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50 m and more, a consequence of the Nyquist criteria.
Importantly, there is no noticeable difference in the data
acquired above the mineralisation and elsewhere. The
near-surface material contains occurrences of the magnetic
mineral maghaemite, which produce very short-wavelength
(high spatial frequency) variations (see
Section 3.9.6
)
. Data
acquired at a smaller station spacing of 0.25 m (minimum
properly represented wavelength 0.5 m) are more useful as
they show not only the longer wavelength variation of the
mineralisation, but also the very short-wavelength
minimal bene
t (and increased cost for a given survey
area) in making measurements closer together than the
size of the footprint, because then a large portion of the
same geology, in the footprint, would also contribute to
the neighbouring measurement. More closely spaced meas-
urements are, however, useful for applying post-survey
signal-enhancement processing (see
Section 2.7.4
).
2.6.3
Survey design
'
variations of the near-surface. The near-surface variations
are under-sampled in the 25-m-sampled dataset and, there-
fore, have created spurious longer wavelength variations;
the true signature of the near-surface response has not been
resolved by the survey. The short-wavelength near-surface
response is properly de
'
spikey
An important consideration in designing a geophysical
survey is the optimum number and distribution of the
measurements to be made. Too many measurements are
a waste of time and money, too few and the survey
s
objective may not be achieved. In deciding the spacing
between measurements it is important to consider the
wavelength of the expected responses and the footprint of
the measurement, i.e. the size of the area in
'
ned in the 0.25-m-dataset and,
therefore, can be accurately removed using data processing
techniques (see
Section 2.7.4
)
, to reveal the longer wave-
example illustrates the need to consider the characteristics
of both the signal and the noise when setting the data
sampling interval, and the implications that the sampling
interval has for the ability to separate the various responses
using data processing techniques.
uencing the
measurement. The distance between individual measure-
ments should be suf
ciently close to satisfy the require-
ments of the sampling theorem (see
Section 2.6.1
)
. It is
important that the survey extends across a large enough
area to de
ne the longest wavelength response of interest
(see
Appendix 2
)
, and that the survey exceeds the limits of
the area of interest in order to determine the regional
response (see
Section 2.9.2
)
, so as to facilitate its removal
from the survey data. Survey design should also account
for the need to minimise all sources of noise (see
Section
2.4
). An example of a data acquisition variable directly
related to improving SNR is the number of repeat meas-
urements (if any) to make at each location. These allow
suppression of random noise (see
Section 2.7.4.1
), but also
increase the time (and cost) to acquire the data. Ultimately,
the survey budget will dictate how many measurements
may be taken and their accuracy, and geological and logis-
tical factors will also constrain their distribution.
Note that it is common for several parameters to be
measured in a single geophysical survey, e.g. airborne
magnetics and radiometrics. Characteristics of all
the parameters being measured need to be considered
when designing the survey, but usually the parameter of
principal
2.6.2
System footprint
The measurement footprint of a geophysical survey system
is the volume of the subsurface contributing to an individ-
ual measurement. Consistent with the source-to-detector
proximity effect on responses (see
Section 2.3
)
, the mater-
ials exerting the most influence on the measurement are
those closest to the sensor, i.e. the point on the ground
surface immediately below the sensor, with influence pro-
gressively decreasing away from this point. The footprint is
often arbitrarily taken as the volume of materials that
contribute 90% of the measured response. For measure-
ments made with a stationary sensor in a geologically
homogeneous area, the surface projection of the footprint
is circular. When measurements are made from a moving
platform (an aircraft, ground vehicle or a boat), the system
will travel some distance during the time taken to make a
measurement, creating a measurement footprint that is
elongated in the survey direction. How far the footprint
extends into the subsurface varies for different types of
geophysical measurement.
The system footprint is a fundamental consideration in
survey design, and it increases with survey height. There is
interest will control
the setting of survey
parameters.
2.6.3.1
Modelling as an aid to survey design
Whether the aim of a survey is mapping, detection or
characterisation, the geological characteristics of the target
and its geophysical response, with respect to the response
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