Bathymetry is a vast field that integrates a multitude of technologies. Whether a hydrological study is conducted on a river, mapping of an area of the ocean floor is required for a pipeline construction or a navigation canal needs an inspection for potential obstacles, several solutions are applicable.
Among the frequently used equipments in bathymetry for detection of objects or obstacles, we found the multibeam echo sounder (MBES) and the side scan sonar (SSS). What are their specific applications, strengths and weaknesses? This article aims at highlighting some differences between these two types of bathymetry instruments in the context of object detection on the seabed.
Let’s start with some general information to have a better understanding of their specificities. The multibeam echo sounder is based on the backscattering of the reflectivity measurement. This makes possible the acquisition of high-precision bathymetric data, but the resulting imagery is of lesser quality than the one achieved with a side scan sonar. The latter, whose operation is rather based on the intensity of the return of the signal, permits only images but of better resolution. Regarding the usual configuration of these instruments, the SSS is frequently used in a “fish” mode that is towed behind the survey ship at the desire depth. The MBES is normally fixed directly under the hull of the survey ship. Let’s now discuss individually these equipments and then comment on their complementarities.
Side scan sonar (SSS)
Towing the SSS behind a survey vessel allows better manoeuvrability since the depth of the instrument relative to the seabed can be adjusted in order to adjust the acquisition bandwidth. The SSS imagery is then more consistent by adapting to variations in the seabed slope. In comparison, the MBES is fixed and can only pick up the backscattering intensity only when it reaches the hull of the ship. The towing configuration of the SSS reduces the accuracy of the positioning of the acquired images. The position behind the ship must be established by trigonometric calculations based on the length of the cable, its depth with respect to the ship and the direction of the vessel. This position thus determined must then be linked to the positioning system of the vessel with the resulting uncertainties.
When used as part of a detection or inspection job, the SSS is a proven tool and the distinctive shading due to the orientation of the SSS transducer in relation to the objects on the seabed is the characteristic enabling it to be so effective for this kind of application. Indeed, it is the “side” aspect of the SSS that makes possible the generation of shades allowing a better definition of objects having different complex heights and shapes.
Multibeam echosounder (MBES)
Contrary to the SSS that only generate images, the MBES is a bathymetric sounders having a complementary imaging function. The extent of surveys done with an MBES is generally greater given the penetration capacity and survey width that can be up to several kilometers. Given its fixed position relative to the positioning device of the ship, the accuracy of positioning of a DTM (digital terrain model) obtained by an MBES is greatly superior of what is obtain from a SSS.
On the other hand, when used for object detection purposes, the resolution and the usable range of acquisition of the MBES are limited. The effective detection zone is only found near the nadir of the MBES transducer. Moreover, several configurations have to be taken into consideration, for example the detection footprint at the bottom versus the dimensions and depths of the objects and the spacing of the survey lines which must be limited to the width of the usable detection bands. Moreover, in comparison with the SSS, the MBES requires the post processing of the data in order to allow the visualization of the data.
By being fixed under the hull of the survey ships, the configuration of the MBES transducers, eliminate any projection on objects creating shadows. So if an MBES is used for object detection purposes, it is necessary to focus on the resultant bathymetry rather than (non-existent) shadows.
Complementarities of using of the SSS with the MBES
Quality of deep sounding collected with the MBES are superior to those of the SSS, but the SSS has a better obstacle detection capability because of its lateral appearance. Their simultaneous use then makes it possible to obtain the best of both worlds. A hull mounted combination of these two instruments that integrates both the accurate MBES bathymetry and the high resolution SSS imagery is a interesting and relatively low costs solution for detailed surveys of harbors, canals, rivers and other shallow areas.
Taking advantage of miniaturization of the components, these two technologies can be embarked on platforms of reduced dimensions such as a launch or an inflatable boat. Moreover, for certain applications, the use of unmanned vessels brings many advantages such as minimal mobilization of personnel, reduction of risks for operators and access to dangerous areas.
For all your bathymetry needs, give Geolocation team a call to determine the optimal solution and overcome the challenges of your projects.
MBES Autonomous hydrographic survey boat
– 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 mounted sidescan sonars with multibeam bathymetry, Kongsberg Maritime.