Numerical investigation of MASW applications in presence of surface topography

Abstract

In the applications of multichannel analysis of surface waves (MASW), dispersion curves are usually picked in an energy tracing manner on dispersion images. They are compared with the theoretical dispersion curves based on a horizontally layered earth model during the subsequent inversion for shear-wave velocities. Surface topography can strongly influence energy distribution on a dispersion image. In theory, static correction should be applied to seismic records before generating dispersion images if there are any elevation variations along a two-dimensional (2D) survey line. The out-of-plane noise from side areas of a survey line in three dimensions (3D) can also contaminate the recorded wavefield. We synthesize the seismograms through finite-difference modeling for 12 types of 2D earth models that represent the basic elements of topography along a survey line. The dispersion images are compared with the corresponding theoretical dispersion curves that are calculated by ignoring the topography of the models. The comparison shows that errors of the picked Rayleigh-wave phase velocities can be constrained within 4% if a slope angle of the topography is less than about 10°. For steeper topography, errors of the picked phase velocities are greater than 4% and static correction are recommended before the dispersion analysis. In the 3D case, we investigate a set of 3D levee-shaped earth models to evaluate the errors caused by the out-of-plane noise from the edge of an embankment. The analysis suggests that the distance between the edge of an embankment and a MASW survey line should be at least 1/10 of the dominant Rayleigh-wave wavelength so that energy distortion on dispersion images due to topography are less significant than that caused by other noises.