"How do we find objects on the bottom of the ocean?" (continued)
When the sound sent by a sonar reaches the seafloor several things can happen to that sound. Some of the sound may be absorbed by the seafloor. Some of the sound is almost always reflected. There are several different ways the sound can reflect. The sound can be reflected directly back or it can be scattered in many different directions (see the diagram below). Sound that is scattered back toward the tow vehicle is called backscatter.
How much scattering, backscattering and absorption happens depends on the properties of the seafloor. Hard materials, like rocks, will scatter more sound while soft materials, like mud, will absorb more sound. Different amounts of scattering produces different amounts of sound returning to the tow vehicle and a different image of the seafloor.
The images below show some examples of how one property of the sea floor (grain size) affects the side scan sonar image.
 |
| Images by David C. Twichell (USGS), from: Twichell, D.C., Cross, V., and Parolski, K.F., 2000, U.S. Geological Survey Open-File Report 00-167, 26 pp. |
The image above shows a side scan sonar image on the left. On the right are two samples taken from areas in the side scan sonar image. The fine sand is darker in the image because more energy is reflected back from the uniform grain size. The gravel is lighter in th image because the gravel scatters more of the sound and less reflects back to the side scan instrument.
Side scan sonar is often used to find objects like shipwrecks on the seafloor. The image below is of the Empire Knight, a British cargo vessel, that sank off Boon Island, Me in 1944. The ship is located in the bottom part of the image. The shadow of the ship can be seen in the upper part of the image.
 |
| The shipwreck of the Empire Knight.
Photo courtesy of Klein Associates, http://www.kleinsonar.com. |
Features on the seafloor create shadows in the side scan sonar images because of the geometry of the sound source and the object. The drawing below illustrates how a shadow forms.
The side scan sonar sends out the sound. The sound will reflect off the object back to the instrument. But the object also "shades" part of the seafloor from the sound. No sound will reach the seafloor behind the object. This will produce a shadow on the side scan sonar image. The shadow will be white. The shadow is black in the image of the ship above because the color has been reversed to highlight the ship.
|
