Multichannel Analysis of Passive Surface Waves – Modeling and Processing Schemes

Characteristics of phase velocity and incoming angle (azimuth) of passive surface waves as they are observed with a multichannel processing scheme are studied through both synthetic modeling and field experiments. They are analyzed specifically in association with the case of multiple sources and multiple modes of dispersion. The features of artifacts and disturbances in phase-velocity estimation and how they can be alleviated in a processing scheme are described. First, a modeling scheme is introduced to simulate multiple sources at different locations as well as multiple modes of dispersion. With such known characteristics as phase velocity, azimuth, strength, and modes of dispersion, these features are illustrated and explained by using a set of modeled data. Then, the scheme to process passive surface waves is briefly described. It transforms wavefields of horizontal plane-wave propgation, measured with receiver spreads along the two orthogonal axes (x and y), into dispersion images. The scheme first transforms the measured wavefields of a particular frequency (ω) into the energy in phase velocity (c)-azimuth (θ) space where multiple sources and modes of surface waves are represented as energy peaks at different azimuths and phase velocities, respectively. The scheme, then, stacks all the energy through the azimuth axis to produce an energy distribution along the phase-velocity (c) axis only. A final image is then created by repeating these steps for different frequencies and displaying the energy in frequency (ω)-phase velocity (c) space. This process greatly alleviates general complications with the passive method such as disturbance of phase velocities due to multiple sources and modes as well as the spatial aliasing.