赝首波特性的模型实验及其与爆破资料的对比

MODEL EXPERIMENTS ON THE CHARACTER-ISTICS OF PSEUDO-HEAD WAVES AND THEIR COMPARISONS WITH SHOT DATA

1. 国家地震局地球物理研究所;2. 北京大学

超声波模型实验能够模拟含高速薄层组模型的特殊震相。本文把含高速双夹层的二维模型作为高速薄层组模型的一个实验例子来加以研究,并结合爆破地震记录的分析,获得赝首波传播的基本特性。沉积层的折射勘探和地壳深部测深可利用赝首波信息反演浅部或深部的薄层构造,本文为解释实际资料中的赝首波震相提供了实验依据,并就其传播机制提出了我们的看法。本文指出,赝首波可由两类性质不同的波叠加形成:1.在通过高速薄层(组)时作波型转换的透射行波(也称被屏蔽波);2.由上覆高速薄层产生的下行首波。在特定条件下,如上覆高速薄层(组)的层厚度远小于首波波长值以及赝首波形成层在层厚和介质弹性等方面占有优势,赝首波可以在振幅上大于超前的波。赝首波的透射能力和波型转换次数受震源的强度、震源在非近场下的波谱特性、介质的吸收特性、转换薄层的层数、各界面的速度反差,以及赝首波形成层的厚度、弹性和连续性等诸多因素的制约。

The specific phase of the models containing a set of thin high-velocity layers can be modelled by ultrasonic experiments. In this paper, the two-dimensional model containing two thin high-velocity layers is used as an experiment example of the model containing a set of thin high-velocity layers'and the basic characteristics of propagation of pseudo-head waves have been obtained by combining with the analyse of explosive seismic records.For the refraction prospecting of the sedimentary layer and the sounding of the deep crust, the thin layer structure of the shallow crust or the deep crust can be inversed by using the signals of the pseudo-head wave. In the paper, the experimental evidence has been provided with interpretation of real data and the view on the propagating mechanisms has been shown.The paper points out that pseudo-head waves can be formed by superposition of the waves with two kinds of different features:1, the penetrating travelling waves (also called screened waves) with the wave types transformed when penetrating the thin high velocity layers;2, the downgo-ing waves yielded by the nearest upper covered thin high-velocity layer.Under a certain condition, the thickness of the upper covered thin high velocity layers is far less than the wave length value of the head wave, and the formed pseudo-head wave layer is prevalent in depth and medium elasticity and the pseudo-head wave can be larger than the head wave and/or the other pseudo-head waves preceding it in amplitude.The penetrating capacity of the pseudo-head wave and the times of types of wave transformation are restricted by such factors as the source intensity, the spectral characteristics of the source, the absorption characteristics of the medium and the number of transformed layers,the velocity contrast on all the boundaries and the thickness, elasticity and continuity of the formed pseudo-head wave layer.