We have newly developed an innovative and practicalmethod as for the multichannel analysis of surfacewaves (MASW) targeting ultra-shallow surfaces. Themethod utilizes oscillation noises generated by walkingalong a linear geophone array. A number of geophones,at least 96, are placed along a survey line at 0.2 to 1 mintervals to record high-frequency surface waves. Themethod is characterized as a quasi-static seismic surveybecause it records active but weak walking noises aswell as ambient microtremors. SPAC (spatialautocorrelation) analysis is applied to each CMP gatherin the linear array to calculate a dispersion curve.Accordingly, the method enables us to reconstruct adense 2D S-wave velocity profile along a line from 50cm beneath the surface to 10 m in depth or more, usingconventional 4.5 Hz geophones. One of the advantagesof walk-along surface wave survey is its robustness tothe traffic noises. It means the method is applicable tothe vulnerability characterization of road embankmentsof highways in service.We have been applying the walk-along surface wavemethod to more than 30 survey lines since 2015. Ourfield tests demonstrated the capability of the method forthe delineation of shallow heterogeneous structure insuch artificial bodies as embankments or levees.Moreover, the method enables us to apply time-lapsemonitoring of S-wave velocity change caused bygroundwater migrations in artificial layers. |