Multibeam echosounders have been used to map the seafloor for almost two decades. Sensor resolution has increased dramatically and resolutions of 0.5 degrees or better are not uncommon. In combination with high ping rates, such high resolutions support acquisition of very dense point clouds, leading to accurate mapping of small features such as pipelines, underwater structures, wrecks, ordinance and debris. Multibeam imagery data further enhances our view of the underwater environment, adding to the ever-increasing data volumes.
Most multibeam transducers employ a swath sector of 120- 150 degrees. Two transducers mounted at angles to the vertical, in a dual head configuration, widen the swath sector beyond 180 degrees. A few sounders offer a swath sector of 210 degrees, using a single transducer. Such wide swaths permit observation of vertical underwater structures right up to the water’s surface.
Extremely detailed 3D images of the underwater environment are routinely produced, including breakwaters, quay walls, platforms and other underwater structures with centimetre-level accuracy. Multibeam systems cannot acquire point cloud data above the water surface. Terrain models of structures are simply ‘cut off ’ above the air-water interface. In many cases we would prefer continuous terrain models that include above-water structures.
Lidar (Light Detection And Ranging)
Lidar (Light Detection And Ranging) techniques have also been employed for almost two decades, mostly for topographic surveys. Airborne laser has long been efficient at mapping large land areas.
For smaller areas, static Lidar techniques are commonly used to map terrain for various purposes, including land development projects. Under pressure to acquire ever more dense data in shorter periods of time, with less manpower, terrestrial mobile laser systems have recently been introduced successfully. In combination with high-end INS systems, they can produce extremely detailed 3D point clouds with centimetre-level accuracy.