Calculation of Rayleigh-wave phase velocities due to models with a high-velocity surface layer |
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| Institution: | 1. Subsurface Imaging and Sensing Laboratory, Institute of Geophysics and Geomatics, The China University of Geosciences, 388 Rumo Rd., Wuhan, Hubei 430074, China;2. Subsurface Multi-scale Imaging Laboratory, Institute of Geophysics and Geomatics, The China University of Geosciences, 388 Rumo Rd., Wuhan, Hubei 430074, China;3. Research and Development Center, BGP International Inc, 10630 Haddington Dr., Houston, TX 77043, USA;1. Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, Prague, Czech Republic;2. Faculdade de Ciências da Universidade do Porto (DGAOT), Porto, Portugal;3. Geology and Geophysics Department, Faculty of Sciences, King Saud University, Riyadh, Saudi Arabia;4. Seismology Dept., National Research Institute of Astronomy and Geophysics, Cairo, Egypt;1. Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan;2. Department of Earth and Environmental Sciences, National Chung Cheng University, Chiayi, Taiwan;3. Environment and Disaster Monitoring Center, National Chung Cheng University, Chiayi, Taiwan;4. Department of Geomatics, National Cheng Kung University, Tainan, Taiwan;5. Earthquake-Disaster & Risk Evaluation and Management (E-DREaM) Center, National Central University, Taoyuan, Taiwan;6. Disaster Prevention Technology Research Center, Sinotech Engineering Consultants, Inc., Taipei, Taiwan;7. Department of Earth Sciences, National Central University, Taoyuan, Taiwan;8. Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan;1. Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, Hubei 430074, P.R. China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, P.R. China;1. Institute of Geophysics and Geomatics, China University of Geosciences (Wuhan), 388 Rumo Rd., Wuhan, Hubei 430074, China;2. Subsurface Imaging and Sensing Lab, China University of Geosciences (Wuhan), 388 Rumo Rd., Wuhan, Hubei 430074, China;3. Hubei Subsurface Multi-scale Imaging Lab, China University of Geosciences (Wuhan), 388 Rumo Rd., Wuhan, Hubei 430074, China |
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| Abstract: | Rayleigh-wave phase velocities have been utilized to determine shear (S)-wave velocities in near-surface geophysics since early 1980s. One of the key steps is to calculate theoretical dispersion curves of an earth model. When the S-wave velocity of the surface layer is higher than some of the layers below, however, the Rayleigh-wave phase velocity in a high-frequency range calculated by existing algorithms approaches the lowest S-wave velocity among the layers above the half-space, rather than a value related to the S-wave velocity of the surface layer. According to our numerical modeling results based on wave equation, trends of the Rayleigh-wave dispersive energy approach about a 91% of the S-wave velocity of the surface layer at a high-frequency range when its wavelength is much shorter than the thickness of the surface layer, which cannot be fitted by a dispersion curve calculated by existing algorithms. We propose a method to calculate Rayleigh-wave phase velocities of models with a high-velocity surface layer by considering its penetration depth. We build a substituted model that only contains the layer with the lowest S-wave velocity among the layers above the half-space and the layers above it. We use the substituted model to replace the original model to calculate phase velocities when the Rayleigh-wave wavelength is not long enough to penetrate the lowest S-wave velocity layer. Several synthetic models are used to verify fitness between the dispersion curve calculated by our proposed method and the trend of the highest dispersive energy. Examples of inversion also demonstrate high accuracy of using our method as the forward calculation method during the inversions. |
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