Multichannel Analysis of Surface Wave (MASW) Method for Geotechnical Site Characterization

Abstract

A surface wave method is often used to map shear wave velocity variation of soil with depth. However, measured seismic wave field usually contains such unfavorable waves as higher modes of surface waves, body waves, and ambient noise. These waves can significantly influence results of the analysis if not properly handled. Since the conventional surface wave method of spectral analysis of surface waves (SASW) is based on the two-receiver acquisition and processing scheme, these complications are usually not effectively accounted and the field procedure tends to be labor intensive. In an attempt to increase confidence in the interpreted Vs profile as a result of the ambiguity in the analyzed dispersion characteristics, multichannel method is used in this research to characterize a test site on the campus of University of Tennessee, Knoxville that is seismically active. The multichannel analysis of surface waves (MASW) method originated from the traditional seismic exploration approach that employs multiple (twelve or more) receivers placed along a linear survey line. Main advantage is its capability of recognizing different types of seismic waves based on wave propagation characteristics such as velocity and attenuation. The MASW method utilizes this capability to discriminate the fundamental-mode Rayleigh wave against all other types of surface and body waves generated not only from the seismic source but also from the ambient site conditions. Dispersive characteristics of seismic waves are imaged from an objective 2-D wavefield transformation. Since the multichannel pattern-recognition ability can tolerate certain extent of adverse influence from the near-field effects as well as from the noise waves, data acquisition procedure is also a simple task, being insensitive to such field factors as seismic source, receiver spacing, and distance from source. In this way, the MASW method can produce a 2-D cross section map of Vs distribution within soil in accurate and efficient manner. The present paper indicates results from MASW survey at a site along the Tennessee river with relatively shallow bed rock. Both SASW and MASW techniques will prove to be important tools for evaluating liquefaction potential for future geophysical and geotechnical engineering community and this paper presents important aspects of MASW technique and its effectiveness in geophysical site characterization.