Depending on the slant range and elevation angle of returned echoes from the bottom, a vessel-mounted multibeam sonar produces a fan, or swath, of acoustic energy across the ship track, then resolves several depth locations throughout the swath. Lidar is a multibeam-like device that uses airborne laser bathymetry to survey shallow clear rivers.
The Sea Mapping Tools Differences
The capacity to scan vast regions of the seabed has improved due to the development of different swathe mapping methods. The following is a comparison between multibeam sonar and Lidar.
In multi-beam sonar systems, multifrequency bathymetry is utilized to broadcast a fan or swath of acoustic energy across the vessel’s track. The swath is narrow along the track (typically 1-2 degrees) and wide across it (often 120 degrees or more). The multibeam receiver array measures the slant range and elevation angle of several seabed echo returns throughout the swath. The word “backscatter” or “amplitude” refers to the echo strength of the returns.
The multibeam sonar system also records the vessel’s precise position and motion data, such as gyro heading, heave, pitch, and roll, to calculate an accurate depth and location for each depth sounding. As long as the vessel goes forward relative to the bottom, a multi-beam swath survey generates a dense ‘point cloud’ of soundings that may be utilized to build a 3D depth model that accurately represents the underwater topography.
The resolution of bottom structures detected by such depth models in shallow waters may be sub-meter. The resolution of identified structures in deeper seas decreases to tens of meters due to the spread characteristics of acoustic radiation over longer distances and the lower frequencies used to ensonify the seabed. Inland surveys ensure the safety of navigation in waterways.
Light Detection and Ranging is a seabed mapping technique that uses low-flying aircraft to scan pulsed laser beams across the bottom and generate a swath of depth soundings. Depending on data density and collection rates, Lidar sensors record around 1000 depth soundings per second with a swath width of 200 m while flying at the height of about 500 m.
At this height, the footprint of the green laser beam on the sea surface is around 2 meters. Even yet, the actual grid spacing may be anything between 2 and 10 meters. Compared to vessel-mounted multibeam sonar systems, lidar can offer fast, high-resolution shallow water surveying capabilities at speeds of about 150-175 nautical miles per hour.
The primary disadvantage of Lidar, as compared to multibeam, is that laser signals are substantially reduced in muddy water. As a consequence, Lidar should only be used to scan clear, shallow waters. On the other hand, tropical coral reef areas provide substantial navigational risks to vessels doing multibeam surveys. Consequently, Lidar has been successfully utilized to map large swaths of the continental shelf where vessel surveys would be impracticable.
Seabed mapping techniques such as multibeam and Lidar have helped chart the world’s oceans during the past few decades. For tropical coral reefs, mapping provides a better understanding of the nature of the deeper seabed and inter-reefal ecosystems. These ground-breaking discoveries open the door for significant improvements in energy generation, fisheries resource management, and ocean environmental protection.