Sources of Noise
As with all geophysical methods, a variety of noises can contaminate our seismic
observations. Because we control the source of the seismic energy, we can control
some types of noise. For example, if the noise is random in occurrence, such
as some of the types of noise described below, we may be able to minimize its
effect on our seismic observations by recording repeated sources all at the
same location and averaging the result. We've already seen the power of averaging
in reducing noise in the other geophysical techniques we have looked at. Beware,
however, that averaging only works if the noise is random. If it is systematic
in some fashion, no amount of averaging will remove it.
The noises that plague seismic observations can be lumped into three categories
depending on their source.
- Uncontrolled Ground Motion - This is the most obvious type of noise.
Anything that causes the ground to move, other than your source, will generate
noise. As you would expect, there could be a wide variety of sources for this
type of noise. These would include traffic traveling down a road, running
engines and equipment, and people walking. Other sources that you might not
consider include wind, aircraft, and thunder. Wind produces noise in a couple
of ways but of concern here is its effect on vegetation. If you are surveying
near trees, wind causes the branches of the trees to move, and this movement
is transmitted through the trees and into the ground via the trees' roots.
Aircraft and thunder produce noise by the coupling of ground motion to the
sound that we hear produced by each.
- Electronic Noise - As you've already
seen, geophones convert the ground motion they detect to electrical
signals. These signals are then transmitted down the cable, amplified by
the recording system, and recorded. Thus, anything that can cause changes
in the electrical signal in the cable or the recording system causes noise
in our recorded data. Electrical noise can come from a variety of sources.
For example, dirty or loose connections between the geophones and the cable
or the cable and the recording system can produce noise. Wet connections
anywhere in the system can cause electrical noise. Wind can also cause
electrical noise. This occurs if, for example, the cable is suspended in
bushes. As the wind blows the bushes, this moves the cable. The cable is
nothing more than a long electrical conductor. As it moves in the Earth's
magnetic field, an electrical current is produced in the cable.
- Geological Noise - Finally, we can consider any type of subsurface
geological structure that we can not easily interpret to be a source of noise.
In seismic refraction surveying, we will assume that the subsurface structure
varies laterally only along the line connecting the source to the geophones.
If the Earth actually varies significantly away from our line, it is possible
for us to misinterpret the seismic waves we record as structure below the
geophones instead of structure to the side of the geophones. Like our resistivity
observations, we will interpret our seismic observations as if they had been
generated from relatively simple earth models. Although these models can be
more complex than those used to interpret resistivity observations (we can
have dipping layers and topography on the layers), in interpreting refraction
seismic observations we must assume that variations occur along the line in
which data is collected only.
Even along the line, geological structure on a scale too small to be accommodated
in our model, or too detailed for our experiment, will give rise to data which
will depart from the predictions of our simple models, and we will treat those
departures (for instance, a gradational boundary between two layers, rather
than a sharp one) as "noise".
