单轴压缩下干燥和饱水岩石中超声P波的衰减

根据脉冲传播和频谱比方法的基本原理建立了一套超声波衰减和波速测量系统。研究了单轴压缩下超声P波在干燥和饱水的小浪底砂岩和房山大理岩中的衰减和速度。常压下,水饱和样品的Q_P值低于干燥样品。初始加载时,在大多数情况下,Q_p值明显增加即衰减减小;当应力水平达破裂强度50-60％后,岩石开始膨胀,Q_p减小即衰减增加;临近破裂Q_p减至最小。Q_p的变化量达一个数量级。单轴压缩下,在干燥和饱水的砂岩、大理岩中,V_p分别增加为27％和28％,44％和11％。径向Q_p和V_p随单轴压力的变化比轴向小得多,呈现明显各向异性。在本实验条件下,波在裂纹表面的摩擦耗散是主要的衰减机制,水的作用主要是潮湿和润滑裂纹表面,促进滑动,增加衰减。

ULTRASONIC P WAVE ATTENUATION IN DRY AND SATURATED ROCKS UNDER UNIAXIAL COMPRESSION

An ultrasonic attenuation measurement system utilizing processing technique by spectral ratios was set up. The QP values as well as the velocities Vp of P wave in dry and saturated specimen of Xiaolangdi sandstone and Fangshan marble are studied as functions of axial stresses. It is found that the Qp value of water-saturated samples in the atmosphere is lower than that of dry ones. When the axial stress initially increases, in most cases the Qpsubstantially increases （attenuation decreases） and Vp increases about 27% （for dry sandstone）, 28% （for saturated sandstone）, 44%（for dry marble）, 11% （for saturated marble）. When the stress level reaches about 50-60% ultimate strength the di-latancy in rocks occurs and the Qp decreases （attenuation increases） about one order of magnitude. The changes of Qp or Vp in the axial direction are much more than that is the transverse direction （anisotropy）. Acoustic emission measurement shows that the changes of Qp and Vp are closely related to the thin crack developed. Under conditions of the experiment the fricition on thin cracks and along grain boundaries may be the dominant attenuation machanism. The water in the eracks and on the grain boundaries may. lubricate them and facilitate the friction slip.