Chesapeake Times, Vol 14 | July 2023
Recently the question came up, “What is the best height to tow my Side Scan Sonar off of the bottom?” The general answer is the altitude should be 10% to 20% of the slant range. While this is a good rule of thumb, or general rule to follow, where does it come from?
First, let us go over the reasons for adjusting the height of the towfish:
- Limiting the amount of water column in the Side Scan Sonar image. This minimizes slant range distortion.
- Placing the center of the beam in the center of the range. This ensures the transducer sensitivity is optimized for the largest area so you get the best image.
- Minimizing side lobes and first surface return. We do not want artifacts in the image.
- Ensuring an optimal shadow length for effective height measurements and identification of underwater objects.
- The down angle, or depression angle (angle from horizontal), and transducer beam pattern varies from manufacturer to manufacturer.
On the way to answering this we should take a look at a typical Side Scan Sonar beam pattern shown in the above illustration. This shows the sensitivity of the transducer as an angle off the center of the transducer. In other words, how well the transducer can hear in any given direction. The highest sensitivity is most often along the centerline of the transducer perpendicular to the transducer face. The manufacturers optimize the placement of the transducer by angling the transducer down. This down angle is commonly known as the depression angle. The depression angle is generally chosen to maximize the area that is ensonified while minimizing the unwanted returns from the surface and things in the watercolumn.
The transducer works better when pointed in a specific direction. If the transducer faces straight out of the side of the sonar the main area sensitivity never covers the sea floor. By tilting the transducer down from the horizontal the transducer can cover more of the seafloor area until it reaches a point where the size of the ensonified area recedes. This is shown in the below illustrations. The first one shows a beam width of 50 degrees and the second shows a beam width of 25 degrees.
In the image the red line represents the seafloor. The gray line is the height of the towfish. As you can see tilting the transducer down gives the best coverage but only to a certain point. In this next illustration we vary the height of the towfish. The blue line represents the center of the beam.
As you can see the higher the sonar is off the bottom the farther out the center of the beam will be. Using this illustration you can see the relationship between the towfish height off the bottom and the range is a triangle. Using a bit of math we can calculate the ideal height off the bottom given a particular down angle.
The ideal height off the bottom is shown as a distance in the following table. While the ranges are shown in popular increments of meters, these numbers also work for feet as well.
The ideal height off the bottom is shown as percentage in the next table. Most sonar system’s depression/down angle vary between 15 degrees and 35 degrees.
As you can see from the tables, the rule of thumb of 10% to 20% covers a majority of the height off the bottom values.