A Marine Seismic Digital Telemetry System (1980)

Abstract Within the past few years, a desire for improved resolution in marine seismic surveys has provided new motivation for increasing the number of recorded data channels. To achieve overall bandwidth of 200 Hz requires a hydrophone group length of 3 meters and sample intervals of 1 millisecond. For streamer cables of conventional length, this group length requires 250 to 1000 individual hydrophone groups. Present marine seismic streamer cables use analog telemetry techniques. Investigation on possible expansion of present techniques for 250 to 500 or 1000 channels showed that the data quality would suffer because of errors in the analog telemetry path. The physical limitations imposed on the streamer section to carry the weight of 500 pairs of wires and the unreliability of cable sections with 1000 or more solder connections presented virtually insurmountable obstacles. This investigation led to the design of a radically different streamer cable. It was decided to perform the filtering, amplification and analog to digital conversion in modules spaced along the streamer. A time-division multiplexing system had to be devised to permit each module to inject the digital data from its 12 data channels and 2 auxiliaries into a coaxial line. Because the analog leads need only connect the hydrophone groups to the next module downstream, the number of active analog conductor pairs in any part of the cable is no more than 14. It is therefore quite practical to use high quality, shielded analog leads, thereby effectively eliminating crosstalk and external pickup. Sections produced from the high-density data obtained with the new cables exhibit improved high frequency response, improved data quality and better resolution of shallow data and of complex geology. Introduction As the search for energy intensifies in the offshore areas, it will be necessary to detect hydrocarbon traps of ever decreasing size. Consequently, the spatial resolution of seismic method must be continuously increased. If the resolution is 1mproved significantly, there even exists a distinct possibility of reliable delineation of actual hydrocarbon, deposits rather than merely indicating possible structural traps. 1 Past attempts to improve resolution were focused on such analog techniques as instantaneous-gain-ranging -and increased resolution. This effort resulted in improved accuracy, reduced preamplifier noise, and decreased per channel cost to allow a greater number of ground stations. The resulting data accuracy pointed up the limited spatial resolution of the existing methods, particularly in cases of complex geological formations where precise velocity information had to be determined. To detect small traps accurately and to improve the signal-to-noise ratio, a ten fold increase in multiplicity is necessary. It will be evident, from the following discussion, that further efforts in improving spatial resolution via analog techniques will yield diminishing returns and that seismic-digital-telemetry systems are required. Limitations of Marine Analog Telemetry Analog methods were quite satisfactory as long as marine streamers remained in the 24 to 48 channel range. A 2 to 1 increase in the number of channels was realized when streamer cables with charge amplifiers were introduced. 2