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 multiple depth locations within the swath. Lidar is a multibeam-like device that uses airborne laser bathymetry to swath map shallow clear rivers.
The Sea Mapping Tools
The capacity to scan vast regions of the seabed has improved with the advent of different swathe mapping methods. A comparison between multibeam sonar and Lidar is shown below.
Multibeam sonar systems utilize a multi-element transducer array to broadcast a fan or swath of acoustic radiation over the vessel’s track. The swath is narrow on one side of the track (typically 1-2 degrees) and wide on the other (often 120 degrees or more). The multibeam receiver array measures the slant range and elevation angle of many seabed echo returns throughout the swath. The word used to describe the echo intensity of the returns is backscatter or amplitude.
The multibeam sonar system also maintains track of the vessel’s precise position and motion data to calculate an accurate depth and location for each depth sounding, such as gyro heading, heave, pitch, and roll. 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 as low as a meter. The resolution of identified structures in deeper seas decreases to tens of meters due to the spreading properties of acoustic radiation over longer distances and the lower frequencies used to ensonify the bottom. Multibeam sonars are also important in mining applications.
Light Detection and Ranging
Lidar, also known as Airborne Laser Bathymetry, 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 about 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 about 2 m. Even yet, the actual grid spacing may be anything from 2 to 10 meters. Compared to vessel-mounted multibeam sonar systems, lidar can offer fast, high-resolution shallow water surveying capabilities at speeds of 150-175 nautical miles per hour.
The primary disadvantage of Lidar over 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 pose 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 are 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 true nature of the deeper seabed and inter-reefal ecosystems. These ground-breaking discoveries open the door for major advances in energy generation, fisheries resource management, and ocean environmental protection.