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The recent use of nonlinear Vibroseis® (trademark of Continental Oil Co.) sweeps became possible with development of an electronics system to drive the vibrators in a nonlinear mode. This new electronics system allows the user to adjust both the vibrator amplitude and the rate of frequency change during the sweep. Nonlinear Vibroseis® sweeps are now becoming popular for the acquisition of high resolution seismic data in
the petroleum industry. However, nonlinear sweeps can seriously degrade seismic data when the amplitude and frequency controls are used incorrectly.
The amplitude controls are used to overcome decoupling, ground roll, and vibrator/ground resonance problems. By suppressing the vibrator drive at the lower frequencies, the typical decoupling effect can be eliminated and the effect of the ground roll minimized. This permits the use of shorter geophone arrays that are more suitable for high resolution recording. In addition, the use of a lower amplitude sweep at the low end of the frequency range helps suppress vibrator/ground resonance. This reduces the large amplitude of the resonance frequency and allows the recording of the weaker high frequency signal.
The ability to control the rate of frequency change during the sweep allows the user to recover an improved signal-to-noise ratio at the higher frequencies. In general, the sweep time is reduced in the lower frequencies and increased in the higher frequencies. The high percentage of sweep time at the high end of the frequency range results in an improved signal-to-noise ratio in the higher frequencies. However, the improper choice of the start and end sweep frequencies can result in recovered data that is actually poorer in signal-to-noise ratio than an equivalent linear sweep.
The nonlinear method is a powerful, but sensitive tool that can be beneficial when used properly. The tool allows the user to recover excellent data in "good" data country and fair data in "poor" data country. We use Pelton's Advance I, Model 5 vibrator electronics together with the FT-1/DFS-V seismic exploration system and test extensively in the field. Areas where we have recovered excellent data include the Williston, Powder River, Big Horn, and Wind River basins.
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