MASW

MASW, or Multi-channel Analysis of Surface Waves, is a geophysical method used to characterize the near-surface shear-wave velocity structure. It uses recordings from multiple sensors along the ground surface to capture Rayleigh waves generated by an active source or by ambient vibrations. From the recorded data, the dispersion of surface waves with frequency is extracted, because Rayleigh-wave phase velocity depends on depth. The dispersion curve (or dispersion image) is then inverted to produce Vs as a function of depth, forming a proxy for subsurface stiffness and lithology.

Acquisition involves deploying an array of geophones or accelerometers along a line, with spacing chosen to balance resolution and depth of investigation. Active MASW uses a source such as a sledgehammer or weight drop; passive MASW uses ambient noise. The length of the array and the sampling rate determine the maximum depth of investigation and the usable frequency range.

Processing and inversion begin with a Fourier transform-based dispersion analysis to yield phase or group velocity versus frequency. The fundamental mode is typically used, though higher modes can be included to improve depth resolution. Inversion aims to fit the observed dispersion by a layered Vs(z) model, often with constraints on layer thickness, velocity ranges, and smoothness to stabilize the solution. The resulting Vs profile is commonly used to estimate soil stiffness and to inform site response analyses.

Applications include geotechnical site characterization, foundation design, seismic hazard assessment, and environmental or groundwater studies. Limitations involve non-uniqueness, sensitivity to data quality and noise, and the influence of lateral heterogeneity; incorporating borehole data or other surveys improves reliability.