Technology for an Ultra-High-Resolution Marine Seismic Cable (1991)

ABSTRACT A second-generation digital marine streamer has been developed with a significant increase in spatial, temporal, and amplitude resolution. The new cable design permits cable lengths in excess of 10,000meters, multiple cables with over 1,000 channels each, bandwidth to 450 Hz, and linearity in excess of 120 dB. Employing a separate oversampling Delta-Sigma Modulator for analog-to-digital conversion of each hydrophone group's signals, the cable resolves and accurately reproduces the seismic signal to over one part per million of the maximum recorded signal. The combination of high resolution and oversampling permits the elimination of analog antialias filters and low-cut filters to make the analog portion essentially transparent to the recording process, Since the A/D converters are assigned to single hydrophone groups, the analog and digitizing circuits are distributed inside the streamer at the center of each hydrophone group. This architecture reduces the electronics in the streamer section interconnecting modules, thus permitting the size of these modules to be reduced to the diameter of the rest of the cable. These modules contain only the telemetry repeaters, system control, and part of the digital antialias filters. The increase in channel count and the extended word length (more bits) required for the converter demand a telemetry bandwidth above what is reliably obtainable with copper wires. This new streamer employs optical fibers for the telemetry link to the boat, providing essentially an errorless telemetry link. The new system has an extensive shipboard testing methodology that digitally generates high accuracy test signals and sends them from shipboard processors to the modules distributed along the cable. Each interconnecting module is capable of recovering the remotely commanded digital test signal and using it to test the sixteen channels serviced by the module, to verify the integrity of the link, and to apply various sine waves, steps, and impulses directly to the hydrophone or to the analog circuitry. Shipboard processing permits rapid evaluation of the cable performance to assure that the seismic data will be recorded with utmost fidelity. A seismic streamer incorporating all these features is currently collecting production seismic data. Multiple production systems are now in the manufacturing process. INTRODUCTION The geophysical community requires very long streamer cables, shorter hydrophone group intervals, and a large number of channels. In the early 1980's, the first digital streamers were introduced to meet this need. In these systems, the standard seismic "front-end" electronics - a preamplifier, analog filters, instantaneous floating point amplifier, and A/D converters - were packaged into a streamer module, which was inserted between the sections of the streamer. The digital samples were passed up to the recording system on the vessel via digital telemetry. The digital streamer offered a significant advantage over an analog streamer as the number of recording channels increased. The digital streamer has fewer wire pairs and, consequently, is much less subject to leakage and crossfeed than an analog streamer. However, it should be noted that the actual technology of the system front end was essentially the same as the instrumentation used with an analog streamer, only repackaged.