,
orientation and conductivity, and quantify the resolution
possible for the available AEM systems. It also helps survey
planning to establish the nature of the overburden and the
background, possibly by initially undertaking petrophysi-
cal studies and/or ground surveys in the proposed AEM
survey area.
Survey parameters are usually set to suit the require-
ments of the AEM system; magnetics and radiometrics are
secondary considerations. Survey lines should be oriented
perpendicular to the regional strike as this provides better
coupling to target conductors, which usually have the same

' tuned '

fly more slowly at 75 - 120 km/h

(20 70 m above the terrain with

the EM system suspended 30 m below the helicopter, i.e. at
about 30

- 33 m/s) and lower at 60 -

40 m above the ground ( Fig. 5.98b ) . The along-
line data interval

-

7 m. The area of the
transmitter loop and the number of turns in it vary from
system to system, with dipole moments ranging typically
from about 100,000 to 600,000 A m 2 , and sometimes as
high as 2 million A m 2 .

is typically 2 -

5.9.2.3 Examples of AEM systems
A variety of TD-AEM systems are in use at the time
of writing. These include fixed-wing systems known as
SPECTREM, TEMPEST, GEOTEM and MEGATEM, and