Seismic diffraction imaging methods and applications
Seismic waves can either be reflected or diffracted by subsurface objects depending on the object's geometry. Diffactions can be used to determine details about the small-scale features that generate them, such as karsts, voids, pinchouts, faults, fractures, and salt flanks. Diffraction imaging can have resolution below the typical seismic wavelength. Scattered waves are recorded as significantly lower-energy signal than reflected waves, requiring that diffractions be separated from reflections. I describe three methods of such separation: data-domain plane-wave destruction, Fresnel zone Elimination, and partial-image plane-wave destruction. Once separated, diffractions can be migrated to create a seismic diffraction image and used in velocity analysis. Common-reflection angle migrated diffractions appear flat in dip angle gathers when migrated with correct velocity. I illustrate how this property can be used to determine migration velocity through a process of oriented velocity continuation (OVC). In OVC framework diffraction data are decomposed by slope and migrated over a range of velocities. Velocities corresponding to the flattest slope gathers are picked using semblance as a measure of flatness. This provides an estimate of migration velocity. Stacking gathers corresponding to this chosen velocity generates a seismic diffraction image. Seismic diffraction images provide interpreters with information about small-scale geologic objects that may not be available in conventional images. Scattering features that are interesting for exploration, like voids, caves, fractures, and faults, cause diffractions and can be resolved with better focus in diffraction images than in conventional ones. This is particularly useful with geologically complex carbonate systems. Carbonates are strongly heterogeneous, making them difficult to image with conventional methods. Reservoir porosity is often contained within caves, or small vugs. These features are difficult to characterize with conventional methods because cave reflections have large geometric uncertainties in cave size and location. Velocity analysis of seismic reflection data in carbonates may not highlight vugular porosity particularity well. I illustrate how diffraction images provide improved characterization by highlighting the edges of caves, thus constraining cave geometry, and highlighting more heterogenous zones by measuring the amount of scattering those zones generate.