Geoscience Reference
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the near-surface feature is the target of interest, but it can
be a major source of interference where measuring the
lower-amplitude, longer-wavelength responses of deeper
features is the main interest. In this case, increasing the
survey height rapidly attenuates the near-surface high-fre-
quency response, but with smaller effect on the deeper-
seated responses.
The amplitude and wavelength of the magnetic response
is sensitive to source
Survey line direction and spacing
Survey line direction requires particular attention when
designing both ground and airborne magnetic surveys as
it is somewhat dependent on the magnetic latitude of the
survey area. In middle to high latitudes, i.e. towards the
poles, survey lines ought to be oriented perpendicular to
the regional strike of the magnetic sources.
In contrast, at low latitudes (less than about 30°), the
sensor separation. Figure 3.66 shows
two magnetic datasets recorded at different survey heights.
The data from Western Australia were acquired at a height
of 40 m and the data from Canada at a height of 70 m. The
obviously smoother appearance of the Canadian data
reflects the greater attenuation of shorter wavelengths with
increasing height compared with the longer wavelengths.
The signi
-
'
of the anomaly are displaced in the direc-
tion of the magnetic meridian to the northern and south-
ern edges of the source (see Figs. 3.8 and 3.26 ), respectively,
irrespective of the shape and strike of the magnetic body.
Survey lines oriented along the magnetic meridian,
high
'
and
'
low
'
i.e.
magnetic north-south, measure both the
'
of the anomaly and provide more information about the
source than lines oriented perpendicular to strike. This is
critically important as it ensures that the survey lines
actually traverse the anomaly dipole, essential for its analy-
sis. However, this strategy assumes the anomalies are
entirely due to induced magnetism. At low latitudes, rem-
anent magnetism can transform the response to that suited
to an alternative line orientation. This is a particular
advantage for long linear sources striking north
'
low
'
and
'
high
sensor separation on mag-
netic responses means that maintaining constant separ-
ation across the survey area is important in order to
minimise noise (Cowan and Cooper, 2003a ) .
Ground surveys by their nature are conducted with the
magnetic sensor at constant (survey) height. Portable
instruments typically have the sensor at a height of
1
cant effect of source
-
2 m, although up to 6 m height may be required where
surface magnetic noise is a problem. A wide range of line
spacing is used depending on the purpose of the survey.
Ground vehicle-borne magnetometer systems operate in a
similar way to airborne systems and provide very dense
sampling, less than a metre, but the locale must be suitable
for vehicles.
Airborne surveys, on the other hand, are conducted at a
constant nominal survey height (see Fig. 2.6 ), but suffer
unavoidable height variations due to topography. Where
weak magnetic responses are anticipated or where high
resolution is required, surveys can be conducted as low as
20 m above the ground with small crop-duster aircraft or
helicopters, topography permitting. Regional surveys
designed to map the longer-wavelength responses of
large-scale geological features are usually conducted at
higher terrain clearances, typically 60 to 150 m, which also
offers easier survey logistics in undulating terrains. In very
rugged terrains it can be logistically simpler to conduct the
survey at a constant barometric height above all the topog-
raphy, in which case the terrain clearance is variable (see
Fig. 2.6 ) .
When designing airborne magnetic surveys it is better to
lean towards lower survey height since it is always possible
to upward-continue the data to a greater height, but the
reverse is much more difficult (see Section 3.7.3.2 ).
-
south,
and particularly at the equator, where induction alone
produces anomalies at its northern and southern edges
with little or no measureable response over the central part
of the body
Survey line spacing is fundamental in determining the
lateral resolution of the survey and a major control on
overall survey cost. The survey lines can be widely spaced
when measuring long-wavelength responses and should be
closer for measuring shorter wavelengths, but in all cases
must be sufficiently close to adequately resolve the features
of interest. Choice of line spacing is not independent of
survey height. A lower survey height will enhance short-
wavelength responses which will require a smaller survey
line spacing to reduce across-line aliasing and allow the
data to be properly gridded (see Section 2.7.2 ). Ideally the
survey line spacing should not exceed twice the survey
height, but a wider spacing is acceptable for areas where
magnetic responses are continuous along strike (Cowan
and Cooper, 2003a )
Line spacings for magnetic survey are typically a few
tens of metres to a few hundred metres, increasing with
survey height. This results in between one and two orders
of magnitude difference in sampling interval in the along-
line and across-line directions. Tie lines (see Section
2.6.3.3 ) are typically spaced 10 times the survey line
-
 
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