Based on the signals, the travel time of the propagation of body waves in soil layers can be calculated, which will be finally used for the profile of the site. In the downhole method, after the surface excitation, body waves propagate in the soil layers and are recorded by receivers inside the borehole. Among invasive methods, the downhole method under surface forward and backward strike is widely used for its several advantages such as fewer boreholes than that of crosshole method and convenient operation. Generally, it is deemed that, by simple arithmetic, invasive methods can obtain more reliable in a thick overlayer site compared to the noninvasive methods which calculate using an inversion process and are of multiple solutions. Many in situ seismic tests such as noninvasive methods (single or multichannel spectral analysis of surface waves) and invasive methods (crosshole or downhole method and PS logging) are now generally used for obtaining of soil layers under a small strain level. S-wave velocity of site soil is a key dynamic parameter in seismic engineering, mainly reflecting site dynamic property determination, site seismic response analysis, and site classification. 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. 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. 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. 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. Therefore, 3D finite element numerical simulations were conducted to study the validity of the theoretical basis for the cross-over method. 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. 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.
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