Investigating wrecks in shallow waters through the effective use of multibeam echosounders, relying on backscatter and water column data.
Off the West Coast of India, approximately 54 nautical miles west of Karwar, a significant operation took place involving the investigation of underwater wrecks. The operation, which marked a milestone in underwater archaeology, utilised advanced technology to gather detailed information about the submerged structures.
The primary tool of choice for this operation was the multibeam echosounder (MBES), a sophisticated and efficient tool for remotely observing and mapping the seafloor. MBES systems, such as the Kongsberg EM 710 and EM 3002, provide high-resolution bathymetry maps and backscatter images simultaneously.
Multibeam echosounders are generally considered more effective than traditional side scan sonar (SSS) for underwater wreck investigations, thanks to several key advantages. MBES provides centimeter-level geo-referenced data, reducing errors like slant-range and layback uncertainties that affect side scan sonar. This means wreck features can be mapped more precisely in 3D space.
MBES systems also offer wider area coverage with higher resolution, enabling faster and more comprehensive coverage compared to towed side scan sonar systems. Additionally, MBES collects simultaneous high-resolution bathymetry alongside backscatter strength, enhancing wreck detection and characterization by revealing both shape and material properties.
The data for the wreck depicted in the position was collected by both the Autonomous Underwater Vehicle (AUV) INS Makar and the side scan sonar ET 4200 FS. The MBES lines were planned at 100m apart along the TSS and were later extended northward. The resolution of the mosaic was set to 1 meter to ensure a detailed representation. Various options for Time Varied Gain (TVG), Auto Varied Gain (AVG) and despeckle were applied to enhance the quality of the mosaic.
Water column data was also acquired on top of the wreck for further analysis. This modern tool is proving to be an effective approach for determining the safe depth over the wreck. The depths in the area ranged from 28-34m, and the achievable swath width was 100m.
The contact was detected in real-time through the SIS's seabed image window. The decision was made to focus on MBES sounding with an emphasis on backscatter imaging. Various normalization and corrections were applied to enhance the quality and accuracy of the backscatter data using third-party software makers.
The integrated approach streamlines the wreck investigation process and enhances its accuracy and efficiency. The S 44 6 edition recommends that, whenever possible, both data sets (bathymetric data for feature detection and data for measuring safe depth) should be collected in conjunction.
The second case study involved a tugboat named Varapradha that sank off Mumbai in May 2021. INS Makar used the EM 710 hull-mounted MBES for data collection, along with the ET 4200 FS side scan sonar. The data underwent processing, and mosaicing was carried out in a manner similar to Case 1.
Modern MBES have been effectively employed by the Indian Naval Hydrographic Department (INHD) over the past two decades. The conclusion is that MBES can be effectively utilized for wreck investigation in shallow waters, provided all the requisite data types, including MBES backscatter and water column data, are acquired.
A comprehensive communication system comprising an Acoustic Communication Link (ACL), and Acoustic Data Link (ADL) was utilized during the mission. The Mill's cross technique was used in MBES, with the transmitting unit oriented along the ship's axis and the receiving unit oriented across the ship's axis.
Understanding the underwater imaging of wrecks or features is significant, and there is an inquiry into whether MBES backscatter produces better results than side scan sonar (SSS) in diverse underwater scenarios. However, side scan sonar remains valuable for certain applications where detailed texture or fine-scale imagery is needed and in shallower or less complex environments due to its simplicity and cost-effectiveness.
In summary, for thorough and precise underwater wreck investigations requiring accurate spatial positioning, comprehensive coverage, and high-resolution 3D mapping, multibeam echosounders are generally the more effective choice compared to side scan sonar. The integrated approach of using both MBES and side scan sonar provides a comprehensive solution for underwater wreck investigations.
- The hydrographic survey operation off the West Coast of India, significant in underwater archaeology, highlights the role of science in uncovering submerged structures.
- The multibeam echosounder (MBES), a sophisticated tool for observing and mapping the seafloor, is the primary tool utilized in the operation.
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- However, side scan sonar (SSS) remains valuable for certain applications, such as those involving detailed texture or fine-scale imagery, in shallower or less complex environments.
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