Rayleigh波勘探中“之”字形频散曲线“起跳点”频率研究

A study on ‘jump point’frequencies of zigzag dispersion curves in Rayleigh wave exploration

当层状介质中存在低速层的时候,实际提取到的Rayleigh波频散曲线往往会发生"之"字形回折.已有研究表明,"之"字形回折与各模式的激发能量有关.特别的,"之"字形回折的"起跳点"(发生"之"字形回折的点)与介质参数有一定关系,因此在应用中它反映了介质的一些特征.但是,这一关系是怎样的还没有人进行详细研究.本文计算了三种不同震源下"之"字形频散曲线起跳点的位置随介质参数的变化情况,发现:"之"字形频散曲线的起跳点频率与震源类型有关,但是对于同一震源,都有相同的规律:对频散曲线的起跳点频率影响最大的是低速层和低速层以上层的横波速度,其次是层厚;而纵波速度、密度、与震源的距离以及低速层以下各层的参数对频散曲线的起跳点频率影响较小;起跳点频率随低速层横波速度的减小或低速层以上层横波速度的增大而降低,反之升高;起跳点频率随低速层厚度或埋深(低速层以上层的总厚度)的增大而降低,反之升高.本文的结论可以作为地下低速层存在情况的定性分析应用于路基压实度检测等.

Zigzag dispersion curves are sometimes found in Rayleigh wave exploration when the media contain low velocity layers. The zigzag dispersion curves are believed to have a relationship with low velocity layers. Especially, there is a relationship between ＇jump pointf frequencies of zigzag dispersion curves and the parameters of the media. So the ＇jump pointf frequencies can be seen as a characteristic of the media. However, the relationship has not yet been deeply studied. In this paper, a detailed research on it is conducted by computing the variation of the ＇jump pointf frequencies with the media parameters under three different sources. It is found that although the ＇jump pointf frequencies depend on the source type, there are the same rules for various sources, the S-wave velocities of the low velocity layer and the layer above it have the dominant influence on the ＇jump pointf frequencies, and followed by their thicknessf the otherparameters have less effect on the ＇jump point＇ frequencies; the frequencies reduce with the decrease of S-wave velocity of the low velocity layer and the increase of S-wave velocity of the layer above the low velocity layerf they reduce with the increase of the thickness or depth of the low velocity layer. The result can be used as a qualitative analysis of low velocity layer in the cases like compaction test of subgrade.