Nonlinear sweeps have often successfully been employed in the 1960s. However, this area of sweep technology has been neglected since the introduction of digital recording techniques in the Vibroseis system. Now the advent of computerized recording instruments yields a new economical possibility of forming approximately nonlinear sweeps by combining several linear sweeps with or without time gaps to a “Combisweep”. The total duration of a Combisweep may be as long as the maximum available recording time, for example 32 s.
Beside the attenuation of correlation noise, the new method has further merits, such as the weighting of predetermined frequency ranges, in order to effect a certain kind of optimum filtering on the emitter side, or in order to compensate to some degree for frequency dependent absorption.
In all these applications the Combisweep is considered as one signal in the correlation process. But by correlating with the individual sweeps or a partial combination of them and by applying automatic switching at predetermined times within the gaps between the individual sweeps additional possibilities arise, such as obtaining in one run with a twenty‐four channel recording unit twenty‐four traces with small distances between vibrators and geophones for shallow reflections and another twenty‐four traces with larger distances for deeper reflections. Various Combisweeps and their applications are presented.
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.
EDELMANN, H.A.K. and WERNER, H. 1982, Combined Sweep Signals for Correlation Noise Suppression, Geophysical Prospecting 30,786-8 12. Comparison of both synthetic and field data shows that considerable suppression of correlation noise can be achieved with the Combisweep technique and with the Encoded Sweep Technique. In the first technique, the spectrum is shaped by superposition of linear sweeps with different frequency range; in the second technique, short sweeps of different polarity are combined to form the " alphabet" of a code.
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