Two sorts of echoes caused by the reradiation of elastic surface wave have been observed in experimentation: the radialsurface backscattering wave and the longitudinal surface backscattering wave.
实验观测到两类由弹性表面波的再辐射产生的回波：径向表面反向散射波和纵向表面反向散射波。
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Radial imperfection. A radial imperfection may be defined as any imperfection whose vertical axis is perpendicular to the material surface.
径向缺欠。径向缺欠可定义为其纵轴垂直于材料表面的任何缺欠。
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Non-radial Imperfection. A non-radial imperfection may be defined as any imperfection whose vertical axis is not perpendicular (it's at an angle) to the material surface.