A 500-Channel Streamer System
Carl H. Savit, Lee E. Siems · Offshore Technology Conference · 1977
ABSTRACT Space age technology makes possible a seismic streamer cable of 500 data channels. Seismic data are converted into floating-point digital format and transmitted down a coaxial cable by microminiature processing units spaced along the cable. A number of ancillary channels are used for status checking, control, and calibration purposes and to transmit data on the depth and orientation of the cable. The 500 channels originate from 50 10-channel modules. All modules are identical and can be freely interchanged without adjustment of circuits or programs. On board the towing ship, command signals may be generated to bypass damaged modules or to change the sampling rate on selected modules. INTRODUCTION Since the beginning of seismic reflection exploration, there has been an inexorable trend toward increasing the number of independent recording channels. The first reflection work in 1927 was done with two channels recorded simultaneously. By 1976 virtually all marine seismic crews were recording with at least 48 channels. Several were using 96 channels and a few were beginning to experiment with 200 or 250. Several forces have been at work to drive the increase in number of channels. At first, the closer spacing of observation points was sought to aid in the observation of continuity of reflecting horizons. At a later time it was recognized that longer spreads could be used to distinguish between primary and multiple reflections. Geophysicists were reluctant to increase the spacing between detector groups because they would then lose the precious continuity they had gained. Hence, the only alternative was further to increase the number of independent channels or traces on each record. IMPROVING RESOLUTION Within the past few years, a desire for improved resolution has provided a new motivation for increasing the number of channels. Improved resolution is sought because hydrocarbon traps discernible by methods having a limited resolution have been essentially exhausted in some areas. Even in areas which have not been combed over with the resolution heretofore available, improved resolution is expected to improve the oilman's ability to distinguish between a productive and a dry trap. Outside the hydrocarbon industry, the need for higher resolution is equally compelling as for example, in delineating faults to determine the safety of a nuclear reactor site. There is now also perceived a real possibility that a high-resolution seismic reflection system could reliably delineate actual hydrocarbon deposits rather than merely indicate possible traps1. Requiring an increase in resolution in the vertical direction is precisely equivalent to mandating the passing of higher frequencies through the seismic system. The minimum thickness of a bed that can be detected under ideal conditions by seismic reflection methods is about one eighth of the wavelength of the highest frequency within the spectrum used in the reflection survey.