Based on the slant range and elevation angle of returned echoes from the bottom, a vessel-mounted multibeam sonar generates 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 scan shallow clear rivers.
The Sea Mapping Tools
The capacity to scan vast regions of the seabed has improved due to the development of different bathymetric mapping methods. Below is a comparison between multibeam sonar and Lidar.
Multibeam Echo Sounder
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 (typically 1-2 degrees) and wide across the track (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 multibeam sonar system records the vessel’s precise position and motion data, such as gyro heading, heave, pitch, and roll, which is utilized to calculate an accurate depth and location for each depth sounding. As long as the vessel moves forward relative to the bottom, a multibeam swath survey generates a dense ‘point cloud’ of soundings that may be utilized to create a 3D depth model that accurately represents the underwater terrain.
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 spread characteristics of acoustic radiation over longer distances and the lower frequencies used to ensonify the seabed. Visit this website to know more about the different multibeam sonars.
Lidar
Light Detection and Ranging is a seabed mapping technique that uses low-flying aircraft to scan pulsed laser beams over the bottom and produce 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 around 500 m.
At this height, the green laser beam’s imprint on the water surface is approximately 2 meters. Even yet, the actual grid spacing may be anything between 2 and 10 meters. Faster than vessel-mounted multibeam sonar systems, Lidar can offer quick, 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 decreased in muddy water. As a consequence, Lidar should only be used to scan shallow, clear water. On the other hand, tropical coral reef areas provide substantial navigational risks to vessels performing multibeam surveys. Lidar has successfully mapped large swaths of the continental shelf where vessel surveys are not feasible.
Conclusion
Seabed mapping techniques such as multibeam and Lidar have helped map the world’s oceans during the past few decades. For tropical coral reefs, mapping provides a better understanding of the features of the deeper seabed and inter-reefal ecosystems. These ground-breaking discoveries open the door for significant advances in energy generation, fisheries resource management, and ocean environmental protection.
