Basic Side Scan Image Analysis
Side scan sonar imagery often appears self-evident, yet there are many subtleties you need to be able to understand. Let's start with the basics. The image to the left is a gorgeous shipwreck. Notice the acoustic shadow? If you haven't looked at a side scan sonar image before please start with the Black Laser Reading a Sonar Image, otherwise back to looking at acoustic shadows.
Any object not lying flat on the bottom imaged by side scan sonar will block the sonar pulse and create a shadow. The shadow provides us with more detailed information, including the height of the object above the sea floor as well as other details not visible in the sonar image.
Compare the surface image of the Paulie Marie (Lower Right) to the sonar image above. From the photograph, we can see the ship has a raised bow and wheelhouse. And yet the sonar image dose not show the difference. However, looking at the shadow, we can clearly see both the bow and wheelhouse have much larger shadows, indicating the relative height of those sections are higher than the middle deck - something the sonar image without the shadow could not show us.
Looking carefully you can see the bow railing visible in both the sonar shadow and the photograph.
In summary the side scan acoustic shadow tells us:
1) If the target rises above the bottom.
2) Details not visible in the sonar image.
Large shipwrecks on open, sand bottoms stand out. It does not take the most experienced side scan sonar operator to identify the target. However, identifying smaller targets or targets with less reflective surfaces such as fiberglass boats requires a higher skill level.
Examine this next sonar image. Do you think it shows rocks or is there something more? Usually the first characteristic of any target to catch your eye is the shadow. There is a small target casting an small acoustic shadow. Let's examine the shadow in more detail.
Zooming in on the target and analyzing it, we see the shadow fairly uniform and straight.
We have purposely increased the intensity of the image to the right to accent the shadow. The shadow indicates we have a target lying on the bottom with some height to it. Looking at the target, we can see the target has some complexity and is not a log or a tree. To further analyze the target, we would need to reduce the gains and bring out more detail in the target. This target looked good enough to dive, and our team added an early 20th century power boat to its wreck list.
For more information on analyzing side scan sonar images go to our Advanced Sonar Image Analysis page or for at-home study, try our Second Edition Not in the Manual Guide® to Side Side Scan Sonar Image Interpretation.
"...truly amazing set of DVD's that have taught us so much!!!" Kevin Heath (Aviation Research Group Orkney & Shetland)