Forward-look 2-D sonar image formation and 3-D reconstruction

Sonar imagery deals with transmitting acoustic signals and measuring the reflected sound from the scene surfaces. The recorded signal encodes information about the shape and material properties of these surfaces. The relationship of the scene geometry to the image irradiance can be established by modeling the physics of the sonar image formation process. The inherent ambiguities in the interpretation of the 3-D world based on visual cues in a 2-D forward-scan sonar image arise as a result of both operating as a ranging device and loss of elevation angle information due to the projection geometry. One aspect of this work is modeling the image formed by a new class of high-resolution 2-D forward-looking sonar systems which supports our complementary aim of recovering the unknown zenith angles from image brightness and thus reconstruct 3-D objects. Our method applies to a single forward-scan sonar image, assuming that the scene objects have smooth surfaces that vary monotonically in terms of distance from the sonar, and cast visible shadows on a flat background. We present the results of experiments with real data to demonstrate the performance of our 3-D reconstruction technique.