The encoded sweep technique for Vibroseis

Abstract

In many areas correlation noise has become a serious problem because of the high dynamic range of more than 60 dB between the strong first arrivals and the weak reflection events arriving several seconds later. Several efforts have been made to reduce correlation noise either by tapering the sweep signals or by deconvolution. The effect of tapering is limited by the strong amplitude distortion of the reflected signals; deconvolution cannot cope with such a large dynamic range in the presence of noise. The sweep signal encoding technique using complementary codes was first proposed for Vibroseis® by Bernhardt (1977). The coding allows a complete correlation noise suppression after a predetermined time interval which is completely independent of the amplitude characteristic of the subsurface and the recording arrangement. Binary and quaternary codes have been investigated. Moreover, the quaternary encoded sweep allows the advantages of the encoding technique to be combined with the preferences of the Combisweep technique in order further to reduce correlation noise in the ± τ range (τ = code member length). The encoded sweep technique has been applied in several areas. The technique effected improvements in those areas where reflections with long traveltime had to be recorded. ®Trademark of Conoco Inc.

The use of pseudorandom sweeps for vibroseis surveys

“…Given the drawbacks of using pulses we could use a swept signal in place of the pulse. Edelmann and Werner (, see also Edelmann and Werner ) describe such a technique, which they refer to as the Encoded Sweep Technique , but it does not incorporate time gaps between pulses. This method involves creating a series of short sweeps whose total length is equal to the length of a conventional sweep.…”

Combined Sweep Signals for Correlation Noise Suppression*

“…They state that a Taylor-weighting offers a good compromise between side-lobe level and pulse-width increase. This technique, called tapering, has become routine since the early 1960s (Edelmann 1966). Goupillaud (1976) made experiments with nonlinear sweeps coming to the conclusion that the use of nonlinear sweeps may be considered equivalent to the use of particular types of filtering which could have been applied to the data acquired by means of a linear sweep of the same bandwidth and time duration.…”

“…Correlation noise is not restricted to the vicinity of the main peak but it is also present far from it. A definition of near-and far-correlation noise is given by Edelmann and Werner (1982). That far-correlation noise can be suppressed by pseudo-random coded signal, as described by Cunningham (1979).…”

Development and application of spread-spectrum ultrasonic evaluation technique

“…This is an equivalent of averaging technique of ultrasonic NDE discussed earlier in section 2.2.2. The various types of nondynamite sources offered operational and economic advantages over dynamite in many types of exploration areas, and as a result their use has 23 risen steadily since their introduction [44,45,46].…”