Dictionary:Vibroseis or vibroseis

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FIG. V-12. Vibroseis. (Courtesy Conoco.)

(vī’ brō sīz,) A seismic method in which a vibrator is used as an energy source to generate a controlled wavetrain.

A sinusoidal vibration of continuously varying frequency (Figure V-12) is applied during a sweep period typically lasting up to 32 s.

In upsweeping the frequency begins low and increases with time, and in downsweeping the highest frequencies occur first. The frequency is usually changed linearly with time.

A non-linear sweep usually involves vibrating longer at the higher frequencies to somewhat compensate for the increased loss of high frequencies in travel through the earth. A vibroseis field record consists of the superposition of many long reflected wavetrains and is generally uninterpretable because of the extensive overlap; it is correlated with the sweep wavetrain to produce an interpretable record which resembles a conventional seismic record such as results from an impulsive source. Developed by Conoco. (Vibroseis is no longer a trademark).

CONOCO geophysicists John M. Crawford, William E. N. Doty, and Milford R. Lee received the SEG Medal Award (today known as the Reginald Fessenden Award), as the original patent holders and developers of Vibroseis.

The original Vibroseis vibrated vertically; it is sometimes be called a "P-wave" vibrator because most of the energy radiated near-vertically travels as P-waves. But, this name is inaccurate, since the vertical vibrator also radiates shear waves, especially at non-vertical angles.

Subsequent variations were designed to vibrate horizontally; hence to radiate shear-wave energy efficiently at near-vertical angles. (But, they also radiate P-wave energy, especially at non-vertical angles.) These horizontal vibrations may be polarized at any azimuthal angle, as chosen by the operator; this flexibility is important for 2C x 2C acquisition.