Estimation of the Soil Layers Thickness Using Multi-Chanel Analysis of Surface Waves and Surface Wave Dispersion Curve

Abstract

One of the most interesting topics in the geotechnical and geophysical engineering is the use of surface waves to characterize the earth subsurface layers. In a vertically heterogeneous media, the phase velocity of the surface wave is a function of the frequency (the frequency-phase velocity relationship is called dispersion curve). The dispersion curve is calculated by the shear wave velocity, compressive velocity, density, and thickness of each of the layers, which their properties can be increasing or decreasing from the surface to the half-space. In this paper, horizontal soil layers were modelled using finite element method based software (ABAQUS). Due to the different layering specifications, the models are divided into two main types: the layers’ characteristics increase with depth and the layers' characteristics decrease and increase with depth. An active impact source was used to create surface waves and the absorption layers with increasing damping (ALID) were applied to the model boundaries to prevent the wave reflection. Based on the gathered surface wave data, the dispersion curve was plotted using Frequency-Wavenumber Transfer method. In addition, the effects of different geophone offsets on the dispersion curve were investigated. The results showed that using the dispersion curve and phase velocity at high frequencies, the thickness of the surface layer can be calculated. Also, the slope of the dispersion curve at low frequencies indicates the number of the layers at different properties, and the steeper and closer to the vertical, means that a few number of layers are exist in the media. Furthermore, the effects of different geophone offsets were investigated and it was observed that geophone offsets should be limited to less than one-fourth of the layer depth in order to prevent the dispersion curve jumping to the higher modes. Furthermore, if the dispersion curve jumps to the higher modes at high frequencies, seismic data can be taken at a less geophones’ offset or the dispersion curve frequency range limitation is only before jumping to higher modes.

Keywords

• surface waves
• dispersion curves
• finite element method
• layer thickness
• geophone offsets
• higher modes