Side Scan Sonar


The side scan sonar creates an image of the bottom using sound waves.  While this can look like a picture, the image depends on the interaction of the sound waves with the bottom.  The system uses the time of the return to compute a distance, and then displays the intensity of the return in a shade of gray.  The intensity of the return depends upon:

Key terms:

Strong returns are now generally shown in white, and no returns in black, denoting the sound shadow.  This was not always the case with the first systems which only used a paper recorder and only put black ink on the targets.  You should always verify the color convention used in the imagery you are looking at.  Colors are sometimes used as well to highlight very strong returns, and keep a watch stander alert as they come in.  A gold color scheme is the most common today.
Grayscale Custom gold color scale

 

Current sidescan sonars often collect use two frequencies simultaneously.

Lower frequencies attenuate less and travel farther; whales use low frequencies to increase the range of their communication.

Higher frequencies have more detail, and for targets close to the fish will have improved contract and details compared to a low frequency image captured at the same time.
Low frequency High frequency  
Less Contrast   More contrast

 

Figure 1 below shows two views of Submarine S5.  The image on the left is smaller, indicating the sonar was operating with a longer range.  This provides less detail, but covers a large area, and is generally how sonars are used in searching.  Side scan sonars are designed to view the seafloor from the side, and provide very poor geometry directly under the towfish (Figure 2).  In both Figure 1 and 2 the track of the towfish is shown by the large pixels.  For the image of the S5, once the NOAA survey ship located the wreck, they switched to a short range on the sidescan to collect a better image, and they returned for a second pass with the ship track oriented in the same direction as the wreck.  In addition, they insured the wreck was in the middle of one channel, and not under the fish.  If the survey that acquired Figure 12 was interested in details of the wreck, they would have taken a second pass and insured they passed to the side of the wreck.  In addition to not wanting to pass over the wreck, the towfish needs to be close to the bottom to enhance the shadows.  Note that in Figure 11 the shadows provide more information than the actual imaged portion of the wreck.

Figure 1.  Submarine S5 (sank in 1920 40 nm off the Delaware coast) sidescan sonar images. Image on the right obtained by knowing the orientation of the submarine and maneuvering the sidescan such that its track ran parallel to the vessel.

The illuminated area is the reflection and dark is the shadow.

Click for higher resolution image.


Image ID: expl4060, expl4061, (NOAA Photo Library.)

Figure 1.  Sidescan sonar record of shipwreck. The track of the towfish went directly over the wreck, and is indicated by the large distorted pixels whichs show the along track direction.


Image ID: theb4045 (NOAA Photo Library.)

 

Survey vessels can acquire multibeam bathymetry and side scan sonar imagery at the same time (Figure 7), and combining this results greatly increases what an analyst can see in the data (Figure 13).

Figure 13.  Comparison of older side scan and multi-beam systems on ship wreck. The newer higher-resolution systems are to the right of the image.

Image ID: cgs00879,  (NOAA Photo Library.)

The sidescan survey has a swath width, which is the area covered.  It is a little less than twice the range, since the quoted range is the slant range for each channel.  If the fish is at the preferred height (15% of the swath width), the horizontal distance on the bottom is about 99% of the range.  If the ship tracks were at exactly twice the range, you would get almost 100% coverage, but the region directly under the towfish would have terrible coverage.  If the ship tracks were at half the spacing, you would cover every point on the bottom twice, and the region directly under the fish on one pass would be at the edge of the next pass.  In either case, you would want the spacing to be a little closer, to avoid gaps.  What spacing you pick depends on time and money available, and the potential cost of missing what you are looking for.

Side scan sonar collects imagery.  A related system, multibeam or swath bathymetry, collects depth information.



Last revised 2/1/2018