| Abstract: | The low-frequency response of the P400 watergun is an improvement over that of the S80 version. It has been further enhanced by deployment in a vertically staggered array. The notch in the amplitude spectrum at about 35 Hz due to the interference between the precursor and main implosion pulse has been virtually eliminated, and depth notches due to the free surface interface at the upper end of the spectrum have been greatly reduced. The spectral band is thus very broad and well-shaped and corresponds to an even shorter signature. After convolution with the streamer ghost, the amplitude of the implosion pulse from the composite far-field signature is more than 30 times the composite precursor amplitude. Thus the signal approaches the minimum-phase condition. Shot-generated noise scattered back by diffractors located at or near seabed is the single greatest impediment to increased penetration, especially in high resolution surveys. Such noise decays much less rapidly than signal, particularly so with broad band data. Furthermore, back-scattered interference coming from a cone of bearings between 15–45° with respect to the line is particularly damaging since in these directions the stack enhances the interference and confuses the primary velocity analysis. The watergun source patterns are therefore element-weighted to maximize directivity within these critical directions in the frequency band 50–155 Hz. When spatial aliasing is reduced by increased streamer spatial sampling and by discrete anti-alias filters during the record cycle,f-k filtering can be applied to the field records as an effective supplement to the source and receiver patterns without the aliased ‘wrapped around’ noise alignments destroying the upper end of the spectrum of genuine reflectors. Furthermore, increased spatial sampling is a pre-requisite in thef-k-migration process if the valuable higher frequencies of broad band data are to be migrated in steep-dip situations without aliasing. Thus, spatial resolution is a necessary complement to temporal resolution if maximum advantage of the broad-band P400 watergun source is to be realized. |
