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Interpreting the Side-Scan Sonar Image

View of the Side-Scan Sonar Display
Tom Rodgers

Many people have written in with questions about the side-scan sonar image, so I’ve put together a quick description of the sonar screen and how to interpret it.

When you look at the screen, you’ll see it is divided into four main panels with smaller strips and windows displaying numerical or graphic information.  The four large panels, with the slowly-moving, orange images, are called “the waterfall.”  This name comes from the fact that each new line of sonar return data is added at the top of a panel, and the older information is bumped down towards the bottom of the display.  The two panels on the left correspond to information collected from the left (or port) side of the towfish, and the panels on the right are from the starboard side.  (More on why there are two panels on each side in a bit!)

If Diana is being towed forward through the sea, the scrolling image of the waterfall corresponds roughly to the moving image of the sea floor.  (However, even if the ship were stationary, the image would still continue to scroll as new data was added at the top.)

The reason there are two panels on each side is that Diana scans at two different frequencies, 600 kilohertz and 300 kilohertz.  The top pair of panels is showing data from the high-frequency sonar, which produces a higher-resolution image, but has less range.  The bottom two panels are from the low-frequency sonar, which can see farther but less well.  Each panel appears the same size on the screen, so the data from the low-res scanner is displayed at a smaller scale.  If you wait until you can see an interesting surface feature in the top panel, you’ll notice that you can see the same feature displayed smaller, and at a lower resolution, in the low-freq panel beneath it.

If you look at the lines above each of the panels, they indicate the distance, in meters, between the center line of Diana and the echo return.  You’ll notice that the high-frequency data, displayed in the top panels, only reaches to a distance of about 100 m to either side of the center line, while the low-frequency signal reaches to a distance of 200 m.

Above each waterfall panel you’ll see the most recent sonar return data plotted as a simple line graph.  The peaks on the graph are plotted more brightly on the waterfall.  These correspond to features on the sea floor which reflect sound most strongly.  That may either be because they are harder, as a rock or an amphora might be, or they could be regions of the sea floor that are angled towards the sonar emitter, as a hill, a cliff, or other geological feature could be.  If the target extends above the sea floor, it will block the sonar “ping”, preventing it from reaching the sea floor beyond it.  This leads to black “shadows” which appear on the far side of the target.

The black swath which appears in the center of both scans is the area directly beneath Diana, where the side-scan sonar signal doesn’t reach.  (Imagine yourself as a passenger in an airplane – you can see the ground out the right and left windows, but you can’t see the ground directly beneath the airplane itself.)

The narrow, computer-generated line which follows the edge of the black swath is the closest place on the sea floor where the sonar software believes the return is strong enough to indicate the location of the bottom.  This is important because this is the point from which Diana’s altitude is calculated.

Which brings us to the last two pieces of information on the screen: altitude and depth.  The “Altitude” window displays Diana’s vertical height above the sea floor, and the “Depth” window shows the vertical distance from Diana to the surface of the water above.  The sum of these two numbers is the depth of the sea at Diana’s location.

I hope that clears everything up – once you know what you’re seeing, it can be a lot of fun to watch the side-scan sonar data scroll past!