Bathymetry is the study of underwater depth of lakes or ocean floors. In other terms, it is the underwater equivalent to topography. From deep-sea cartography to canal inspection, bathymetry is a solution for many types of projects. One of its miscellaneous applications is the detection of objects buried in seabeds. Awesome, isn’t it? Because bathymetry is part of our job, we will briefly discuss two of the equipments available in object detection : the multibeam echosounder and side-scan sonar.
Multibeam echosounder (MBES)
The MBES is a high-tech bathymetric device that measures simultaneously many water levels under the vessel, along a port and starboard direction. It is based on reflectivity measuring, and the sensor is placed under the ship’s hull. This can be compared to airborne LiDAR as it scans a surface line after line. Thanks to a powerful penetration capacity, it can cover a large area and survey several kilometers in a row. However, it provides a limited spatial resolution and acquisition width in object detection. Indeed, the signal transmitted by objects has to hit the transduction mechanism at an optimal angle. Plus, the echosounder doesn’t collect any shadow as the sensor is installed under the boat. So we better use the transmitted waves instead of the the images to detect hidden objects. Once the acquisition completed, the post-processing will help in further analysis, for example the production of an accurate DTM.
Sidescan sonar
Sidescan sonar is also used for bathymetry needs. It works with the intensity of the rebound signal as a way to measure depth. Contrary to the MBES, this sonar is usually towed behind a ship, making it easy to operate. Especially since you can adjust the device’s depth under the waterline. In comparison, MBES rebound signal’s measurement can only be collected when hitting back the vessel’s hull. The accuracy of the MBES is however lower than for the sidescan sonar. Since the sonar is set behind the ship, its position is calculated relatively to the boat, and so decreases the accuracy. Nevertherless, it is an effective tool for detection or inspection because the shadows are easily identifiable on the images.
Complementarities between the two techniques in bathymetry
Both methods collect bathymetric data of high quality, even if images from the sidescan sonar look better. The simultaneous use of these two instruments on a boat makes it possible to mix the precision of the MBES with the high resolution imagery of the sonar. This combination is a very cost-effective solution for detailed surveys of harbors, canals, rivers and other shallow areas. Miniaturization of technological devices now allows both devices to be installed on a single, easy-to operate vessel, even an inflated boat. In particular, aboard an unmanned vessel, access to restricted areas is no longer at risk for operators.
For any of your bathymetry projects, don’t hesitate to communicate with Geolocation. We will analyse your project and find the best way to fit and even exceed your expectations.
SOURCES
– M. B. Brissette and J. E. Hughes Clarke, Side scan versus multibeam echosounder object detection : a comparative analysis,
– SEABED HABITAT, Multibeam backscatter vs. sidescan sonar imagery, https://seabedhabitats.org/2014/02/16/multibeam-backscatter-vs-sidescan-sonar-imagery/, novembre 2016
– LE SHOM, Les outils d’acquisitions : Sonar latéral, sondeurs multifaisceaux, https://www.shom.fr/les-activites/activites-scientifiques/sedimentologie/imagerie-acoustique/#, novembre 2016.
– F. Pohner, J. O. Bakke, K. E. Nilsen, T. Kjaer, Integrating imagery from hull mouinted sidescan sonars with multibeam bathymetry, Kongsberg Maritime.