Development of an Underwater Frontal Imaging Sonar, Concept of 3-D Imaging System

As it is shown in the Fig. 1, a frontal imaging device obtains its information from the reverberation of an acoustical beam furnished by the submarine bottom at low site angles in the same way than side scan sonars. The acoustical beam is wide open in the vertical plane and focused in the horizontal direction at the intercept of the ground floor. Information is achieved through the modifications of the reverberation or through the shadows created by obstacles laid on the floor. These shadows reveal the relief in an exaggerated way as low angled light does. As for side scan sonars, the distance R of the obstacles is related to the time of flight of the acoustical information and instead of the parallel scanning of the beam obtained by the motion of the side scan sonar vehicle, a rotation of the beam is operated electronically with an array of transducers in the θ-azimutal direction to obtain a sectorial R-θ image of the ground floor. For theoretical and practical reasons, we have chosen to operate the azimutal rotation of the beam with the electronic scanning of a focused aperture along a circular array of transducers. This technique permits to use relatively low delays to focus the beam and reduces the electronic rotation of the beam to a simple commutation of the transducers retained for the focused aperture. The focusing operation depends only on the R variable and may be controlled in a better way.