We will use our Simrad EM 3000 multibeam echosounder [1], operating at 300 kHz, to produce a high-resolution map of the sea bed bathymetry and backscatter in the SAX99 study area. The interaction of sound with ocean floor sediments depends on many factors, including the shape of the sea floor and the distribution of roughness elements. Our survey will characterize bottom topography, bottom roughness and backscatter patterns prior to the emplacement of bottom equipment. We will also seek to understand the factors that cause the observed backscatter patterns.
The EM 3000 collects bathymetric and backscatter data across a swath that is about four times the water depth. We propose a minimum track spacing of 1x water depth, justifying a gridding interval of 25–30 cm for backscatter and bathymetry in a water depth of 18 m, and allowing backscatter images to be generated using a consistent illumination direction. This trackline spacing will also allow us to measure backscatter at several different incident angles for each portion of the study area. Bathymetry will not change with track orientation, but backscatter may change with track orientation if smaller-scale topography (e.g., ripples) is present. Therefore we will run surveys at several different line orientations so that backscatter is known as a function of direction. We will determine optimal survey directions by collecting data while turning in a circle and noting orientations of maximum and minimum backscatter. We plan two transit days and three survey days to allow a 750 m x 750 m study area to be surveyed with tracks in more than one direction. We will use the University of Southern Mississippi Institute of Marine Sciences R/V Tommy Munro, a 97.5 foot vessel based in Biloxi, MS, and survey from about Sept. 28 to 30, 1999.
For this project, the MSRC/SUNY EM 3000 system will be installed on the R/V Tommy Munro with a pole-mounted transducer. The EM 3000 system includes the Simrad EM 3000 (echosounder transducer, surface electronics, logging computer), a ship attitude and DGPS navigation package (POS/MV made by TSS [2]), a CTD (for determining the sound velocity profile during the survey), and a mooring with a bottom-mounted pressure gauge for measuring water surface elevation (tide) changes during the survey. We will also bring additional computers for near real-time data reduction and a printer for data display. Preliminary data products will be available to other investigators by the end of the cruise.
This effort will be cooperative between MSRC/SUNY (Flood) and the Ocean Mapping Group at New Brunswick (Mayer). MSRC (with assistance from Dale Chayes at the Lamont-Doherty Earth Observatory of Columbia University) will build the transducer mounting, ship the multibeam equipment to Mississippi, install the equipment on the vessel, and operate the system during the survey. MSRC will also use standard OMG SwathEd programs [3] to reduce the bathymetric and backscatter data. Larry Mayer (OMG) will provide additional analysis of the 300 kHz backscatter data. Mayer and Flood will work jointly to interpret the backscatter and bathymetric information in terms of sediment characteristics to be determined by others as part of the acoustics experiment.
[1] http://www.kongsberg-simrad.com/Products/Hydrographic_Survey/multibeam/EM3000/
[2]http://www.intnlind.com/TSS/tssindex.html
[3]http://www.omg.unb.ca/~jhc/SwathEd.html
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