INFLUENCES OF DATA ACQUISITION AND LAYER PARAMETERS ON FUNDAMENTAL DISPERSION CURVE IN ACTIVE MULTICHANNEL ANALYSIS OF SURFACE WAVE (A-MASW) METHOD

Abstract

Obtaining a continuous and high resolution dispersion curve in a wide frequency range is one of the most important steps to determine a reliable shear (S) wave velocity-depth profile of the shallow subsurface in active MASW method. In this article, the effects of layer (or physical) (S- and P-wave velocities, density and thickness) and data acquisition (source offset-X0, receiver spacing-dx and number-N, spread length-L=(N-1)*dx) parameters on the fundamental mode dispersion curve of the surface wave field were investigated by analyzing the synthetic shot gathers. The synthetic data were generated by harmonic mode summation technique at each receiver with using of the dispersion curve calculated by the layer reflection/transmission coefficient technique. Accordingly, it was observed that low S-wave velocity interlayers may cause shifting the dispersion curve to low velocities in the low frequency band, while high velocity interlayers may cause both discontinuity on the dispersion curve at high frequencies (zig-zag effect) and high-mode effect error. It was seen that while the source offset is effective in the formation of full waveforms of surface waves, the distance between the first and last receiver affects the frequency band and resolution of the dispersion curve. In addition, it is observed that wide receiver sampling increases the reliability of the dispersion curve in the low frequency region, though it disrupts the continuity of the dispersion curve at high frequencies due to wavenumber aliasing. As a result, the analyses made on the synthetic data generated for various subsurface layer models with different wave velocities demonstrate that the combinations of N