Keywords: geotechnical site characterization, S-wave soil profile, surface waves, multichannel analysis of surface waves, inversion.

A soil characterization in terms of the vertical shear-wave velocity profile of the soil column is crucial for the estimation of dynamic soil response. Vertical shear-wave velocity profiles are obtained by inverting the dispersive phase velocity using the multichannel analysis of surface waves (MASW). The MASW procedure for calculating the vertical shear-wave velocity profile consists of three steps: acquiring ground motion data in the field, processing the data to determine a dispersion curve, and calculating the shear-wave velocity for different depths using inversion algorithms. Two types of inversions are used: one involves using a dispersion image and the second one uses dispersion curves. The dispersion curve is used as an input to a gradient-based iterative method and Monte-Carlo search for dispersion images. In order to overcome the problem of obtaining different inversion solutions, an inversion strategy with a high level of confidence was proposed to evaluate dominant trends in the vertical shear-wave velocity profile. The following strategic steps that were used can be identified as:

• Preprocessing done on seismograms and the dispersion image. Field experiments have to be repeated several times to enhance the definition of an image, enlarge the bandwidth of the dispersion pattern and reduce noise.
• To avoid any bias, two different methods of dispersion curve extraction were used. In the first method, points on the dispersion curve were identified and in the second approach the entire dispersion image was used as an input to the inversion.
• Additionally, two different types of inversion methods were applied to obtain the shear-wave velocity profile. The dispersion curve was used as an input to a gradient based iterative method. A Monte-Carlo method was used to randomly search for a layered model with a dispersion curve that best matches the dispersion image.
• Finally, a forward model was used to compare the theoretical dispersion curves of the first four modes with the experimental dispersion curve.

This paper presents surface wave testing conducted at a roadside in Pretoria, South Africa. The inverted shear-wave velocity is constant to a depth of 2.5m then all the inversion profiles show a systematic decrease in velocity with depth. At a discontinuity of 12.2m all the profiles show a significant increase in shear-wave velocity. The observed reversal of the shear-wave is present in all the models, therefore, this can confidently be considered as real. Uncertainty in the shear wave velocity model is also evaluated.

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