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利用超声实验技术,对含高速薄层的两类不同形状的二维模型所产生的广义首波进行了研究。发射和接收装置采用SYC-2或SYC-3型超声仪,模型材料采用泡沫塑料、塑料、有机玻璃和铝,模型板厚度均为6mm,长度大于1200mm。实验表明,在此条件下,不仅能够观测到临界角射线形成的初至首波,也能观测到非临界角射线形成的初至首波,这种初至首波称为广义首波,它的传播不遵守斯奈尔(Snell)定律。与此相应,在高速薄层中还存在一组反向对称的下行首波。广义首波的最大振幅点不出现在临界点上,而是偏离临界点外侧约等于临界距离一半的位置上。根据观测振幅曲线,可以求出拟合此观测值的解析函数。非临界条件下广义首波的存在,表明首波在其入射、折射和侧向传播的路径上,将以具有一定空间的能流形式出现。在广义首波振幅最大点附近,能流密度最大,随着偏离此点的距离加大,能流密度迅速减小。下行首波的运动学和动力学特点与广义首波十分相似。但是,下行首波的存在给波场的研究带来新的复杂性。上述实验结果对研究由爆炸激发的地震记录震相的动力学特征是有意义的。
Using the ultrasonic experiment technique, the generalized first wave generated by two kinds of two-dimensional models with high-speed thin layers is studied. The SYC-2 or SYC-3 sounders are used for the transmitting and receiving devices. The model materials are made of foam, plastic, plexiglass and aluminum. The thickness of the model plate is 6mm and the length is more than 1200mm. Experiments show that under this condition, not only the first arrival wave of the critical angle ray can be observed, but also the first arrival wave of the non-critical angle ray can be observed. The first arrival wave is called the generalized first wave. The transmission does not follow Snell’s law. Correspondingly, there is also a set of reverse symmetrical downlink first wave in the high-speed thin layer. The maximum amplitude point of the generalized first wave does not appear at the critical point, but deviates from the critical point by about half the critical distance. Based on the observed amplitude curve, an analytic function fitting this observation can be found. The existence of generalized first wave under noncritical conditions shows that the first wave appears in the form of energy flow with a certain space on its path of incidence, refraction and lateral propagation. In the vicinity of the maximum point of the generalized initial wave amplitude, the maximum energy flux density decreases with the distance from this point increasing. The kinematic and kinetic characteristics of the downlink first wave are very similar to the generalized first wave. However, the existence of the downlink first wave brings new complexity to the study of the wave field. The above experimental results are meaningful for studying the dynamical characteristics of the seismograms triggered by explosions.