Shear Wave Velocity Profiling: Comparing MASW and SPT for NEHRP Site Classification

Abstract

Discrepancies between shear wave velocity (Vs) profiles derived from Multichannel Analysis of Surface Waves (MASW) and the Standard Penetration Test (SPT) pose significant challenges for seismic site classification under the National Earthquake Hazards Reduction Program (NEHRP) standards. This study investigates the effects of velocity contrasts and irregular subsurface stratification on the alignment of MASW and SPT Vs profiles. A MASW survey was conducted using an active seismic source and an array of geophones, with data processing involving preprocessing, dispersion curve extraction, and shear-wave velocity inversion using a Monte Carlo approach. The extracted dispersion curve spans frequencies from 25 Hz to 80 Hz, with phase velocities ranging between 200 m/s and 500 m/s. The inverted Vs profile indicates a progressive increase in shear wave velocity with depth, suggesting a compacting sedimentary sequence transitioning into bedrock at greater depths. The computed Vs30 value of 541.7 m/s classifies the site as NEHRP Site Class C (very stiff soil or soft rock). The correlation with regional borehole data confirms the presence of clay and silty deposits in the upper layers, followed by claystones and sandstones at depth. Despite the reliability of the MASW for seismic site characterization, discrepancies with SPT-derived Vs profiles highlight the challenges posed by velocity contrasts, irregular stratification, and higher-mode Rayleigh waves. Refining inversion techniques and integrating multiple geophysical methods are crucial for improving subsurface characterization and seismic hazard assessments. These findings emphasize the importance of considering seismic design regulations for infrastructure development in Site Class C soils, which can exhibit moderate ground motion amplification.