3-D seismic imaging in crystalline rock environments: An approach based on diffraction focusing

Diffracted waves are seismic waves that backscatter from localized discontinuities in the earth. They describe backscattering from interfaces with either a curvature locally approaching infinity or occur if medium properties change on a scale smaller than the predominant seimic wavelength. Both types of diffracted events are of great importance in seismic processing as they allow to identify the presence of small inhomogeneities, truncations, faults, or pinch-out layers. However, to reliable interpret such subsurface features, diffractions should be properly imaged. An inherent part of the imaging is therefore a velocity model building tuned to diffractions. We present a method for 3-D velocity analysis based on the medium's diffraction response. We propose to evaluate a focusing norm along diffraction traveltimes in the time domain. The focusing analysis considers both amplitude and phase of the diffracted events which results in several output volumes: three migration velocities for point diffractions, and one migration velocity and azimuth for edge diffractions. As additional information, the focusing analysis provides two coherence volumes which can be used to classify the back-scattering geology. Application of the proposed imaging strategy to 3-D seismic field data, acquired in the context of geothermal exploration in eastern Germany, reveals that crystalline rocks cause a rich diffraction response whose focusing can provide complementary insight into structures that are notoriously hard to image conventionally.