Simulation on Improving Imaging Resolution of SAFT
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Simulation on Improving Imaging Resolution of SAFT
Li Qiufeng, Jin Xinhong, Zhao Min Key Laboratory of Nondestructive Test of Ministry of Education, Nanchang Hangkong University Nanchang, China qiufenglee@163.com Shi Lihua, Shao Zhixue Engineering Institute of Corps of Engineers PLA Univ. of Science and Technique Nanjing, China shilh@jlonline.com
Abstract—Concrete is one of the main construction materials in civil engineering. Among the existing Nondestructive Testing (NDT) methods, the imaging techniques that can give a graphic display of the inner structure of the concrete elements have received great attention. Because of several kinds of disturbing signals in the concrete structures imaging by low frequency ultrasound, low signal-noise ratio and poor resolution are engendered. Synthetic Aperture Focusing Technique (SAFT) is one of effect methods in the ultrasonic NDT area, and can focus effectively to reflected waves, and improve the reconstructed image’s resolution through rough B-scan method. For low-frequency excitation signal, however, wavepacket distortion is produced during the focusing process, which decreases test resolution. Therefore, Wavepacket Decomposition Technique (WDT), which can put the whole reflected wavepacket into focusing calculation, is introduced here to improve the reconstructed resolution. This technique solves the problem of wavepacket distortion, and better focusing effect and resolution are obtained. Furthermore, calculating velocity is increased because the points joined into SAFT calculation are cut down greatly. The presented technique has been verified by the simulation experiment. From the results, the method can improve more the ability of localizing object and give better imaging effect.
Keywords- ultrasonic test; concrete; SAFT; WDT I. INTRODUCTION Due to the important role reinforced concrete elements play in the structures such as buildings, bridges and other infrastructures, non-destructive testing (NDT) and health monitoring during its whole life-cycle need to be considered [1,2]. Usually, two methods are applied to test of concrete structures. One is Ground Penetrating Radar (GPR). Intensely scatter of electromagnetic wave from the reinforced bar results in badly effect for detection of inner objects of concrete structures. The other is ultrasonic test. Because of ultrasonic diffraction, scattering of ultrasonic wave from the reinforced bar is quietly weak compared to the former method. Furthermore, its sensitivity to the abnormity in the continuous medium is high. It is a promising method [3,4]. But some challenges appear when it comes to real engineering application. Because hardened concrete is a strongly scattering medium with porosity and inhomogenous, several kinds of scattering sources, such as steel bar and coarse or fine aggregates, which are the essential parts of the concrete structures themselves, can cause real problems in the detection [5-7]. To resolve this
problem, Synthetic Aperture Focusing Technique (SAFT) is first introduced here to enhance the signal-to-noise ratio (SNR) and improve the reconstructed image’s resolution through rough B-scan method. This algorithm derives from the imaging technique of Synthetic Aperture Radar (SAR) [8], and the imaging theory of SAFT is explained in next section.
II. THE IMAGING THEORY OF SAFT
The SAFT algorithm focuses the received signals to any point of the reconstructed image by coherent superposition. In this way, a large virtual transducer with variable focus is synthesized, and the schematic diagram of SAFT imaging is showed in Fig. 1. Because of angle of divergence of ultrasonic transducers, each reflective point inside concrete structure can be detected by transducers on different apertures. In Fig. 1, for example, P(xi,yj) is a random reflective point, and the
vertical distance from P to the scanning line is R=yj. P
begins to enter into the detecting area while transducer is placed at position A, and P is closest to transducer while the transducer is moved to position O, and when the transducer is moved to position B, P leaves the area gradually. If the aperture number between A and B is M, the reflection of P occurs in the mth aperture at distance rm:
MmdRrmm...2,1,22=+=
(1)
Where R is fix value, dm is the transverse distance from each aperture to P, and rm is expressed as a curve L in Figure 1.
The time of flight (TOF) tm of reflected echo in each
