Multibeam imaging of the environment around marine renewable energy devices

The drive towards sustainable energy has seen rapid development of marine renewable energy devices, and current efforts are focusing on wave and tidal structures. However, little is known of the effect of installing and operating these devices. The NERC/DEFRA collaboration FLOWBEC-4D (Flow, Water column & Benthic Ecology 4D) is investigating these effects at test sites in Orkney (EMEC) and Cornwall (Wave Hub), combining the data from bird observations, shore-based marine X-band radar surveys of wave and current data, detailed modelling of the flow and water column, passive acoustic monitoring and the deployment of a remote-sensing sonar platform. The latter contains a multibeam imager, whose design and operation will be presented in this talk. The first field deployments are planned for May-July 2012 with first underwater tests earlier in spring. Self-contained and intended to be deployed for 2 weeks at a time with short turnaround times between deployments, this platform is based on the Imagenex Delta T multibeam sonar (260 kHz) with associated power supplies, instrumentation and data recording devices. The sonar faces vertically upwards with the multibeam swath orientated parallel to the tidal flow. Versatile duty cycling allows acoustic snapshots at specific intervals of interest. Range and volume backscatter intensity measurements are used to reconstruct the evolution of the environment around the sonar. This allows imaging of targets (e.g. fish and diving seabirds) interacting with the marine renewable energy device of interest and moving in the tidal flow.