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a)
Flight direction
a)
800
Nominal
clearance
Sensor
height
Sensor height
Sensor height
600
400
200
0
Ideal geometry
Reading too low
Reading too high
600
b)
400
200
Sensor
height
0
Constant
clearance
Sensor height
Sensor height
Location on flight-line
Ideal geometry
Reading too high
Reading too low
b)
Flight direction
Figure 2.7 In
uence of topography on geophysical measurements. (a)
Effects when sensor height varies, as with loose-draped airborne
surveys. (b) Effects when the sensor is maintained at constant terrain
clearance, as in ground and close-draped airborne surveys in rugged
terrain. Note how the effects of the topography on the reading are
opposite in (a) and (b).
800
600
400
200
0
600
400
200
measured geophysical response is a distortion of that for a
horizontal survey surface.
Other features of the terrain producing similar effects to
topography include open-pit quarries and mines, tall
obstructions such as buildings and other infrastructure,
and trees and thick vegetation. The availability of high
resolution digital terrain information from airborne and
satellite sensors, and stereo photography, is an important
development in terms of compensating for terrain effects.
The height of the terrain above sea level can be obtained
from airborne geophysical surveys by combining the air-
craft height above the terrain, measured with a radio
altimeter, with the GPS-derived height. A digital elevation
model (DEM), also known as a digital terrain model
(DTM), for the survey area can be created in this way.
A useful source of radar-derived terrain information is the
Shuttle Radar Topography Mission (SRTM) dataset
(Cowan and Cooper, 2005 ) . As a general rule, it is good
practice to have a terrain model available when interpret-
ing geophysical data.
0
Location on flight-line
0
2000
Draped flight path
Constant altitude flight path
Metres
Figure 2.6 Survey height and associated variations in terrain
clearance for draped and constant barometric height surveys flown
in opposite directions across a ridge. The draped paths are actual
flight paths from an aeromagnetic survey in Western Australia.
Based on diagrams in Flis and Cowan ( 2000 ).
low-level helicopter-borne surveys conducted in rugged
terrains. Both situations can cause anomalous responses
which might be misinterpreted (Mudge, 1998 ) . Making a
measurement in a gully or adjacent to a cliff means more of
the
is closer to the sensor and can create anomal-
ously high readings, with the opposite occurring when on
the top of ridges etc. Note that this is exactly the opposite
of the effects on imperfectly draped airborne surveys in
Fig. 2.7a .
Another problem caused by topography is the distortion
of the geophysical response. This is particularly a problem
for electrical and electromagnetic surveys where the near-
surface flow of electrical current is strongly in uenced by
the shape of the (conductive) terrain (see Section 5.6.7.3 ).
Also, surveys conducted on sloping terrain need to account
for the terrain slope in the analysis of the data, because the
'
geology
'
2.4.1.2 Near-surface and deep-seated responses
A signi cant source of geological noise is the near-surface
environment ( Fig. 2.5b ). Regolith can present major prob-
lems for geophysical surveying (Doyle and Lindeman,
1985 ) , as can cover such as sand dunes, glacial deposits
and snow (Smee and Sinha, 1979 ), and permafrost
 
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