Shear-wave velocity structure

Ritsema J., van Heijst H. J., and Woodhouse J. H. (1999) Complex shear wave velocity structure imaged beneath Africa and Iceland. Science 286, 1925-1928. 

Fig. 2. Large-scale velocity structure beneath the Canadian Shield as determined by Grand et al. (1997). The scale indicates the shear-wave velocity anomaly as a percentage relative to IASP91 (Kennett Engdahl 1991). O, locations of the TWiST seismic stations A, from south to north, the permanent seismic stations ULM (CNSN), FFC ffRIS) and FCC (CNSN).

Stress levels 1 and 2 mean about 50 and 100% of the bearing capacity, respectively. These valnes are typical and may be used for soils with the shear wave velocity less than 1000 m/s. References [30-31] are recommended to consider the soil structure effects in a more exact manner. 

As mentioned, accurate models of surface layers are necessary for high-resolution prediction of wave amplification in mediums with low shear wave velocities. However, constructing an accurate model is difficult due to limited availability of borehole data, some of which are poor in quality. Often these borehole data are coming fi om scattered locations and are inconsistent borehole data is considered inconsistent if the sequence and the number of distinct layers differ fi om site to site (see Fig. 3a). To make a model of underground structures, based on borehole data, Hori (Hori 2011) has proposed a method which involves two steps find a reference sequence of soil layers consistent with the data of each borehole site and interpolate the obtained consistent layers to find soil layer information at any arbitrary point. The proposed scheme, which is used in geo-information geology, for estimating a consistent set of soil layers at each site involves the following four steps ... 

For simple earth-retaining structures, the outcrop seismic action is more commonly expressed as an elastic response spectrum, and the local site response is considered through simple amplification coefficients, usually based on the classification of the subsoil according to the shear wave velocity of the topmost, say, 30 m. In this case, acceleration time histories are not available a direct calculation of the displacements is not possible, and it is necessary to use a simphfied approach. 

Seismic analysis of underground structures starts with site-specific definition of its seismic environment. A detailed field and laboratory investigation program is necessary the field investigation program should include definition of the site stratigraphy and direct measurements of shear wave velocity profiles and cone... 

Hashash et al. (2010) provides a simplified 2D dynamic soil-structure interaction procedure that makes computational effort manageable for design purposes of transverse response of rectangular tuimels. The first step is to perform a 1D site response analysis to obtain the acceleration and displacement time history throughout the soil profile and then obtain the strain-compatible shear wave velocities and damping ratios for the 2D model layers. 

Schon et al. (2005) applied a modular model concept for electrical, hydraulic, and elastic anisotropy studies that allows a joint interpretatiOTi of anisotropic formations. The model crmsideration and analysis of real logging data show that shear wave velocities depend strongly on the shale distribution and that the difference of the velocity of shear waves with different polarization can be related to the shale distributirHi (laminar or dispersed, structural). A shear wave-based method can discriminate between laminated and dispersed shaly zones and provide an estimate of the sand reservoir properties. 

Current information on dynamic soil properties indicates that the soil properties used. in> the soil structure interaction (SSI) analyses for the K-Reactor building were estimated from the SPT blow count data from boring K-5. These data, in turn, were correlated with low-strain, shear wave velocity data for sand, which was obtained from the open literature. A comparison of, the data w cross-hole measurements obtained from other areas, of SRS indicates that, on the average, the low strain data estimates are reasonable. As described in Section 8.4.1 of this SER, the latest SASSI calculations, which include soil layering effects, indicate that the response. spectra calculated using the uniform property data are conservative. 

As shown in Fig. 4.19, in anisotropic medium, a surface acoustic wave represents a combined longitudinal (4.19a) and shear (4.19b) motion of the lattice in the y-(0)-z plane this is the saggital plane. In anisotropic media, in certain multilayer structures and at some interfaces, the surface wave velocity exceeds the velocity of the shear wave and the energy continuously leaks from the surface to the bulk of the material. In such cases, we talk about pseudo- or leaky waves. Various energy-loss... 

Meltzer A. and Christensen N. (2001) Nanga Parbat crustal anisotrophy implication for interpretation of crustal velocity structure and shear-wave splitting. Geophys. Res. Lett. 28(10), 2129 -2132. 

Let us analyze the space and time structure of the elastic displacement field in detail. We will demonstrate that equation (13.26) describes the propagation of two types of body waves in an elastic medium, i.e., compressional and shear waves travelling at different velocities and featuring different physical properties. To this end, let us recall the well-known Helmholtz theorem according to which an arbitrary vector field, in particular an elastic displacement field U(r), may be represented as a sum of a potential, Up(r), and a solenoidal, Us(r), field (Zhdanov, 1988) ... 

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