Goueygou M, Piwakowski B, Fnine A, Kaczmarek M, Buyle-Bodin F
Electronics and Acoustics Group, IEMN DOAE UMR CNRS 8520, Ecole Centrale de Lille, BP 48, 59651 Villeneuve d'Ascq Cedex, France.
Ultrasonics. 2004 Apr;42(1-9):889-95. doi: 10.1016/j.ultras.2004.01.075.
The Spectral Analysis of Surface Waves (SASW) is a popular technique in seismics for imaging the ground subsurface. It uses the dispersive properties of Rayleigh waves in a transversely homogeneous, multilayered medium. The SASW approach is being transposed into the civil engineering domain to characterize subsurface damage in concrete structures. Such a damage consists in a few millimeters thick surface layer with porosity slightly higher than in the sound material. It is induced by contact with moisture or chemicals at the surface of the structure and may facilitate penetration of aggressive agents. In this study, two-layered mortar samples are made to mimic concrete cover damage in real structures. The dispersive behavior of Rayleigh waves arises when the wavelength is comparable to the thickness of the first layer. Given the small thickness of this layer, it requires increasing the frequency up to several hundreds of kHz, which means high attenuation and low signal-to-noise ratio. Rayleigh waves with 0.5 MHz central frequency are generated into the samples by the wedge method. Phase velocity dispersion curves are obtained by broadband phase spectroscopy from signals received at various distances from the source. The signal processing is first validated on simulated signals with known dispersion law. Then, the measured dispersion curves are compared with the theoretical curve for a two-layered medium, following Haskell's approach. The measured curve displays the general behavior expected from theory. However, a three-layered, visco-elastic model would be necessary to get a better fit and to estimate more accurately the parameters of each layer.
表面波谱分析（SASW）是地震学中一种用于地下成像的常用技术。它利用瑞利波在横向均匀多层介质中的频散特性。SASW方法正被应用于土木工程领域，以表征混凝土结构中的地下损伤。这种损伤表现为几毫米厚的表层，其孔隙率略高于完好材料。它是由结构表面与水分或化学物质接触引起的，可能会促进侵蚀性介质的渗透。在本研究中，制作了两层砂浆样本以模拟实际结构中的混凝土保护层损伤。当波长与第一层厚度可比时，瑞利波会出现频散现象。鉴于该层厚度较小，这就需要将频率提高到数百千赫兹，这意味着高衰减和低信噪比。通过楔形法在样本中产生中心频率为0.5兆赫兹的瑞利波。通过宽带相位谱从距源不同距离处接收到的信号中获得相速度频散曲线。首先在具有已知频散规律的模拟信号上验证信号处理方法。然后，按照哈斯克尔方法，将测量得到的频散曲线与两层介质的理论曲线进行比较。测量曲线显示出理论预期的一般行为。然而，需要一个三层粘弹性模型才能得到更好的拟合，并更准确地估计各层参数。
