Geoscience Reference
In-Depth Information
may, however, have to be removed from SP data and
downhole electromagnetic data. Masking of responses of
interest may occur where there are conductive geological
features which are not of interest. Examples include
conductive faults and shears, rock units such as shales
and graphitic zones, and the contacts between rocks with
contrasting conductivities across which the background
response changes.
Range for
electromagnetic surveys
a)
10 1
Geomagnetic
noise
10 0
Sferic noise
10 -1
50 Hz
Power-line
noise
10 -2
VLF
noise
10 -3
Atmospheric noise
Non-geological sources of environmental noise are time-
varying natural electromagnetic fields originating from the
magnetosphere, the region around the Earth which
includes the atmosphere and the ionosphere. The time
variations occur over a wide frequency range. Because of
their very low frequency they penetrate to great depths (see
Section 5.2.3.1 ), inducing circular current
10 -4
10 -5
10 -2
10 -1
10 0
10 1
10 2
10 3
10 4
Frequency (Hz)
b)
+
ow systems,
several thousand kilometres across, into the Earth
s crust
and mantle. The currents flow as horizontal layers and are
known as telluric currents.
Below 1 Hz the fields are due mainly to current systems
set up in the ionosphere by solar activity. They remain
fixed in position with respect to the Sun and move around
the Earth as it rotates. From 1 Hz to 10 kHz the currents
are mainly from the atmosphere and mainly due to pulses
known as sferics ( Fig. 5.26b ) , which are caused by lightning
discharges associated with worldwide thunderstorm activ-
ity in the lower atmosphere. They are random and vary
throughout the day, and they show pronounced seasonal
variation with activity decreasing away from the equator
since the major sources are in tropical equatorial regions.
Storm activity is nearly always occurring somewhere on
Earth, and their fields propagate in the Earth-ionosphere
cavity to great distances.
Telluric currents are a source of noise in resistivity, IP
and most EM surveys, particularly in conductive
environments, although they are the signal for magneto-
telluric measurements (see Appendix 4 ) . Noise due to
geomagnetic phenomena, i.e. micropulsations occurring
below 0.1 Hz (see Micropulsations in Section 3.5.1.1 ), is
of suf ciently low frequency to be important at only the
very low frequencies used for magnetotelluric measure-
ments. The strength of sferics at a particular site depends
on the location, strength, distribution and density of the
lightning strikes at the time, and the season. They are a
major source of noise in electrical and EM surveys, so
surveying in equatorial regions may be more desirable
during winter months when noise levels are lower. Also,
'
0
1
-
ms
Figure 5.26 Noise in different frequency bands. (a) The
electromagnetic noise spectrum. VLF
very low frequency.
Redrawn, with permission, from Macnae et al.( 1984 ) . (b) Two
examples of sferics. Redrawn, with permission, from Macnae et al.
( 1984 ) and Buselli and Cameron ( 1996 ) .
-
the different frequency bands vary markedly ( Fig. 5.26 ) .
Some types of noise are restricted to particular parts of the
EM spectrum, so digital
filters are included in the survey
equipment to attenuate the noisy part of the spectrum (see
Frequency/wavelength in Section 2.7.4.4 ).
5.4.2.1 Environmental noise
Geological sources of noise are usually in the form of
variable, and especially high, conductivity of the near-
surface (see Section 5.3.4 ). Examples include regolith,
permafrost, swamps, lakes, rivers and palaeochannels.
These can produce strong responses, particularly when
they are close to the system transmitter and/or receiver.
When a conductive overburden is present, the ability of
electrical and EM surveys to detect electrically anomalous
zones in the bedrock can be severely compromised.
Long-wavelength variations, on which target responses
are superimposed, are generally less of problem with elec-
trical and EM methods than, for example, gravity and
magnetic methods (see Section 2.9.2 ) . These variations
 
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