SHALLOW-WATER IMAGING MULTIBEAM SONARS - A NEW TOOL FOR INVESTIGATINGSEA-FLOOR PROCESSES IN THE COASTAL ZONE AND ON THE CONTINENTAL-SHELF

Hydrographic quality bathymetry and quantitative acoustic backscatter data are now being acquired in shallow water on a routine basis using high frequency multibeam sonars. The data provided by these systems pr oduce hitherto unobtainable information about geomorphology and seaflo or geologic processes in the coastal zone and on the continental shelf . Before one can use the multibeam data for hydrography or quantitativ e acoustic backscatter studies, however, it is essential to be able to correct for systematic errors in the data. For bathymetric data, arti facts common to deep-water systems (roll, refraction, positioning) nee d to be corrected. In addition, the potentially far greater effects of tides, heave, vessel lift/squat, antenna motion and internal time del ays become of increasing importance in shallower water. Such artifacts now cause greater errors in hydrographic data quality than bottom det ection. Many of these artifacts are a result of imperfect motion sensi ng, however, new methods such as differential GPS hold great potential for resolving such limitations. For backscatter data, while the syste m response is well characterised, significant post processing is requi red to remove residual effects of imaging geometry, gain adjustments a nd water column effects. With the removal of these system artifacts an d the establishment of a calibrated test site in intertidal regions (w here the seabed may be intimately examined by eye) one can build up a sediment classification scheme for routine regional seafloor identific ation. When properly processed, high frequency multibeam sonar data ca n provide a view of seafloor geology and geomorphology at resolutions of as little as a few decimetres. Specific applications include quanti tative estimation of sediment transport rates in large-scale sediment waves, volume effects of iceberg scouring, extent and style of seafloo r mass-wasting and delineation of structural trends in bedrock. In add ition, the imagery potentially provides a means of quantitative classi fication of seafloor lithology, allowing sedimentologists the ability to examine spatial distributions of seabed sediment type without resor ting to subjective estimation or prohibitively expensive bottom-sampli ng programs. Using Simrad EM100 and EM1000 sonars as an example, this paper illustrates the nature and scale of possible artifacts, the nece ssary post-processing steps and shows specific applications of these s onars.