The Vibroseis seismic energy source system was developed by the Continental Oil Company of Ponca City but may be used by other operators under a license arrangement. It differs from practically all other forms of seismic reflection exploration in that the signal which is sent into the ground is not a pulse but is a carefully controlled sweep frequency. ( Fig 7.1 ) The input signal is long, usually 7 sec or more, and normally ranges smoothly from one end to the other of some defined frequency band, as for example from 17 to 67 Hz.
The signal is impressed into the ground by one or more vibrators driven under control of a sweep signal generator. The signal can be chosen so that the band which is emitted corresponds closely to the natural response of the area. Thus, those frequencies which would in any case be attenuated need not be transmitted. The amplitude spectrum of the particular sweep signal may differ very little in bandwidth from that of the unit impulse response. The important difference in the characteristics of the input lies in the well defined phase components of the sweep signal. This makes a tremendous difference from the processing standpoint because the input signal now has both the amplitude and the phase spectrum well defined. The amplitude spectrum is white ( flat) over the sweep bandwidth and the phase spectrum is a smooth acceleration function. In other words, the conditions of the transmitted signal now meet the basic assumptions