Establishing Confidence in Surface Wave Determined Soil Profiles

Surface waves can be used to determine the shear velocity profile from the ground surface to some depth limited by the spectral band of the seismic source. A number of factors influence the uncertainties of the determined profile. The field acquisition factors include the deployment geometry of geophones, the spectral characteristics of the geophones, recording instruments, and seismic source. A key data processing factor is the determination of a dispersion curve from the field recordings. Finally, there are important choices in conducting the inversion of the dispersion curve which leads to the final soil profile. Even if the field factors and acquired data are fixed, determination of the dispersion and the inversion decisions will have a strong influence on the final result. Different engineers will make different decisions, and a range of soil profiles can be expected. Assessment of this variability was the goal of the Surface Wave Benchmark Study sponsored by the Geophysical Engineering Committee of ASCE. Participants were invited to analyze as little or as much of the data as they wished. This paper documents one participant’s analysis of a selected set of the data taken at a single location. A key finding documents how a lack of low frequency content limits the maximum depth for which one can have confidence in the soil profile. INTRODUCTION The near surface shear-wave velocity profile is of interest in a number of applications. These include earthquake soil dynamics, the response of foundations and structures to both earthquakes and vibrations due to construction, pile driving, and blasting. The method has also been applied to the evaluation of pavements. Frequently employed non-invasive surface wave methods used to determine the shear-wave soil profile include Spectral Analysis of Surface Waves (SASW) and Multichannel Analysis of Surface Waves (MASW) (Penumadu & Park, 2005). There has been interest in evaluating these non-invasive techniques (Tran & Hiltunen, 2008). Additionally, the uncertainty in the analysis and the impact on application risks has begun to be considered (Hiltunen et al. , 2006; Marosi & Hiltunen, 2004). Typical practice involves the acquisition of Rayleigh wave data using a surface seismic source and geophone receiver deployment. Recordings are digitally processed to yield phase velocity dispersion, and the dispersion curves are inverted to a 1-D shear-velocity profile (Yuan & Nazarian, 1993). The depths penetrated depend on the recorded signal spectrum. Recent advances include an effort to extend the low frequency content of data to increase the maximum depth for which reliable results can be obtained (Rosenblad et al. , 2008; Rosenblad & Li, 2009). In an attempt to further understand the variable factors leading to risk assessment and uncertainty with surface wave methods, the Geophysical Engineering Committee (GEC) of the ASCE Geo-Institute sponsored a benchmark study. Practitioners were invited analyze sets of surface wave data collected at the National Geotechnical Experimentation Site (NGES) at Texas A&M University (TAMU). This paper is one of those investigations into the risk assessment of surface wave analysis. SELECTED DATA This paper focuses on one of the data sets, test site 98-220. These data were collected for Multi-channel Analysis of Surface Waves (MASW). The active source was a sledge hammer located at station 78, 6.10 meters (20 ft) from the nearest geophone. Geophones were placed 0.6096 meters (2 ft) apart extending away from the source to a maximum offset of 43.28 meters (142 ft). The 62 geophones had a natural frequency of 4.5 Hz and were oriented vertically. The data were digitized at a .00078125 second sample interval, anti-alias filter of 500 Hz. The pre-trigger delay was about 10% of the 12.8 second record length. A selected portion of the data are shown in Figure 1. Figure 2 shows an all-pole estimate of the available signal spectrum.