Geoscience Reference
In-Depth Information
Section 5.7.5 ). Anomaly shapes for the in-loop and
separated-loop con gurations described previously (see
Pro le analysis in Section 5.7.5.3 ) apply also to AEM data.
a)
90 Hz
X component
(ppm)
7000
6000
Bedrock target
response
5.9.4.1 Interpretation pitfalls
AEM data have the same interpretational problems as EM
data in general and are described in Section 5.7.6 .Inadd-
ition, variations in survey height above the ground, common
in rugged terrains, cause variations in the size and strength
of the primary field in the ground with subsequent effect on
lateral resolution and strength of the secondary field. Inte-
grating survey height data and terrain data with the EM data
allows these effects to be recognised for what they are. The
survey-direction dependent asymmetry of the towed-bird
separated-loop response needs to be accounted for when
working with pro
5000
4000
3000
Overburden
response
2000
Earlier
channel
Later
channels
1000
0
14,000
15,000
16,000
17,000
18,000
19,000
Location (m)
b)
30 Hz
X component
(ppm)
le data from adjacent survey lines.
Bedrock target
response
7000
6000
5.9.5 Examples of AEM data from
mineralised terrains
5000
Overburden
response
4000
Earlier
channel
3000
Witherly ( 2000 ) reviews 50 years of AEM surveying and
credits AEM with directly aiding in the discovery of more
than 80 mineral deposits. The most common targets are
massive metal sulphides. Examples of AEM responses from
nickel sulphide deposits are described by Wolfgram and
Golden ( 2001 ).
2000
Later
channels
1000
0
14,000
15,000
16,000
17,000
18,000
19,000
Location (m)
Figure 5.100 GEOTEM responses for two different system base
frequencies. (a) 90 Hz and (b) 30 Hz. Note the enhanced target
response and reduced overburden response at the lower frequency.
Redrawn, with permission, from Smith and Annan ( 1997 ).
5.9.5.1 Butcherbird Supergene manganese deposit
This example illustrates the use of AEM for mapping
shallow conductivity structure and for targeting conductive
manganese mineralisation in the Mesoproterozoic Collier
Basin of Western Australia. EM and magnetic data were
acquired with the XTEM helicopter system, using a dB/dt
sensor, along survey lines spaced 200 m apart. Manganese
oxide mineralisation occurs as sub-horizontal sheets sev-
eral hundred metres in length usually within a few tens of
metres of the surface. Figure 5.101 shows the magnetic data
and gridded TDEM channel amplitude data. These data
are chosen to illustrate shallow, intermediate and deeper
responses. CDIs were created from the pro le data
( Fig. 5.102 ) and maps were created by combining the
results for selected pseudo-depths ( Fig. 5.101 ). Known
mineralisation at Yanneri was shown to coincide with a
conductive response, and other similar target responses
were identi ed in the data. Subsequent drilling discovered
additional manganese mineralisation. Responses related to
non-economic targets were also mapped, for example a
palaeochannel (a source of water) and a pyritic shale unit.
strike. Line spacing is determined by the strike length of
the target, and the spacing should be set so that at least
two, and preferably three, survey lines pass over the target.
Survey line spacing is usually chosen as a compromise
between survey cost and the expected strike length of target
conductors with terrain clearance kept as low as possible
whilst maintaining safe operations.
5.9.4 Display and interpretation of AEM data
Display and interpretation of AEM data is essentially iden-
tical to that for ground data (see Sections 5.7.4 and 5.7.5 ),
although the characteristically large data volumes have led
to AEM data being routinely displayed as imaged conduct-
ivity distributions obtained by inversion of the data (see
Sections 2.11.2.1 and 5.7.4.3 ) . The analyses otherwise
proceed in an identical way to that for ground data (see
 
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