Wavenumber unwrapping for the automated analysis of SASW measurements

Abstract

Depending upon the surface wave methods, the measured surface wave velocities are either normal mode velocities or apparent velocities. The “apparent” surface wave velocity corresponds to the superposed mode combining the available normal modes. The spectral-analysis-of-surface-waves (SASW) method measures the apparent surface wave velocities. However, these velocities render SASW analyses complicated because of the mode jumps induced by several wave groups. To alleviate such complications, this study introduced a beamforming technique and produced information on unwrapped wavenumber, which is vital in unwrapping the phase spectrum in SASW analyses. Additionally, incoming wave directions filtered out multiple reflections and irrelevant seismic events to retain only the surface waves directly propagating from the source. The proposed wavenumber unwrapping technique was verified using field applications, dynamic finite element analysis, and synthetic waveforms. Notably, an optimal beamforming technique was established using four geophones, making the proposed method highly practical and reducing the hardware requirements. Hopefully, the proposed analysis technique provides SASW practitioners with a more comfortable and more reliable environment in utilizing the SASW technique.