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.