On the Accuracy of Harmonic Estimation from Weighted-sum Ground Force

The harmonic noise generated by hydraulic seismic vibrators is not a major concern in conventional vibroseis acquisition with upsweeps and sweeps longer than the listening time. However, its significance is magnified when the acquisition is conducted with simultaneous shooting techniques based on phase encoding or slip-sweep. We show that the use of the ground force estimated at the vibrator location leads to a determination of the response of the earth's interior to the harmonics which is a worse approximation than that determined with the seismic data. The inaccuracy of the estimated ground force in characterizing the harmonic noise increases with frequency and, for the considered dataset, leads to an increase rather than attenuation of the harmonic noise above 40 Hz. It is therefore recommended to use the surface data to determine the harmonic noise.