Geoscience Reference
In-Depth Information
high as possible and that the amplitude of the noise, the
noise level, is as low as possible in order to obtain an
accurate measurement of the parameter of interest. As a
general rule, if SNR is less than one it will be very dif
cult
to extract useful information from the measurement,
although data processing techniques are available to
improve the situation (see
Section 2.7.4
).
Suppression of noise is of utmost importance and must
be considered at every stage of the geophysical programme,
from data acquisition through to presentation of the data
for interpretation. Active geophysical methods usually
allow the SNR to be improved by changing the nature of
the output from the transmitter, e.g. increasing its
amplitude or changing its frequency. This advantage is lost
with passive methods, where the geophysicist has no con-
trol over the natural
Wind is a common source of noise from the atmos-
phere. It causes objects attached to the ground to move,
e.g. trees and buildings, which produces noise in seismic,
electromagnetic and gravity surveys. The movement of
wires linking sensors to recording equipment may also
create noise because of voltages induced by their move-
ment
'
position and orientation of geophysical sensors during
airborne surveys which affect the measurements.
As well as creating noise, natural phenomena may
reduce the amplitude of the signal: for example, radioactiv-
ity emitted from soil is attenuated when the soil is satur-
ated by rainfall. The variability and unpredictability of
natural phenomena cause noise levels to vary during the
course of a geophysical survey.
Cultural noise includes the effects of metal fences, rail-
ways, pipelines, powerlines, buildings and other
infrastructure (see
Section 2.9.1
). In addition, cultural fea-
tures may radiate energy that causes interference, such as
electromagnetic transmissions (radio broadcasts etc.),
radioactive fallout and the sound of machinery such as
motor traf
c. Mine sites are particularly noisy environ-
ments, and noise levels may be so high as to preclude
geophysical surveying altogether.
The two most troublesome forms of geological environ-
mental noise are those associated with the shallow subsur-
face and with topography; the latter are known as
topographic or terrain effects. In both cases it is possible,
in principle, to calculate their effects on the data and
correct for them. To do so requires very detailed infor-
mation about the terrain and/or physical properties of the
subsurface, which is often lacking. This is an example of
the geophysical paradox (see
Section 1.3
)
. To fully under-
stand the geophysical signal, and the noise, requires infor-
mation about the subsurface. However, it was to acquire
such information that
through the Earth
.
The signal depends solely on the objective of the survey,
and geological responses not associated with the objective
of the survey constitute noise. Of course, any response of
non-geological origin will always be considered noise. As
data are revisited the information required from them may
vary, in which case so too do the representations of signal
and noise in the data. A useful de
nition of signal is then
'
'
transmitter
'
what is of interest at the time
'
, whilst noise would then be
'
everything else
'
; just as the saying goes,
'
One man
'
is trash is
'
'
'
another man
s treasure
, so also one geoscientist
is signal is
another geoscientist
s noise.
Two basic types of noise affect geophysical measure-
ments. Firstly, there are effects originating from the local
environment, i.e. environmental noise. Secondly, there is
methodological noise, which includes unwanted conse-
quences of the geophysical survey itself and of the process-
ing of the geophysical data. A feature in the data that is
caused by noise is referred to as an artefact. It goes without
saying that identi
'
cation and ignoring of artefacts is crit-
ical if the data are to be correctly interpreted.
the
geophysical
survey was
undertaken.
2.4.1
Environmental noise
2.4.1.1
Topography-related effects
Some examples of topography-related noise are shown
creates noise by causing variations in the distance between
geophysical transmitters and/or sensors and features in the
subsurface. This changes the amplitude and wavelength of
the responses (see
Section 2.3
). These effects can some-
times be accounted for by modifying the measurements,
during data reduction. The accuracy with which this can be
The main types of environmental noise affecting the
different types of geophysical survey are summarised in
origin; as either geological or non-geological. Geological
environmental noise is produced by the geological envir-
onment, including topography. Non-geological environ-
mental noise includes sources in the atmosphere and
outer space, plus cultural responses associated with human
activities.
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