Surface-wave methods (SWMs) are very powerful tools for the near-surface characterization of sites. They can be used to determine the shear-wave velocity and the damping ratio overcoming, in some cases, the limitations of other shallow seismic techniques. The different steps of SWM have to be optimized, taking into consideration the conditions imposed by the small scale of engineering problems. This only allows the acquisition of apparent dispersion characteristics: i.e. the high frequencies and short distances involved make robust modelling algorithms necessary in order to take modal superposition into account. The acquisition has to be properly planned to obtain quality data over an adequate frequency range. Processing and inversion should enable the interpretation of the apparent dispersion characteristics, i.e. evaluating the local quality of the data, filtering coherent noise due to other seismic events and determining energy distribution, higher modes and attenuation. The different approaches that are used to estimate and interpret the dispersion characteristics are considered. Their potential and limits with regard to sensitivity to noise, reliability and capability of extracting significant information present in surface waves are discussed. The theory and modelling algorithms, and the acquisition, processing and inversion procedures suitable for providing stiffness and damping ratio profiles are illustrated, with particular attention to reliability and resolution.

Surface-wave method for near-surface characterization: a tutorial

Measurements of surface wave attenuation provide a means to determine the in situ material damping ratio profile of near-surface soils. Frequency-dependent surface wave attenuation coefficients are determined from measurements of seismic wave amplitudes at various offsets from a swept-sine source. The accuracy of the measured attenuation coefficients is improved by properly accounting for the geometric attenuation of multimode Rayleigh waves. Once the frequency-dependent attenuation coefficients are determined, the shear damping ratio profile is calculated using a constrained inversion analysis. Application of the method is illustrated at the Treasure Island National Geotechnical Experimentation Site. Values of shear damping ratio, obtained using surface wave measurements, were less than those measured using cross hole tests, possibly because the higher frequencies used in cross hole tests result in more apparent attenuation due to scattering and because fluid losses contribute to damping at higher freque...

In situ measurement of damping ratio using surface waves

Ambient vibration techniques are promising methods for assessing the subsurface structure, in particular the shear-wave velocity profile ( Vs ). They are based on the dispersion property of surface waves in layered media. Therefore, the penetration depth is intrinsically linked to the energy content of the sources. For ambient vibrations, the spectral content extends in general to lower frequency when compared to classical artificial sources. Among available methods for processing recorded signals, we focus here on the spatial autocorrelation method. For stationary wavefields, the spatial autocorrelation is mathematically related to the frequency-dependent wave velocity c ( ω ). This allows the determination of the dispersion curve of traveling surface waves, which, in turn, is linked to the Vs profile. Here, we propose a direct inversion scheme for the observed autocorrelation curves to retrieve, in a single step, the Vs profile. The powerful neighborhood algorithm is used to efficiently search for all solutions in an n -dimensional parameter space. This approach has the advantage of taking into account the existing uncertainty over the measured curves, thus generating all Vs profiles that fit the data within their experimental errors. A preprocessing tool is also developed to estimate the validity of the autocorrelation curves and to reject parts of them if necessary before starting the inversion itself.

We present two synthetic cases to test the potential of the method: one with ideal autocorrelation curves and another with autocorrelation curves computed from simulated ambient vibrations. The latter case is more realistic and makes it possible to figure out the problems that may be encountered in real experiments. The Vs profiles are correctly retrieved up to the depth of the first major velocity contrast unless low-velocity zones are accepted. We demonstrate that accepting low-velocity zones in the parameterization has a dramatic influence on the result of the inversion, with a considerable increase in the nonuniqueness of the problem. Finally, a real data set is processed with the same method.

Direct Inversion of Spatial Autocorrelation Curves with the Neighborhood Algorithm

Detecting different water table levels in a shallow aquifer with combined P-, surface and SH-wave surveys: Insights from VP/VS or Poisson's ratios

Surface-wave propagation in unconsolidated Quaternary alluvium has been investigated within the valley of the Chiusi Lake tributary (Central Italy). The seismic experiment was originally designed to determine the structural response of an Etruscan tomb to a 200-kg dynamite shot, planned as part of a refraction survey within the CROP-DSS (an Italian project for the investigation of deep crustal structures). The availability of many recording instruments allowed us to design a more complete experiment, and we decided to use surface waves generated by a small explosion (10 kg dynamite) to infer the shear-wave-velocity-attenuation model in the alluvium layer.

We deployed a linear array aligned with the shot point on the river sediments. Sixteen digital, three-component recording stations were used; the minimum distance from the source was 1.9 km, and we chose an interstation separation of 50 m, for a total array length of 750 m. Clear Rayleigh waves were recorded along the array, and we were able to recognize two propagating modes in the waveforms. We calculated the dispersion curves of group velocities using a multiple filter technique. Dispersion curves of phase velocities were calculated using a p -ω stacking technique.

The whole procedure was run as follows: (1) the inversion of the group-velocity dispersion; (2) the use of synthetic seismograms for mode recognition; (3) the extraction and the inversion of phase-velocity values from the p -ω stack performed on the observed waveforms; (4) the use of a trial-and-error approach for Qβ to obtain “good” synthetic waveforms; and (5) the application of a conjugate gradient inversion technique to the anelastic absorption model (no inversion was run on γ dispersion curves). Values of Qβ of ranging from 2 to 20, and a shear-wave velocity in the range 280 < β < 400 m/sec were found in the Holocene alluvium. Moreover, in our modeling we used the relation Qα = Qβ for unconsolidated, water-saturated sediments. Our results suggest that surface waves generated by explosions can be successfully used to determine shear-wave-velocity and attenuation models in soft sediment coverages.

D. Jongmans

Papers

The importance of surface waves in vibration study and the use of Rayleigh waves for estimating the dynamic characteristics of soils