基于偏振特性的下孔法剪切波到时判别问题探讨

基于偏振特征的剪切波到时判别在下孔法波速测试中被广泛应用,然而实际测试中时有发现压缩波初动出现反相的现象,因此针对压缩波不偏振而剪切波偏振的理论基础进行论证很有必要。通过三维有限元分析模型模拟了地表敲击下孔法波速测试,并基于时域逐步积分动力有限元方法求解了地表正反向水平敲击下不同深度观测点振动响应。结果表明:地表水平正反向激励下,各深度观测点中与激励方向平行的水平分量中压缩波也出现反相。为进一步探讨这与实际工程的认知相悖的现象,数值模拟了荷载倾斜、探头旋转、探头倾斜等实际测试可能出现的情形。结果显示:荷载倾斜与探头倾斜的综合影响会导致正反向敲击激励下压缩波的初动相位不反相。进一步,提出了一种剪切波旅时计算新方法以减少每次敲击之间敲击角度、探头偏转和倾斜变化带来的影响,数值计算得到基于新方法求解的剪切波速接近模型介质剪切波速值,其优于利用峰-峰法和互相关法确定旅时而得到的波速结果。

Discussion on determination of shear wave arrival time based on the polarization effect in downhole method

Polarization effect of shear wave is mainly utilized to obtain shear wave arrival time in downhole method of shear wave velocity test. However, the polarity of the P-wave of received signals is observed to be reversed in in-situ tests. Therefore, it is vital to study the theoretical basis that the P-wave keeps the same polarity while the shear wave changes polarity. A three-dimensional finite element numerical model was established in order to simulate the polarization effect of the downhole method excited by surface forward and reversed hammer strike. The vibration responses under surface excitation at different depths were analyzed by time-domain lumped mass dynamic finite element method with an explicit step-by-step integration. It is shown that both S-waves and P-waves are clearly observed to be 180 degrees phase difference from the horizontal signal traces with the direction of excitation generated by reversed impulse, which is contrary to the practical engineering cognition. In order to find out the reason behind this phenomenon, numerical simulations of three possible scenarios (inclined excitation, geophone deflection and geophone inclination) were carried out. The results indicate that the combined influence of inclined excitation and geophone inclination is the main cause that leads P-wave to keep the same polarity while the shear wave changes polarity. Furthermore, a method based on the time interval during load peak and response peak is proposed to obtain reliable shear wave velocity, which is verified to have a shear wave velocity closer to preset velocity of the model in numerical simulation and better than that from the peak to peak method and cross-correlation method.