Numerical modelling of pseudo-random land seismic sources

Abstract

Environmental, logistical and security considerations mean that non-explosive, surface seismic sources must assume increasing future importance. The challenge is to make such sources more competitive with dynamite in terms of resolving power and signal-to-noise ratio. We use numerical modelling to explore possible improvements in Vibroseis reference-signal design, and algorithmic approaches to Mini-SOSIE stacking. We revisit an alternative Vibroseis sweep comprising a constant frequency carrier which suffers polarity reversals according to a pseudo-random coding sequence. Numerical models allow various comparisons with the conventional swept-frequency approach. Visually, the correlation wavelet from the pseudo-random reference appears less affected by side lobes than the conventional Klauder wavelet. On the other hand the correlated pseudo-random trace is noisier away from the wavelet itself. A pseudo-random sweep built from half-cycle components has interesting theoretical possibilities, but practical implementation may be difficult. Pseudo-random design concepts extend naturally to the Mini-SOSIE source, which stacks, in real time, numerous low-amplitude impacts, occurring at approximately random time intervals. We demonstrate the undesirable effect of non-randomness, and examine the feasibility of using predictive deconvolution to improve the randomness of the impact sequence prior to stacking. Sign-bit stacking provides better attenuation of noise bursts than standard Mini-SOSIE stacking, although it may be prone to some amplitude distortions. A stacking procedure which incorporates a median-filtering stage appears to provide good noise-burst attenuation whilst maintaining reflection amplitudes.