Experimental analysis of the polarity of vibrator and impulsive source data

Abstract

Although our understanding of vibrator source characteristics continues to increase, the fundamental issues of signal polarity and phase remain controversial. Improvements in the accuracy of vibrator control have reduced the potential for phase inconsistencies in processed data, but field setup procedures and variations in data processing still create confusion. This paper describes an experiment comparing a vibrator and two impulsive sources. Downgoing and reflected waveforms from these three sources are analyzed at selected stages in the processing sequence. Emphasis is placed on polarity procedures during data collection and phase considerations during digital processing. Polarity issues are examined in detail. Downhole tests compare first-arrival energy from dynamite, an ARIS* impulse source, and a 45,000 pounds peak force vibrator. Stacked sections and VSP data recorded using the ARIS source and vibrator over a well are compared to a synthetic seismogram derived from the lithology of the well. The results show that controlling the vibrator ground force permits the polarity of the vibrator signal to be defined consistently with respect to impulsive signals for both downgoing and reflected energy.