Towards 3D SPM Monitoring in the North Sea Using Multibeam Sonar

Monitoring suspended particulate matter (SPM) in coastal areas is essential for research, management and protection of coastal ecosystems. In the Belgian part of the North Sea, the dynamic nature of SPM variability and the increasing human activities (offshore windmill parks, dredging and dumping) call for 3D monitoring of these natural and human-induced SPM changes.Multibeam echosounders (MBES) provide, in addition to bathymetry and seafloor backscatter data, a 3D dataset of acoustic measurements in the water column, which can be used to monitor SPM in coastal waters. Although MBES water column data are commonly used by fisheries and gas seepage research, only a handful of studies focus on the quantification of SPM in the water column.During the Timbers project, we developed a novel methodology to convert MBES water column data into 3D SPM maps. In contrast to most studies that deploy the MBES from stations, we quantified SPM using MBES from a sailing vessel. Simultaneous optical and acoustic measurements were collected during ship transects to yield an empirical relation using linear regression modeling. This relationship was then used to convert the acoustic measurements into a 3D grid that displays the mass concentration of SPM. The large spatial coverage of these SPM maps allows us to observe phenomena in the water column that otherwise would be missed by traditional monitoring approaches. Furthermore, several valuable lessons were learned. In particular, the interpretation of the acoustic signal is not straightforward, which makes it difficult to distinguish between different types of scatterers (sediment, plankton, flocs, bubbles, fish, etc.) captured by the MBES. Hence, additional research efforts focusing on discriminating scatterers in the water column are needed to unlock the full monitoring potential of MBES water column data.In the ongoing Turbeams project, we are exploring multi-frequency approaches to differentiate between various scatterers and their wide spectrum of sizes. Additionally, we are applying imaging tools on collected water samples and we are using underwater cameras that capture particles in their natural environment. These improvements will help to move towards operational use of MBES as a common tool for SPM monitoring in the future.