225 7. Seismic inversion and characterization applied to geothermal energy migration technique used. The delay with the offset, called NMO effect, is corrected. For the same time (TWT), they image the same subsurface point. Traces are classified with an offset key, defined by the distance between the source and the receiver (Figure 7.2). Figure 7.2 Conceptual view of a seismic gather after the process sequence, including NMO. In practice, gathers are stacked to reduce random noise: • Full stack: Information is summed over a wide angle or offset range, allowing a strong reduction of random noise. This result is the basis of seismic interpretation, but in this process, the variation of amplitude with offset and, if available, with azimuth is lost. • Angle-stack: Offsets are converted with a velocity trace into angles, from which theoretical responses can be modeled by wave equations. They are often denominated Near, Mid, Far and UFar to the reference of their angle ranges. 7.1.2 The subsurface as an isotropic elastic medium In seismic reflection, in a supposed homogeneous isotropic medium, the reflectivity of the PP (incident P, reflected P) wave at an interface between two layers is governed by the Zoeppritz equation. It depends on three elastic properties, P-wave velocity (VP), S-wave velocity (VS), and density, as well as the incidence angle at which the wave arrives at the interface. In addition, P-impedance, product of velocity (P) and density, is the capacity of a compressive wave to cross a medium. S-impedance, linked to S-velocity, is a similar property, but related to the shear waves. When seismic data are fully stacked, the amplitude variation with incidence angle is lost. The considered hypothesis is therefore of normal incidence: the amplitude of the reflected wave only depends on the gradient of P-impedance. The optimization
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