As seen on the left in this image, some of the reflections, or echoes, are returned in the direction of the sonar. The sonar interprets two important characteristics of these echoes, the amplitude and the time it is received, relative to the initial array pulse. Using this information, the sonar generates an image by plotting the radial location based on the speed of sound and the round trip time, and the angular location based on which beam receives the reflection. The amplitude is converted into a color and plotted at that point. Generally, the larger the amplitude the brighter the color. What results is a top down view of the scene as seen on the right.

Some things to note about the sonar image on the right. First, the edges facing the sonar usually return the loudest echoes and are rendered with the brightest colors. The second is that areas behind the object are in acoustic "shadows" and the sonar receives no information about these areas. These acoustic shadows are displayed as dark areas on the distant side of each object. If there was treasure chest behind the cube, we would not be able to tell from this direction.

As noted earlier, the sonar is higher than the cube is tall, so the shadows are not that long and we can see more details beyond them. Eventually however, the pulse attenuates to a point such that the sonar can no longer receive echo data, limiting the sonar to a fixed maximum range.

While the bright spots tell us an object is present and where it is, the shadows often give us as much or more information about the details. The pipe is a good example of this. If you look at the bright spot representing the pipe on the sonar image, you can see it is a straight line which conveys no information that it is bent or sticking up. This is because the plane of the bend is perpendicular to the location of the sonar and from that angle, every part of the pipe is roughly the same distance from the sonar, and the result is a straight line. But, if you look at the shadow of the pipe, you can clearly see it is bent and that the pipe is angled up from the bottom. The lesson here is that sonar image interpretation relies on understanding both the light and dark regions to make the most sense of the image.

This has been a very brief introduction, but we will return to a few more image interpretation exercises at the end of this lesson.