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 affect 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 catagories 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 affect 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.

• Geologic Noise - Finally, we can consider any type of subsurface geologic 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.