Un système d'imagerie 3-D

Abstract
Within the EUROMAR 406 project, we have designed an acoustic camera that uses 2 orthogonal linear arrays. Site and azimuth scanning, at transmit and receive respectively, provides tridimensional data of the aquatic medium. The technology and morphology of the arrays are derived from a study on cylindrical antennas. Each array is 256 mm long and is made of 64 elements working at 500 kHz. These antennas are able to scan a 30°x30° sector with a 0.75° resolution. The grating lobes artefacts are rejected under -35dB. A fast dedicated numerical electronic system provides sectorial images in quasi-real time for ranges up to 100 m. The transmit fan beams scan the 30° wide sector according to 64 site directions and are wide open in the associated azimuth planes. Moreover they can be focused according to 32 finite distances. The 64 received signals undergo a continuous complex demodulation. A complex numerical computation (for 64 azimuths and 32 focal zones) over the 128 demodulated signals provides the sectorial image of the illuminated site at a rate of 2µs per pixel. The 3-D sector is acquired after N site shots (N<64) providing as many sectorial images. For example, with N=32, 1 s is needed to collect the domain between 9 m and 12 m range. In association with a microcomputer the system can provide front views. A frontal imaging mode may be obtained in real time by opening the transmit beam in both site and azimuth.