ELIMINATION OF HARMONIC DISTORTION IN VIBROSEIS DATA

We present a new approach to eliminate the harmonic distortion in vibr oseis data. This technique is based on the definition of the sweep for the case where the instantaneous frequency varies linearly with time. The method requires no more than the computation of a purely determin istic phase-shift filter that has the phase spectrum of a hypothetical ''kth harmonic distortion.'' With this particular phase-shift filter, all harmonic distortions in the base-plate signal and those caused by the first strong arrival in the vibroseis trace (which leads to the ' 'correlation ghost sweeps'') are removed. The process consists of (1) applying the deterministic pure phase-shift filter to the vibroseis tr ace and the base-plate signal; (2) setting time values equal to zero f or negative times in both the filtered base-plate signal and the filte red vibroseis trace; and (3) applying the inverse pure phase-shift fil ter to both base-plate signal and vibroseis trace prior to performing the usual crosscorrelation. The correlated trace thus obtained will be freed substantially of correlation noise; i.e., the correlation-ghost sweeps (produced by severe harmonic distortion both at negative and p ositive correlation times) are eliminated without degrading the seismi c information content of the trace. The new method of elimination has been successfully applied to synthetic and real harmonically distorted vibroseis data and is compared with the standard correlation techniqu e that uses the pilot sweep as the correlator.