Nobuyuki Ishikawa
Yoshio Fujii
Control & AI Lab., Japan Atomic Energy Res. Inst., Tokai-mura, Naka-gun, Ibaraki, 319-11 Japan
Yoshikuni Shinohara
Nuclear Power Eng. Cooperation, Toranomon, Minato-ku, Tokyo, 105 Japan
Ultrasonic synthetic aperture imaging is a useful technique for detecting cracks in the material and viewing the underwater environment. Since high-frequency ultrasonics are utilized for such purposes, transducers with sufficient transient response have been made available. On the other hand, ordinary transducers for measurements in air are of the resonant type because of comparatively low practical frequency used. However, the utilization of resonant type transducers for synthetic aperture imaging in air makes the measurement resolution poor. This degradation comes from the narrow-band characteristics of transducers, which cause signal distortion in transduction. In this study, to compensate for the narrow-band characteristics of transducers, a deconvolution technique is applied for preprocessing of synthetic aperture imaging and improvement is attained thereby in the case of using the resonant type transducer. The filter used for deconvolution is of the FIR type which is designed in the time domain. The desired filter output has bandpass characteristics with a center frequency of 40 kHz corresponding to the resonant frequency of the transducer and the passband is 6 kHz. Experimental results show that resolution of synthetic aperture imaging is improved by a factor of 3.0 in the range direction and 3.5 in the lateral direction by using deconvolution processing.