SUPPORT > SIDE SCAN IMAGERY > CALCULATING DEPTH BELOW THE SONAR
STARFISH CALCULATING DEPTH BELOW THE SONAR (ALTITUDE)
A useful measure to know when towing a side scan sonar is the height above the seafloor, so a safe distance can be maintained and hazards avoided.
Looking at the sonar image to the right, we can see a large black 'hour-glass' shaped area in the middle of the display, where no echoes have been received.
In fact, this area is actually showing us the height of the sonar above the sea bed.
As the figure to the left shows, the nearest object to the sonar will always be the seabed below it (assuming a relatively flat seabed).
However, it will take some time for the 'bottom echo' to be returned, and as the display shows the received echoes from when the transmission started, this area appears black.
Therefore, the narrower the band of black is, the closer the sonar is to the seabed. As the image is built up over time, if the sonar's altitude changes we can start to see the profile of the seabed that the sonar has passed over. This means in the side scan image above, the seabed gets shallower in the middle of the display, and then gets deeper again at the bottom.