Far-field harmonic measurement for seismic vibrators

Abstract

Seismic vibrators in normal operation are programmed to produce a swept-frequency signal. They invariably radiate energy not just at the fundamental frequency of the sweep, but at harmonics of that frequency as well. These harmonics are normally measured by instrumenting the seismic vibrator. This paper introduces a method for estimating the harmonics using the uncorrelated seismogram measured at far-field geophones. This estimate may be used to attenuate harmonic artifacts (“ghosts”) in the correlated seismogram by using deconvolution. In tests, systematic differences were observed when comparing far-field harmonic estimates with direct measurements made on the vibrator baseplate. A comparison of deconvolution using the far-field harmonic estimates the with deconvolution using the baseplate signal shows conclusively that the far-field harmonics are correct. This is a direct consequence of the non-linear elastic response of the earth's near-surface.