Seismic Sources

There are many types of seismic sources, but the most often used sources for land surveys are vibrator trucks (see Figure 1.6), and for marine surveys are air guns. Vibrator trucks continuously shake the ground, starting from a low frequency rumble at about 5 Hz and then progressively sweep to higher frequencies up to 150 Hz. The sweep time ranges from 20 s to 40 secs or so, and the recorded signal is crosscorrelated with the vibrator truck signal to produce an impulsive source wavelet.

  

Figure 1.6: Typical land and marine sources. Vibroseis truck above is workhorse of land surveys, where truck lifts up on center pad and vibrates up and down for about 20-40 secs per station. Sometimes 5 or more trucks vibrate simultaneously, and are evenly spread out over an interval equal to the wavelength of a surface wave so that the groundroll is canceled by destructive interference. Air guns are the preferred source for marine surveys, and muscle power is the preferred cost-effective source for U of Utah surveys at the bottom.