The drive towards sustainable energy sees 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), with the first field deployments in June-July 2012. The project combines 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 an innovative autonomous sonar platform. Self-contained and deployed for 2 weeks at a time with short turnaround times between deployments, this platform includes an Imagenex Delta T multibeam sonar (260 kHz) with associated power supplies, instrumentation and data storage. The sonar faces vertically upwards with the multibeam swath orientated parallel to the tidal flow. 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.