A Fully Transportable Sea Beam Complex Acoustic Data Acquisition System

Abstract

ABSTRACT Although bathymetric contour mapping is the primary function of the Sea Beam bathymetric survey system, its multi- narrowbeam characteristics (2°213 angular resolution) make it a potentially important tool for seafloor acoustic backscattering studies. For such studies, it is necessary to preserve both the amplitude and the phase of the echo signals so as to be able to remove the sidelobe interference inherent in the multibeam geometry. As no provisions exist within the Sea Beam system to preserve the acoustic signals received, a parallel acoustic data acquisition system is needed to record the acoustic data. This paper describes such a data acquisition system which has been designed and built at the Marine Physical Laboratory of the Scripps Institution of Oceanography to record the in-phase and quadrature components of complex echo signals received by Sea Beam. This data acquisition system is fully transportable and is adaptable to all ships equipped with Sea Beam. It has been used successfully aboard the R/V Thomas Washington and the French Oceanographic Vessel Jean Charcot. INTRODUCTION In the late 70's the General Instrument Corporation introduced the Sea Beam bathymetric survey system, a 12 kHz multinarrow- beam echo-sounder designed for high-resolution swath mapping of the deep seafloor. Since then a dozen such systems have been installed on oceanographic vessels worldwide and the bathymetric data they have yielded has greatly advanced our understanding of the ocean floor. In addition, the multi- narrow-beam characteristics of these systems make them potentially useful tools for seafloor acoustic measurements which could yield information about the nature of the seafloor. So when in 1981 a Sea Beam system was installed aboard the RlV Thomas Washington of the Scripps Institution of Oceanography (SIG), the Marine Physical Laboratory (MPL) explored the possibility of measuring deep-seafloor acoustic backscatter with this system. Because the Sea Beam system has no internal provisions to store the echoes it receives, it was necessary to build an acoustic data acquisition system which digitized and recorded on magnetic tape the envelopes of the signals received on the Sea Beam system's 16 preformed beams (de Moustier, 1985). Data recorded with this envelope acoustic data acquisition system have proven very useful for assessing the performance of the Sea Beam system as a mapping tool (de Moustier and Kleinrock, 1986). Analyses of echo envelopes recorded over a variety of ocean floors indicated that information about the characteristics of the seafloor could be extracted from the signals received by Sea Beam. However, inherent in the multibeam geometry is a problem of sidelobe interference in which a strong specular bottom echo entering the main lobe of anyone beam also leaks into the sidelobes of all the other beams, and is present at a level comparable to those of the bottom returns on these beams. Because in many cases bottom return and sidelobe interference overlap, and because only envelope data were recorded on tape, it was not possible to remove the sidelobe interference without degrading the underlying return.