30 Seismic Imaging Multiple reflections are produced within the rock layers due to laminations of the rock formations, giving rise to internal multiples. Figure 1.7 Marine constant offset section (data provided courtesy of Craig Fullthorpe, University of Texas). Other notable spurious arrivals on the records are diffractions. A seismic diffraction is produced when the seismic wave impinges on an angular point or heterogeneity of a size that is less than the wavelength of the incident wave. The heterogeneity or point diffractor radiates energy in all directions. On a seismic record, the point diffractor appears across different traces as a hyperbolic alignment. Figure 1.7 shows diffracted waves due to the heterogeneity of the sea bottom. 1.4 Wave separation Direct and refracted arrivals have linear time-distance curves, reflection events are hyperbolas to a first approximation (Figure 1.4-b). The time-distance curves of surface waves can be treated as locally straight lines. Diffractions have a hyperbolic time-distance curve. Locally a wave can be characterized by its: • period T expressed in s or ms, • apparent velocity Va = Δx / Δt. The quantity Δt is the difference in arrival times of the wave at various receivers at a distance Δx apart.
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