Bathymetry Estimation and Marine Mapping

Bathymetry is recognized as an essential component in marine applications because it is used in many coastal erosions tracking and engineering projects. Traditionally, it was obtained via shipboard echo sounding, but nowadays, multispectral satellite imagery is also commonly used with various remote sensing-based algorithms. The surface reflectance of shallow coastal waters is related to the depth of the water column using satellite-derived bathymetry (SDB).

Satellite-derived bathymetric (SDB) remote sensing analysis employs an ocean imaging system to evaluate near-shore bathymetry elevation values using Landsat 8 or DigitalGlobeWorldView satellite imagery. Bathymetric surveys are typically carried using Single-beam and Multibeam technology to obtain finer insight. Such an approach necessitates extensive fieldwork and is both time-consuming and costly.

Remotely sensed images with a high spatial resolution (one meter) and Digital Elevation Models (DEMs) are extensively used in marine ecosystems monitoring and are a significant tool to enhance Integrated Coastal Zone Management (ICZM) research. Bathymetric data were extracted from UAV multispectral images to test different band mixes and evaluate accuracy as a function of the density and amount of sea bottom control points.

Multibeam sonars and Side-scan sonars

A multibeam sonar system is an active sonar system that is used to map the seafloor and recognize objects in the water column or along the sea bottom. Backscatter measurements can also be captured by multibeam sonars for features that reflect sound in the water column.

The reflectivity estimation is of multibeam backscatter, while the sidescan sonar imagery is the actual severity of the reflected signal. Because the depth of the tow-fish just above sea bed can be adjusted concerning the swath width, the Sidescan sonar towing arrangement offers a better range of movement. As the slant range increases in the far range, the footprint length of the sidescan sonar is higher than at the nadir beam, enhancing the two-way travel time of the acoustic signal. A wider footprint has a higher degree of uncertainty in sensing its first return and a lower resolution.

 

Sidescan imagery, for instance, is less likely to be influenced by the slope of the seafloor because it can be aligned, whereas the multibeam can only receive backscatter frequency as it approaches the survey vessel. Interferometric sonars can be very efficient for imaging, but due to technical constraints, they are especially suitable for collision avoidance. Usually, the shape and size of those objects are projected by analyzing the size of their shadows.

Applications

• Benthic habitat mapping
• Hydrographic surveys
• Civil infrastructure investigations
• Charting of the seas
• Dredging operations
• Archaeological expeditions
• Route mapping