A 3D IMAGING-SYSTEM

Within the EUROMAR 406 project, we have designed an acoustic camera th at uses 2 orthogonal linear arrays. Site and azimuth scanning, at tran smit and receive respectively, provides tridimensional data of the aqu atic medium. The technology and morphology of the arrays are derived f rom 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 sca n a 30-degreesx30-degrees sector with a 0.75-degrees resolution. The g rating lobes artefacts are rejected under -35dB. A fast dedicated nume rical electronic system provides sectorial images in quasi-real time f or ranges up to 100 m. The transmit fan beams scan the 30-degrees wide sector according to 64 site directions and are wide open in the assoc iated azimuth planes. Moreover they can be focused according to 32 fin ite distances.The 64 received signals undergo a continuous complex dem odulation. A complex numerical computation (for 64 azimuths and 32 foc al zones) over the 128 demodulated signals provides the sectorial imag e of the illuminated site at a rate of 2mus per pixel. The 3-D sector is acquired after N site shots (N<64) providing as many sectorial imag es. For example, with N=32, 1 s is needed to collect the domain betwee n 9 m and 12 m range. In association with a microcomputer the system c an provide front views. A frontal imaging mode may be obtained in real time by opening the transmit beam in both site and azimuth.