103 2. Surface geophysical methods Figure 2.37 Velocity and temperature full waveform inversion (after Yang et al., 2024). Initial models: velocity (a) and temperature (b). Figure 2.38 Velocity and temperature full waveform inversion (after Yang et al., 2024). Inversion results: velocity model (a) and temperature model (b), velocity at trace 135 (c) and temperature at trace 135 (d). Passive seismic method and MASW method In addition to body waves (P- and S-waves) which propagate within the subsurface, a surface seismic source generates surface waves (Love and pseudo-Rayleigh waves). The seismic method based on the analysis of surface waves is called MASW (Multiple Analysis of Surface Waves). Surface waves, which are mainly sensitive to the shear modulus of the formation, propagate through the earth with their energy concentrated near to the surface. Their propagation velocity is frequency-dependent (dispersion). The degree of dispersion is a measure of seismic wave velocity as a function of depth and can be used to calculate the thickness of surface layers. 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. (a) (b) (a) (b) (c) (d)
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