Multi-channel Analysis Of Surface Wave And Its Utility

The shear wave velocity of near surface materials and its effects onseismic wave propagation are of fundamental interest in many engineeringand research domains. Such as seismic-resistant design of buildings, seismichazard evaluation of a region, evaluating seismic site response. Surfacewave methods are sorts of shear wave velocity in situ test methods. Based onthe dispersive property of the Rayleigh wave, which means the Rayleighwaves with different frequencies will transmit with different phase velocities,these methods can be applied to obtain the shear wave velocities and otherelastic modules of the sub-materials using an artificial source to generate theRayleigh waves. Multi-channel Analysis of Surface Wave (MASW) method,which is one of the surface wave methods and is still in the front ofinteresting, is growing in many geophysical and geotechnical applicationsdue to its low cost and efficiency. In this paper, the utility details of theMASW method is discussed, and some process techniques and algorithmsetc, which comprised in the MASW method, are improved. The maincontents and results of this research are as follows:(1) The Rayleigh wave dispersion curves for flat layered models arecalculated by means of the transfer matrix method (Dunkin,1965). Thedispersion curves’ characters in the phase-velocity and the half-wavelength domain for the flat layered models are summarized. Meanwhile,the Rayleigh wave fields for some flat layered models and complicatedmodels are simulated using the staggered grid finite difference method.The characters of the Rayleigh wave fields and their dispersion curvesare summarized too.(2) Improvements are made for the three commonly used dispersion curveimaging methods, namely the F-K transform, the τ-p transform and thephase-shift methods. The results show that the three methods afterimproved can get the same resolution in imaging the Rayleigh wavedispersion curves. Besides, the improvements can help the three methodsto enhance the separating capability of different modes.(3) The implementation details of the linearized inversion algorithm ofDamped Least Square (DLS) and the nonlinearized inversion algorithmof the Very Fast Simulated Annealing (VFSA) in surface wave disper-sion curve inversion considering high modes are analysed, and a hybridinversion algorithm of DLS with VFSA is proposed by this paper. Theproposed algorithm can not only overcome the defects of DLS that couldbe captured by local minimum, but also more efficiency than the VFSA algorithm.(4) The CMP Cross-Correlation method (Hayashi and Suzuki,2004),which is used to construct two dimensional shear wave velocity profile,is verified using simulated wave field data. Results show that thismethod could only be used to approximate step flat layered model andcould not be used to approximate dipping layered model. In addition,using this method, only the fundamental mode of the Rayleigh wave’sdispersion curve can be obtained.(5) Finally, the research results of this paper are applied to the in situ testedrecords from the two regions of Chengdu basin and that of Dujiangyanmunicipal area, the equivalent shear wave velocities (VS20) andoverburdens of the two regions are obtained. Using the time averageshear wave velocity (VS30) estimating relation lgVS30=f(lgVSz) proposedby Boore (2011) which was used to estimate the VS30by VSzwhen VS30isunavailable by seismic methods or borehole tests, the relation’scoefficients of the Chengdu basin was determined. The results show thatthe relation of Chengdu basin is similar to that obtained by Boore (2011)using KiK-net borehole data. This relation is reliable for one to evaluatethe time averaged shear wave velocity (VS30) among the Chengdu basinwhen test VS30directly is unavailable, and for one to estimate the siteeffects while establishing ground motion attenuation relationship for theChengdu basin.