HR: 09:45h
AN: NS11A-06    [Abstracts]
TI: Current Development of Estimation of Near-Surface Elastic Moduli by Analysis of Rayleigh Waves
AU: * Xia, J
EM: jxia@kgs.ku.edu
AF: Kansas Geological Survey, The University of Kansas, 1930 Constant Ave., Lawrence, KS 66047 United States
AU: Xu, Y
EM: xyxian@cug.edu.cn
AF: China University of Geosciences, Lu Mo Rd. #31, Wuhan, Hub 430074 China
AU: Miller, R D
EM: rmiller@kgs.ku.edu
AF: Kansas Geological Survey, The University of Kansas, 1930 Constant Ave., Lawrence, KS 66047 United States
AB: The shear (S)-wave velocities of near-surface materials (such as soils) are of fundamental interest in many environmental and engineering studies. There are three basic ways to perform shear tests in-situ in soil mechanics: in-situ shear box, shear vane, and penetration. During these tests, either a soil sample must be carefully cut, remolded, or invasive penetrations needed to be performed. Analysis of surface waves offers a non-invasive and cost-effective alternative to rapidly evaluate shear strength. Multichannel Analysis of Surface Waves-MASW, a method that estimates near-surface S-wave velocity from high-frequency (> 2 Hz) Rayleigh waves, has been applied to more and more near-surface problems. The differences between MASW results and direct borehole measurements are 15% or less and random. Several studies have shown that the accuracy and resolution of estimated S-wave velocity can be increased by inverting the Rayleigh wave fundamental mode with higher modes simultaneously. A feasibility study in determining near-surface quality factors had promising results. Acquiring high quality surface-wave data and increasing acquisition efficiency have attracted the attention of the near-surface geophysical community. The MASW method was combined with the standard roll-along acquisition format to generate pseudo-2D S-wave velocity sections. Understanding the resolving power of MASW techniques and improving the resolution of S-wave velocity results were studied. The previous studies mentioned above were all focused on a 1D layered-earth model. We have completed two projects recently: estimating S-wave velocities from Rayleigh waves in a non-layered earth model and modeling high-frequency Rayleigh waves in a 2D earth model. A compressible Gibson half-space is a model of the shear modulus variation linearly with depth. In a half-space of sedimentary granular soil under the geostatic state of initial stress, the density and Poisson's ratio do not vary considerably with depth. In such an earth body, the dynamic shear modulus is the parameter that mainly affects dispersion of Rayleigh waves. An analytical dispersion law of Rayleigh waves in this half-space is in an algebraic form, which makes our inversion processing extremely simple and fast. The main advantage of using this model is that only three Rayleigh wave phase velocities are required in defining this half-space. We developed a scheme using the finite-difference (FD) method to model high-frequency Rayleigh-waves in near-surface mediums. Although many FD programs existed for earthquake research and oil exploration, none have been developed to model high-frequency Rayleigh waves in near-surface elastic mediums with a source and all receivers on the free surface. The scheme used a decoupled system of first-order differential equations. Combined elastic and acoustic free-surface conditions and a combination of one-way sponge filtering and anisotropic filtering methods were implemented to minimize edge effects. Modeling results were proved satisfactory using dispersion analysis of Rayleigh waves in a homogenous or layered half-space. A synthetic seismogram for a 2D corner-edge model matched favorably with dispersion analysis. This method is simple, stable, fast, and accurate and provides a practical tool for improving confidence of 2D interpretation and a basis of 2D inversion. Our future studies will mainly focus on Rayleigh-wave inversion of a 2D subsurface model, determination of dispersion curves with arbitrary geophone settings, and Love-wave inversion.
DE: 0902 Computational methods, seismic
DE: 0935 Seismic methods (3025)
DE: 5470 Surface materials and properties
DE: 7255 Surface waves and free oscillations
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly