Pseudo-random Sweep Optimisation for Broadband Vibratory Seismic Measurements

Broadband vibratory seismic measurements are suited for deep signal penetration, improving reflector continuity and inversion. Seismic vibrators have serious mechanic and hydraulic limitations in emitting very low frequencies, hence special low-dwell non-linear sweeps are the most common drive signals, but pseudo-random sweeps are also considered to achieve satisfactory results. In this study a global optimisation technique used tailored to include several optimisation targets concurrently to reach a workable solution. Since a simple random sweep generated by vibrator manufacturers’ software has serious disadvantages, our optimisation minimises amplitude fluctuations in frequency domain, correlation side-lobe amplitudes and maximises total seismic energy of the sweep. By controlling maximum amplitudes below a certain vibrator specific lower frequency and by minimising instantaneous frequency changes vibrator mechanic and hydraulic limitations are taken into account. The resulting optimised pseudo-random sweeps have much better correlation and spectral properties than a simple random sweep. Also sweep length could be reduced substantially compared to a simple random sweep and a typical non-linear broadband sweep due to the increased total seismic energy achieved by the optimisation process. As a consequence optimum pseudo-random sweeps could be used in high productivity broadband seismic acquisition where resonance effect reductions are important, too.