A mechanism for generating the acoustic wave that travels through the earth is required for seismic acquisition. In all the preceding diagrams the source and reflection were depicted as spikes. While this is desirable it would require source to produce a signal with an infinite frequency bandwidth, which is not practical. The result is that the signal acquired is not a series of spikes but a waveform. The figure below shows a series of layers in the subsurface on the left with the idealized spike reflection series that is would produce in the center and the seismic signal that would be obtained from a real source on the right.
Figure 11 Rock layers, left, and the corresponding ideal and actual seismogram.
A number of types of sources have been used for seismic acquisition. Drilling shallow holes and exploding dynamite in the holes, called shot holes, was one of the earliest techniques used. Dynamite produces a sharp signal, but the consistency of the signal from shot to shot may vary and consequently the reflections vary from shot to shot. This variation from shot to shot causes many problems, for example, a reflection will not have the same shape from trace to trace making it difficult to recognize and the reflections would no longer add constructively when performing a CMP stack.
Airguns are generally used as sources in marine acquisition. An airgun consists of a chamber that is filled with compressed air that can be quickly released through a port in the side . When the airgun is placed in the water and the air is released it creates seismic wave in the water. The signal produced by an airgun is not as sharp or as loud as dynamite but it is more consistent.
An airgun is recharged from air tanks or compressors and can be fired rapidly. Typically several airguns of different sizes are combined in an airgun array or source array to increase the strength if the signal and to improve the frequency content.
Figure 12 An airgun.
Vibrator trucks, shown in the drawing, are often used as seismic sources on land. These trucks are equipped with a vibrating unit on the bottom of the truck that is pressed against the ground by a large mass and used to shake the earth. The shaking, referred to as a sweep, lasts from 20 to 40 seconds. In the course of a single sweep the frequency of the vibration starts at approximately 5 Hz, or cycles per second, and progressively increases to a maximum of approximately 150 HZ.
Figure 13 Vibrator Truck
The signal recorded must be cross-correlated with the sweep to produce a trace that has the response that would be obtained with an impulsive source. Several vibrator trucks may be used to simultaneously shake the earth with the sweeps synchronized, to increase the signal to noise ratio.
Marine vibrator units such as the one at the below are also available for marine use, though this is not a common practice at this time. These units produce vibration sweeps, however the unit is placed in the water instead of being clamped to the earth.
Figure 14 Marine Vibrator