当管子壁厚与外径之比大于0.2时，通常的横波法不能有效地发现内壁缺陷。采用使入射角小于第一临界角的办法，利用折射纵波在管子外壁反射时产生波型转换所得到的横波探测内壁缺陷；也可利用折射纵波扫查外壁缺陷，但探测灵敏度偏低。试验利用两路变型波的叠加来探测外壁缺陷，达到了使内、外壁缺陷的探测灵敏度基本一致的目的。

The usual shear wave can not effectively detect the inner-wall defect if the ratio of the wall thickness to outer diameter of the pipe is more than 0.2. At this time, we can take the principle that the incident angle is less than the first critical angle. That is when the refractive compressive wave reflects on the outer-wall of the pipe, it will produce the S-wave, which can be used to detect the inner-wall. However, the sensitivity of this detecting method is much lower. As a result, in order to keep the sensitivity of detecting the inner and the outer wall, we choose to superpose two anamorphic waves to detect the outer-wall.