Acoustic multibeam system for 3-dimensional bedform evolution studies

The equipment requested is a high-frequency acoustic multibeam system for obtaining high-resolution profiles of the seafloor at frame rates approaching that of video systems. The bed elevation profile is obtained by steering the acoustic beam spot electronically over the seafloor. Based on an earlier design operating at an acoustic frequency of 260 kHz (6-mm wavelength), this newly-developed Canadian-built sensor operates at 2.25 MHz (0.7-mm wavelength), the shorter wavelength providing much higher spatial resolution. To obtain a 3-dimensional map, the acoustic sensor must be mechanically driven in a direction orthogonal to the beam-steering plane, and thus funds are also requested to purchase a rotator unit for this purpose. The combined multibeam/rotator system will be capable of generating 3-d maps of the small-scale seafloor topography over a 5-m by 5-m area with a horizontal resolution of a few cm or better, and better than 1-cm vertical resolution, in tens of seconds. Given the 10-s typical periods of wind-generated sea-and-swell waves, this acquisition time is commensurate with the passage of several surface gravity waves. Thus, the system will enable first-time investigations of the evolution of mobile seafloor topography on the time scales of wave-groups. The system will be deployed in the fall of 2007 for 2 months as part of an investigation of the temporal evolution of wave-formed ripples on inner shelf in coarse- and fine-grained sands.