Sediment classification based on repetitive multibeam bathymetry surveys of an offshore disposal site

A disposal site near Saint John, New Brunswick, Canada, has been surveyed at six-month intervals for the past three years with Simrad EM3000 multibeam bathymetry systems. Analysis of the bathymetry data from surveys in the autumn show accumulation of sediments over the site due to disposal activities in the summer and autumn; surveys in the spring show that the sediment has undergone transport and redistribution due to the effects of currents and waves over the winter months. Quantitative measures of sediment accumulation and erosion have been made and show definite changes in the nature and distribution of the disposal pile. Acoustic backscatter data collected simultaneously with four of these surveys were analyzed with the QTC MULTIVIEW/spl trade/ software, which segmented the multibeam sonar images into six regions of homogeneous acoustic character. The Multiview process leading to the six classes is described. Seafloor grab samples and photographs were used to characterize these acoustic classes physically. The depths and the classes were interpolated onto the same grid to allow subtractions and comparisons. Interpolating classes was done with a categorical interpolation process. Comparisons among the sequence of maps of acoustic classes show the spreading of new spoils and consistent seasonal changes over large areas. Because high-frequency backscatter depends heavily on surface conditions, seasonal changes in the local hydrodynamic regime could be responsible for these changes in acoustic character.