Locating a large steel shipwreck like the 460-ft victory ship pictured left should be a piece of cake. Anyone who can turn on the machine should be able to see the wreck when towing the system properly in good weather. But poor weather conditions with excessive towfish motion is one of the main causes of distorted sonar imagery. Notice how the shipwreck appears warped? The twisted wreck is a result of the curved sonar path. A large swell forced the boat captain to correct course which translated into a twisted shipwreck image.
High sea states and pitching survey boats increase the towfish motion. Heavy seas, faster tow speeds, longer sonar ranges, high towfish altitude and the wrong aspect make the large shipwrecks appear much smaller.
It’s hard to believe the side scan image to the right is the same shipwreck. It is much easier to miss a large shipwreck when conditions are combined with higher speeds and longer sonar range settings. Think about the effect when searching for much smaller targets. The experienced sonar operator understands the limitations of weather conditions and designs his or her survey/search plan accordingly.
Examine the side scan sonar image on the left. One would think there is some type of structure on the bottom rising up into the water column. Notice the lack of an acoustic shadow. Actually, the bottom is a flat lakebed and the stronger reflections are ghost images we call second sweep returns. The image on the lower right shows the actual island. Identifying ghost returns, especially for homeland security folks working in crowded harbors, is a critical sonar analysis skill.
Need a better understanding of how to enhance your sonar skills? Try our Second Edition Not in the Manual® Guide to Side Scan Sonar Image Interpretation DVD. Or if you prefer, you can stream the video.
These programs will provide techniques such as:
- How to detect ghost images
- Determine if targets are on the bottom
- How to conduct surveys between piers and other obstructions
Need to know a specific side scan sonar term? Check out the Black Laser Learning Sonar Glossary
Have a side scan sonar related question? Go to our Side Scan Sonar Forum
Side Scan Sonar Manufacturers & Sonar Data Acquisition/Processing Software
MARINE SONIC TECHNOLOGY, LTD.
© 2018 Black Laser Learning, Inc.