At engineering practice, the theoretical basis for the cross-over method, used to obtain shear wave arrival time in the downhole method of the wave velocity test by surface forward and backward strike, is that the polarity of P-wave keeps the same, while the polarity of S-wave transforms when the direction of strike inverted. However, the characteristics of signals recorded in tests are often found to conflict with this theoretical basis for the cross-over method, namely, the polarity of the P-wave also transforms under the action of surface forward and backward strike. Therefore, 3D finite element numerical simulations were conducted to study the validity of the theoretical basis for the cross-over method. The results show that both shear and compression waves are observed to be in 180° phase difference between horizontal signal traces, consistent with the direction of excitation generated by reversed impulse. Furthermore, numerical simulation results prove to be reliable by the analytic solution; it shows that the theoretical basis for the cross-over method applied to the downhole wave velocity test is improper. In meanwhile, numerical simulations reveal the factors (inclining excitation, geophone deflection, inclination, and background noise) that may cause the polarity of the P-wave not to reverse under surface forward and backward strike. Then, as to reduce the influence factors, we propose a method for the downhole wave velocity test under surface strike, the time difference of arrival is based between source peak and response peak, and numerical simulation results show that the S-wave velocity by this method is close to the theoretical S-wave velocity of soil.

中文翻译：

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交叉法应用于井下波速测试的理论基础探讨

在工程实践中，地表正反走波速测试井下法中用于获得横波到达时间的交叉法的理论依据是纵波的极性保持不变，而当冲击方向反转时，S波的极性发生变化。然而，测试中记录的信号特征往往与交叉法的这一理论基础相冲突，即纵波的极性在地表向前和向后撞击的作用下也会发生变化。因此，进行了 3D 有限元数值模拟以研究交叉方法的理论基础的有效性。结果表明，观察到横波和压缩波在水平信号轨迹之间的相位差为 180°，与反向脉冲产生的激励方向一致。此外，数值模拟结果通过解析解证明是可靠的；说明交叉法应用于井下波速测试的理论依据是不恰当的。同时，数值模拟揭示了可能导致P波在地表前向和后向撞击下极性不反转的因素（倾斜激励、检波器偏转、倾角和背景噪声）。然后，为了减少影响因素，我们提出了一种地表冲击下的井下波速测试方法，到达时间差是基于源峰和响应峰之间的时间差，数值模拟结果表明，由此产生的横波速度方法接近于土壤的理论 S 波速度。