Modal Separation Before Dispersion Curve Extraction By Masw Method

Abstract

Accurate extraction of dispersion curves is the most critical part with any surface-wave method. Although a multichannel method like Multichannel Analysis of Surface Waves (MASW) method proves most effective for this purpose, there are several acquisition parameters that need to be set properly for the optimal extraction. One of them requires a long range of receiver spread to separate phase velocity of one mode from those of other modes. Sometime, however, this requirement can not be met due to an unacceptable lateral inhomogeneity in the near-surface materials being surveyed and therefore multichannel records are often collected with a relatively short spread length. In this case the interference between the different modes of surface waves can be so severe that neither fundamental nor highermode dispersion curve can be extracted with a reliable confidence. In addition to this multimodal effect by surface waves, other types of seismic event such as channel (guided) waves can cause similar harmful effect during the analysis. When this occurs, a simple multichannel processing technique that mutes the interfering wavefields in the offset-time (x-t) domain can significantly enhance the resolution of multimodal dispersion curves. This is demonstrated by using both synthetic and real shot gathers.