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How does side-scan sonar work?

We are using the DSL-120 kHz side-scan sonar instrument to map the surface of the Puna Ridge.   The DSL-120 vehicle is towed 100 m (300 ft) above the seafloor.  Sound is transmitted into the water by the side-scan sonar instrument at an angle rather than just straight down as shown. 

DSL-120 vehicle

The sound transmitted by the DSL-120 kHz instrument travels to the seafloor, bounces off of the seafloor, returns to the instrument, and is recorded.

The intensity or strength of the returning acoustic signal is controlled primarily by the slope of the seafloor and what the seafloor is made of.  A stronger return is received if the seafloor slopes toward the instrument.  Also, the return is stronger if the seafloor is made of bare rock. The strength of the return is much lower if the seafloor is covered by mud or sand.

Volcanoes and other features that stick up above the surrounding seafloor will cast acoustic shadows.  These shadows are just like the shadow behind a person when you shine a flashlight on them.

Ship towing DSL-120 kHz vehicle

How is echo strength turned into an image?

The strength of the sound recorded by the side-scan sonar instrument is converted to shades of gray.  A very strong return, say from bare rock, is white; a very weak return is black.   The echo strengths that fall between these two extremes are converted to different shades of gray.

The two figures below show a seafloor volcano that has a large crater on its top.  To the left is the bathymetry.  The contours are lines of equal water depth;  the color also represents water depth with reds being the shallowest and dark greens the deepest.  The volcano stands about 150 meters (450 feet) above the surrounding seafloor.  To the right is the side-scan sonar image of the same volcano illuminated from the right.  Notice that the volcano casts a shadow to the left, and the slope facing to the right is very bright.  Smaller bumps also cast small shadows making the topography look lumpy.  Each image is 3 km (1.8 miles) wide.

Contour map of volcanoSide-scan sonar image of volcano

Test Your Knowledge

Will a recent lava flow that covers the side of a volcano facing the DSL-120 kHz instrument look bright, medium gray, or dark on the side-scan sonar image?

Check your answer below to see if you are right.

bright
medium gray
dark