A comparison of minimum phase `correlation' and its role in seismic data misties

Abstract

Minimum phase conversion is a common and often routine first step in the processing of zero phase correlated Vibroseis data. Over the period of March 1983 to December 1985 the Cooper Basin joint venturers used five different recording contractors and as many as nine different processing contractors. Many misties, due to apparent phase differences, were noted and a study was initiated to determine the cause of these. A data set was sent to seven different processing contractors and the results compared. Major differences appeared during the correlation and minimum phase conversion of the data. The degree of difference depended primarily on the contractor but was also affected by the choice of sweep parameters. The results of the study are presented in this paper. The minimum phase conversion operators are designed from the amplitude spectra of the Vibroseis pilot sweep. These differ from contractor to contractor due to the various levels of additive noise and base levelling of the amplitude spectrum outside the swept frequency range. The degree of phase rotation is dependent on the levels of noise and is not necessarily linear. Sweep non-linearity can also affect the minimum phase conversion procedure and the level of phase distortion caused by this is shown. A standardisation of the minimum phase conversion to avoid phase shifts between the various contractors was introduced by the Cooper Basin operators in 1986 and 1987.