Digital Synthetic Aperture Acoustic Imaging for NDE
Real time synthetic aperture or synthetic focus techniques for acoustic imaging have been investigated and a prototype digital imaging system has been developed. It operates by exciting, with an impulse, one element from a transducer array, digitizing the return echoes, and storing them in a Random Access Memory. When this process has been repeated for all the array elements, the focus information is loaded from a mini computer. The system then generates a series of swept-focus lines, which are arranged perpendicular to the array face. Our processor handles typical input data at rates sufficient to generate real time images. As only one transducer at a time is excited it has been necessary to develop a high efficiency broadband transducer array with quarter wavelength matching layers. The array we have developed has an 11 dB return loss, a 2.7-4.3 MHz frequency range with a pulse response approximately 5 half cycles long. The digital processor operates at a 10- 16 MHz sample rate with 8 bit quantization. Theoretical and experimental images will be presented for a system with a 96 line display employing 8 and 32 active transducer elements, which has a resolution of < 1 mm. We will also discuss methods of reducing the sidelobe responses in these systems. We have carried out experiments and theory, and we can considerably reduce the sidelobe level with input gain compression from the current experimental value of -12 dB to beyond -20 dB in our prototype 8 transducer system. In addition, we are investigating inverse filtering techniques for shortening the effective pulse length to 1 rf cycle to further improve the image quality and range resolution.