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Pacific Coastal & Marine Science Center

Monterey Bay Studies

Side-scan sonar and acoustic profiling on the NOAA ship McArthur May 26 - June 2, 1996

fine-tuning the sonar fish

Overview: Scientists from the Western Marine and Coastal Surveys team of the USGS boarded the NOAA ship McArthur on May 26 in Santa Cruz, California to conduct a side-scan sonar and shallow acoustic profiling survey of the Monterey Bay shelf. The area is part of the Monterey Bay National Marine Sanctuary.

One objective of the Monterey Bay National Marine Sanctuary project is to map the seafloor geology of the Monterey Bay shelf. Side-scan sonar mapping and shallow acoustic profiling are key ingredients in achieving this objective. The side-scan sonar data collected this year add to similar data collected on the McArthur in 1995.

The type of side-scan sonar used was a Klein 100 kHz transducer fish mounted in a weighted tow body. Vertical subbottom profiling was done with a dual-transducer "Geopulse" 1-kHz sound source array mounted on a surface-towed sled. All the data was recorded on the USGS data-acquisition system "Mudseis". The navigation used was differential GPS feed into the USGS "YoNav" navigation system which provided a line following display in the lab and on the bridge. Acoustic Doppler Current Profiler (ADCP) data was also collected.

Some results: 1012 line-km were mapped with the side-scan sonar, and 662 km with both side-scan sonar and 1-kHz vertical reflection profiling. Lines were spaced at 400 m intervals on the outer shelf in two areas, the first south of Santa Cruz and the second west of Ano Nuevo, resulting in a total of 405 square km mapped.

Data quality was mixed, with high winds and rough seas in the early part of the cruise degrading the data. With good weather in the latter part of the cruise, data quality improved. At the full swath range of the side-scan, 300 m from center line, only strongly reflective targets were recorded, so that with the 400 m line spacing, some weakly-reflective seafloor targets may have been missed during the survey. It is felt that all moderately-reflective targets were recorded, except during periods of most severe sea conditions. Up to about 40 m of penetration was achieved with the Geopulse profiling system with high sea conditions producing lower than satisfactory signal-to-noise ratios. The Benthos 30-element streamer produced less satisfactory results than the 3-element array, contrary to earlier experiences with these two arrays.

The ADCP system appeared to work well, with consistent results appearing from point to point of data recording. For much of the early lines of the cruise a persistent strong (about 20 cm/sec) easterly flow was observed in a bottom layer from about 80 to 100 m depth while the upper water column, from about 10 to 70 m, recorded westward flow. These observations could represent a bottom onshore upwelling flow balanced by an upper water-column offshore flow.

This data-gathering cruise was part of a cooperative program with the NOAA Sanctuary Office in Monterey and was coordinated through the Research Activities Panel of the Sanctuary Advisory Council.

deploying the fish

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