关键词: 下孔法, 数值模拟, 偏振效应, S波, P波

Abstract: 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.

Key words: downhole method, numerical simulation, polarization effect, S-wave, P-wave