| Obtaining S-wave velocity profile is the key point of surface wave methods and the purpose for field surveys. In near-surface investigations, suspension logging has been adopted for years to estimate shear (S)-wave velocity structures in demand in engineering and environmental problems. However, it is costly and not convenient because it requires boreholes information and it is not friendly to the environment and ecosystem. A new method called multi-channel analysis of surface/Love wave (MASW/MALW) was developed. This new method has been widely used by the near-surface geophysical community to solve the near-surface geological and geophysical problems because it is non-invasive, low cost, and relatively highly accurate. Surface waves (Rayleigh wave and Love) are guided and dispersive in a multi-layer earth model. For a homogeneous half-space earth model, the Rayleigh wave is not dispersive and travels with a specific phase velocity (for example, a velocity of approximately 0.92 times with S-wave velocity, when the Poisson’s ratio is 0.25). However, in the case of a multi-layer earth model, Rayleigh waves are dispersive. For a given mode, the longer wavelengths will penetrate deeper and are more sensitive to deeper layers; for shorter wavelengths are more sensitive to the surface layers. S-wave velocity can be derived by inverting dispersion phase velocities of the surface waves (Rayleigh and Love wave). Surface waves have multi modes in a multi-layer earth model, according to the values of their phase velocities, they are named as the fundamental mode, first higher mode, and second higher mode, etc. S-wave velocity can be obtained by inverting signal surface-wave dispersion mode phase velocities or jointly inverting multi-mode phase velocity. Picking accurate surface wave phase velocities is important to obtain reliable inverted results.Mode misidentification happens when we identify the surface-wave modes. The dissertation studies the characters of surface-wave mode, and finally provides the following conclusions:(1) Non-geometric wave exists uniquely in near-surface materials, especially in unconsolidated sediments. Numerical tests have proved that non-geometric wave is leaky wave. Leaky surface wave could cause misidentification when treating the leaky-wave energy as the fundamental or higher modes of Rayleigh waves. Such misidentification will result in wrong inversion results. (2) "Mode-kissing" phenomenon will also cause mode misidentification. "Mode-kissing" phenomenon is common in Rayleigh-wave surveys and affects the picking of the fundamental mode, and results in higher S-wave velocities compare to the true model. (3) In field surveys, quality of the dispersion images will be affected by clipped waves, which caused by the uncorrected process or unreasonable offset. The numerical tests or (and) real data have suggested that muting and filtering should been applied to raw seismic records prior to generating dispersive images to prevent mode misidentification and reasonable offset and dynamic range should be chosen to make sure of the quality of surface-wave data.MASW method can be used to delineate shear (S)-wave velocity structures (tunnel, fault, collapse, etc.) by a standard CMP (common midpoint) roll-along acquisition system. However, it is time consuming. I propose a new method named virtual source multi-channel analysis of surface waves (VSMASW) aimed to decrease the number of shots in the field work by generating a series of pseudo shot gathers with virtual sources located at receiver locations. This method is based on extracting empirical Rayleigh-wave Green’s function. I propose that mode separate and mode reconstruction are needed in the processing of extracting Rayleigh-wave Green’s function. Because spurious events will arise due to the multi-modes of Rayleigh wave. Analyses of numerical modeling and real world test indicate that the proposed method possesses high potential in increasing the efficiency of field work, compared with the conventional MASW method. |
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