ABSTRACT

The need to avoid interference between sound sources during seafloor surveys led to the design and assembly of a device which automatically controls the firing rate of different sound sources on board a ship. This synchronizer is built around a Z80 microprocessor which schedules the timing of a single channel seismic system, a 3.5 kHz subbottom profiler and a Sea Beam bathymetric survey system. The paper describes the design characteristics and the implementation of this synchronizer aboard the R.V. Thomas Washington.

INTRODUCTION

Given the high cost of ship time, it is essential to optimize survey work by gathering data simultaneously from a variety of sensors. With sound sources, this can only be done effectively if the sources are kept from interfering with each other. While it is theoretically manageable for a human operator to manually synchronize multiple echo-sounders, it is unreliable in any but the most ideal and geologically uninteresting conditions, e.g. constant depth, constant sediment layer, and flat bathymetry. The complexities of changing water depths, ranges to sub-bottom reflectors, slant ranges to side reflectors, and outer beams in a multibeam echo-sounder, create a difficult synchronization task for an operator. To effectively prevent interference among multiple sound sources a computer controlled echo-sounder scheduling device is needed.

During surveys using multiple sound sources on the R/V Thomas Washington the on board Sea Beam multibeam echosounder was subject to data loss due to interference from the other sound source events and echoes. To eliminate this problem the Shipboard Computer Group of the Scripps Institution of Oceanography built a multiple sound source synchronizer using a Z80 microprocessor system in conjunction with a DEC VAX 11/730 computer running under the Berkeley UNIX 4.3 operating system. The Sea Beam system is a multiple narrow-beam sonar comprised of echo-sounding and echo-processing subsystems. The echo-sounder transmits a sonar pulse and receives the resultant echoes. These are relayed to the echo-processor which automatically controls system operation by tracking the bottom over changing bathymetry. The bottom tracking process is quite effective when presented with a signal from strongly scattering or specular reflectors. However, echoes received from sections of the sea bottom which are neither specular nor strongly scattering (e.g. outer beams from a flat, sedimentary sea bottom) are susceptible to interference from source events of even the weakest sound source, or from bottom echoes from a strong sound source. For detailed descriptions of the Sea Beam system the reader is referred to the works by Renard and Allenou (1979), Farr (1980), and de Moustier and KIeinrock (1986).

This interference has two possible effects on Sea Beam. If the interfering sound source is operating at a repetition rate different than that of Sea Beam, the random interference points could cause the Sea Beam bottom tracking algorithm to momentarily lock on the intruding sound and miss the actual bottom return. After the interference point had passed Sea Beam would be unable to relocate the sea bottom and would require appropriate operator action to resume bottom tracking.

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