各向同性薄层反射理论地震图

Synthetic seismograms of reflection from isotropic thin layer

本文在各向同性介质假设下,计算了薄层顶、底反射P波和PS波的理论地震图.理论模拟发现薄层的反射P波与PS波是一复合波,包括层内的多次透射和反射转换波型,且具有类似单一界面反射的、脉冲式的波形特征.两类薄层反射整体振幅随着薄层厚度的降低而缩小;在地震子波主频40 Hz条件下,2 m左右厚度的薄层反射相比单一界面反射具有等同的振幅水平.1 m以下极薄层仅有弱反射甚至无反射;薄层反射复合波振幅与炮检距的关系依然成立,但不惟一;单界面反射AVO原理、方法不适用于薄层反射解释与反演.

In this paper the synthetic seismograms of P-and PS-wave reflections from the top and bottom of a thin layer are calculated in case of isotropic media. The results show that both P-and PS-wave reflections are composite waves resulted from different types of conversion as transmitted and reflected on two interfaces of the thin layer, and have the pulse wave form similar to reflections from single interface. The amplitudes of two kinds of reflections decrease as layer getting thinner, but are on the same level as that of single interface reflection, even if the thin layer decreases to 2 m thick when the dominant frequency is 40 Hz. The reflections from extremely thin layer below 1 m thickness are weak and even disappear. There is a relationship between composite amplitudes of thin layer reflections and offset, however, which is not unique. The theory and method for single interface reflection, about the amplitude versus offset （AVO） is not suitable to interpret and invert for thin layer parameters.