Vibroseis encoding

Abstract

The FM signals, called sweeps, used in the Vibroseis method of seismic exploration show a considerable amount of energy in the sidelobes after correlation detection. These sidelobes represent signal generated noise and if not kept low in amplitude they miggit mask subsequent reflections, thereby reducing the detection capability of the Vibroseis system. The purpose of this research has been to investigate new coded signal design techniques for the use with Vibroseis, in order to achieve sidelobe suppression. Some of the codes examined have already been known to radar and communication theory, whilst some codes are original developments of this research exercise. Binary and quaternary complementary series are found to be especially suitable for a Vibroseis encoding technique. A new and simple algorithm for the generation of quaternary series from known binary complementary sequences is given and the concept of correlation matrices is introduced to complementary series, permitting signal design in the detection window. The encoded Vibroseis input signals were tested on a computer and showed perfect sidelobe suppression a certain distance away from the main compressed pulse, when detected by a matched filter. Field tests with the coded signals were conducted, taking advantage of a computerized Vibroseis field system. The tests showed promising results. However, it became clear that the vibrator control devices will have to be adjusted to the transmission of such sophisti cated signals, in order to allow substantially better results than in the conventional Vibroseis system. A ‘Continuous Vibroseis Transmission System’ is suggested, transmitting energy during the normal listening period. Such a system has been developed witht h help of so-called ‘Mutually-Orthogonal-Complementay Sets of Sequences’ and although not yet practically tested its anticipated advantages and disadvantages are described. Finally, ‘Predistortion’ as a method of Vibroseis signal design is examined. Providing the correct predestortion parameters are chosen, the signal-to-correlation noise ratio an be increased. A spectrum whitening effect observed an addition of selected perdistorted sweeps can be of advantage in a quaternary comlementary coded Viboseis system, permitting an optimal wavelet design in the detection window.