24 Seismic Imaging Box 1.3 The various types of seismic surveying. At distances equal or greater than the critical distance, refracted waves or wide-angle reflections can be captured from the reflector. In addition to the body waves generated by a surface seismic source, which propagate through the subsurface, surface waves also propagate through the earth with their energy concentrated near to the surface. Surface waves, which are mainly sensitive to the shear modulus, do not represent a new wave type; they are interference phenomena resulting from body waves (Figures 1.3-a, b). Travelling only within a few seismic wavelengths from the surface of a solid, the lower the frequency of a surface wave, the deeper its penetration depth into the earth. Therefore, in the same medium, waves of different wavelengths affect different depths (Figure 1.3-c). These waves, which usually exhibit larger amplitudes and lower frequencies than body waves, are used in civil engineering to determine the mechanical parameters (shear velocity and shear modulus) of the first tens of meters below the ground surface. Among surface waves, which are the main component of ground roll, there are two classes of great interest to characterize the shallow near surface: a) Rayleigh waves, (R-waves) – travel on the free surface of a semi-infinite solid medium. They are generated from the interference of multiple reflections of P and SV-waves. The particle motion has a retrograde elliptical orbit travelling in a vertical plane parallel to the propagation direction, this is polarized in a vertical plane through the propagation direction. The minor axis of the ellipse is parallel to the wave motion direction and equal to two-thirds of its major axis (Figure 1.3-a). On the surface of a solid medium with a Poisson’s ratio of ν = 0.25, the Rayleigh waves travel with a velocity VR, which is slightly lower than shear wave velocity VS, VR = 0.92VS.
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