Pseudorandom Sweeps Research Articles

The use of pseudorandom sweeps for vibroseis acquisition

The use of pseudorandom sweeps (rather than the more conventional sweeps whose instantaneous frequency changes linearly) has received periodic attention over the past 45 years. In a previous article (Dean, 2014) I summarised the work published in this area including the different motivations for their use as well as different methods for generating a variety of different sweeps. At the time, I attributed their lack of use to the difficulty in transmitting them and their relatively low energy levels when compared with conventional, linear, sweeps. In this article I summarise recent work (i.e. not covered in Dean (2014)) involving pseudorandom sweeps before describing some of our own test results. I then discuss the implications of this recent work on the future of the method.

A seismic vertical vibrator driven by linear synchronous motors

A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSM-driven vibrator, we successfully designed and built a multi-LSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the low-frequency regime, down to 2 Hz, rather easily.

The use of pseudorandom sweeps for vibroseis surveys

ABSTRACTPseudorandom vibroseis sweeps have long been suggested as an alternative to standard linear sweeps due to their potential for having superior orthogonality, a lower likelihood for infrastructure damage, and increased low‐frequency content. In the past, they were also attractive as they have a better autocorrelation shape, although that is less important today. Their use has been limited but the increasing popularity of simultaneous acquisition techniques has rekindled interest as they offer the ability to reduce interference noise. A wide variety of methods for generating pseudorandom sweeps have been developed over the last 45 years. This paper gives an overview of the motivations for their use before classifying and describing the different sweep types. Finally, the sweeps will be compared in terms of their major attributes including their suitability for simultaneous surveys.

Pseudo-random sweeps for built-up area seismic surveys

Optimized pseudo-random sequences proved effective, both theoretically and empirically, in increasing data quality while reducing possible damage to buildings by vibratory sources during seismic reflection data acquisition. The linear sweeps commonly used for vibratory measurements can cause resonance in nearby infrastructure; hence, there is a potential for property damage. A pseudo-random sweep signal can be designed to decrease resonance effects and therefore reduce damage thresholds. The sweep sequences produced by simple random number generators, like the ones given by the vibrator manufacturers, have marked disadvantages, such as high correlation noise, large fluctuations in spectral amplitudes, and reduced seismic energy. To overcome some of these problems, an optimization process can be developed to produce pseudo-random sweeps for site-specific conditions. Two strategies are considered for preprocessing: crosscorrelation and deterministic deconvolution. Analysis of examples for an optimum pseudo-random sweep and simple pseudo-random sweep demonstrates that the total seismic energy is increased, while side-lobe energy is decreased and spectral fluctuations are reduced when the pseudo-random sweep is optimized. A limited-scope field test reveals that the peak particle velocity values are lowered substantially, while correlated and deconvolved records generated by the optimized pseudo-random sweep are of similar quality to a linear-sweep record. Application of optimized pseudo-random sweeps has the potential to increase productivity, while maintaining data quality and reducing resonance in surveys of built-up areas.

Numerical modelling of pseudo-random land seismic sources

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.

SIGNAL DESIGN IN THE “VIBROSEIS”® TECHNIQUE

After more than twenty years of field practice, the most widely used signal in the Vibroseis technique remains the original so‐called linear sweep. To many, it seems that substantial improvement should be expected from taking advantage of more sophisticated signal design techniques. The purpose of this paper is to present a tutorial survey of the basic concepts involved in signal design, tying together the known features of the linear sweep with those of the more general type. Several particular types of nonlinear sweeps are analyzed and the results of a field test seem to confirm the conclusions drawn from the conceptual analysis. After a brief consideration of still another type of nonlinear signals, a particular variety of pseudorandom sweeps, the author concludes by giving his views of what might be the role of nonlinear signal design in the future development of the Vibroseis technique.