摘要
选取3类不同宏观特征的武当群片岩岩样,分别制取不同片理角的标准圆柱试样。采用波速测试获取烘干、浸水处理后的试样的纵波速度,分析不同含水状态下武当群片岩波速各向异性特征,并结合偏光显微镜、扫描电镜下3类岩样的矿物组成与微观结构特征,探讨内外影响因素下的波速各向异性机制。结果显示:武当群片岩主要矿物为硬质粒状石英、长石与软质片状白云母,硬质矿物含量越高、孔隙率越小,则片岩纵波速度越大,反之越小;干燥样的纵波速度表现出显著各向异性,传播方向由垂直片理面向平行片理面变换时,波速逐渐增大,不同方向的波速值可用波速圆公式进行预测;波速各向异性本质上是微裂隙的定向展布、间隔分布的结果,可间接用白云母定向系数δ_2评价各向异性程度k,k~(1/2)与δ_2呈线性正相关,武当群片岩的k值上限为2.90;浸水后试样的纵波速度普遍增大,这是水充填空隙后对片岩等效体积模量的增加起主控作用的结果,片岩吸水后的波速增长效应受其孔隙率与微裂隙分布特征影响,引起不同类片岩间的纵波速度差异,导致同类片岩的波速各向异性程度与浸水时长呈负相关。
Three types of Wudang group schist with different macroscopic characteristics were selected to make the standard cylindrical test specimens possessing different schistosity angles. Firstly, P-wave velocity of the specimens were tested after dried and soaked to investigate the velocity anisotropy of Wudang group schist in different water bearing states. Then, combined with mineral composition and microstructure characteristics of three kinds of schist obtained by polarizing microscope and scanning electron microscopy, he wave velocity anisotropy affected by internal and external factors were further investigated. The results indicate that the major minerals of Wudang group schist are hard granular quartz, feldspar and soft muscovite. The higher content of hard mineral or the smaller porosity of schist, the larger P-wave velocity and vice versa. The P-wave velocity of dry samples shows significant anisotropy. As the angle between the direction of wave propagation and foliation decreases, the wave velocity increases gradually.Wave velocity in different directions can be predicted by the formula expressed as wave velocity circle. The wave velocity anisotropy is essentially caused by the directional and the interval distribution of micro fissures, of which the degree k can be evaluated indirectly through the orientation coefficient δ_2 of muscovite. There is a linear positive correlation between k~(1/2) and δ_2, and the upper limit of the p-wave anisotropy of the Wudang group schist is 2.90. Due to the increase of the equivalent volume modulus of schist after the pores filled by water, the P-wave velocity of the soaking sample is generally increased. And the increasing effect of velocity depends greatly on the porosity and distribution of micro fissures, leading to the difference of P-wave velocity among different kinds of schists and a inverse correlation between the anisotropy degree of wave velocity for the same kind of schist and the soaking time.
引文
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