Abstract
An underwater real-time imaging system based on acoustical holography utilizing an encoded wavefront is constructed.
The imaging system has a coaxial circular array consisting of ultrasound transmitters and receivers. The transmitters are simultaneously driven by Walsh function modulated signals. Transmitted pulses generate an ultrasound encoded wavefront while the echo waveforms returning from distinct points are approximately uncorrelated. Both the transmitted and received beams are synthesized by a numerical decoding and delay-and-sum process.
The system can reconstruct images with a single transmission, but to reduce artifacts in one-shot images, a pulse repetition method has been proposed in the previous paper.15) This method utilizes a zero cross-correlation property of repetitively transmitted signals.
In this paper, the prototype imaging system is described. The characteristics of newly developed transducers are demonstrated. The range of the area where the zero cross-correlation holds for given number of the repetitions is derived theoretically.
The system has 16ch arbitrary waveform generators and 16ch A/D converters. Transmitters and receivers of wide frequency band-width and wide angle of view were developed. A performance test was carried out in a large towing tank. The system's spatial resolution is 5mm longitudinal and 0.3 degrees in azimuth. Images of targets made of 50φsteel pipes were reconstructed in the range of 6m to 110m, and a pipe of 50×50 square cross-section was reconstructed at 170m distance. B-mode cross-sectional images and C-mode focal-plane images are reconstructed and displayed within one second for the targets at a range of 20m; the images being obtained with a single ultrasound pulse transmission. The test also demonstrated that image quality was improved with 4 repetitive transmissions, along the ability reconstruct 3-D images.
