Multi Beam Echo Sounder Survey at London

High Resolution Multibeam Bathymetry data were collected by Robin Fiddler of Aequorea and Matt King of Hemsley Hydro on the 22nd October 2025. The data were collected to a specification produced by MSDS Marine. The Multibeam data were collected using a Norbit Winghead B53s Multibeam Echo Sounder (MBES) mobilised onto a dedicated survey vessel. At 400 kHz the B53s has a beam width of 0.5� x 0.9�, a real time user selectable swath sector of 5� to 210� and a depth resolution of up to 6 mm. These features ensure high resolution, high density data collection the parameters of which can be adjusted in real time to ensure optimum ensonification of the seabed and any features of potential archaeological interest. Positioning and motion for the MBES were controlled using an Applanix POSMV WaveMaster with real time RTK corrections. The Applanix system with RTK corrections can produce positional accuracy of >0.1 m, roll and pitch to 0.02�, heading to 0.03� and heave to 2 cm or 2%. Prior to data collection a patch test was undertaken to determine any offsets between the MBES, the Inertial Measurement Unit (IMU) and heading sensor. Offset corrections were then applied to the dataset to ensure minimal errors in the positioning and overlap of the data. MBES data were collected by running predetermined lines, broadly east-west, based on the depth of water to achieve 100% ensonification of the study area, with a minimum data density of one point per 0.1 m cell. The deeper the water the wider the coverage at a fixed swath sector, however, beam footprint will increase and data density will decrease. The data recorded was displayed in real time; as such online QC took place and lines were re-run or data gaps filled in where required. Sound velocity was recorded continuously at the MBES head with a Valeport Mini Sound Velocity Sensor (SVS) and at intervals through the water column with a Valeport Sound Velocity Profiler (SVP). Sound velocity measurements are required, and applied to the MBES data, in order to correct errors that may be created due to variations in the speed of sound through the water column. Data processing The data were supplied to MSDS Marine as full density .xyz files with patch test and tide corrections applied. The lines of data were then cleaned in various programs including Cloud Compare to remove noise, data artefact and unwanted features such as fish. Once the data were cleaned the lines were imported into software including Cloud Compare and Esri ArcPro where the data were visualised and effects such as shading applied to help highlight potential anthropogenic features. The data were presented as georeferenced, depth encoded, Tiff images and also exported as a processed point cloud in .xyz format. All depths were reduced to Lowest Astronomical Tide (LAT). The data had good positional correlation between previous datasets, and the specification of one point per 0.1 m cell was exceeded. Site One Site One of the London broadly comprises three distinct areas of wreckage, covering an approximate area of 29.1 m x 17.2 m (Figure 4) and is located within the northern section of the designated area. The southern most feature measures 17.2 m x 7.3 m and is a prominent mound comprised of visible, but incoherent, structure and accumulated seabed, scour is prominent to the north and the east. A number of linear features, assumed to be structure, link the southern feature to a central feature which measures 9.6 m x 3.7 m, and comprises a number of parallel linear features oriented north to south and each measuring c.3.2 m. The linear features represent a mound of iron guns. Approximately 5.0 m to the north there is another incoherent area of structure measuring 11.2 m x 6.2 m, scour is visible to all sides of the feature with the exception of to the west.