Attenuation of vibrator harmonic ghosts

Abstract

Vibrator sweep signals, transmitted into the subsurface, contain harmonic components due to nonlinearities in the vibrator system and due to the ground's non-linear response to the vibratory force. The harmonic energy produces a distortion in the correlated records and appears as a noise train displaced from each primary reflection. The noise trails the primary when the pilot sweep begins at a high frequency and ends at a low frequency (downsweep). For upsweeps, the noise appears as a forerunner. This paper evaluates the characteristics of harmonic ghosts in terms of time of arrival, strength and frequency content. Adverse affects of the distortion, on adjacent reflections, are shown to be minimized by: (1) increasing the sweep length, (2) reducing the sweep bandwidth, (3) utilizing an upsweep, (4) stacking alternate polarity sweeps, (5) stacking variable phase sweeps and (6) stacking variable frequency sweeps. Advantages and disadvantages of each attenuation method are evaluated.