An introduction to seismic diffraction

Abstract

Despite its unique properties the diffracted seismic wavefield is still rarely exploited in common practice. Although the first works on seismic diffraction date back at least as far as the 1950s, a first rigorous theoretical framework for diffraction imaging only evolved decades later and many important questions still remain unanswered until the present day. While this comparably slow progression can partly be explained by the lack of densely sampled high quality recordings, recent advances in acquisition and dedicated processing suggest we might be at the door step to a paradigm shift in which seismic diffraction could play an important role. Despite the fact that most major progress-in terms of data acquisition and processing-has been achieved for the reflected wavefield, upon closer inspection it becomes obvious that the concept of diffraction is deeply ingrained in migration-type seismic imaging. With the aim of complementing existing contributions on the topic, this chapter is an attempt to provide an intuitive introduction to the process of seismic diffraction. Discussed are the deep conceptual roots in optics, physical links to the Kirchhoff integral as well as diffraction types and their importance in different contexts of application. By means of controlled synthetic and academic as well as industry-scale field data examples, I suggest a simple integrated framework for non-invasive diffraction separation and high-resolution imaging, which remains computationally affordable and can be reproduced by the reader. Different applications suggest that the faint diffracted background wavefield is surprisingly rich and, once it is given a voice, it announces highly resolved features such as faults, fractures, and erosional unconformities, which remain notoriously hard to image conventionally. Extending the dominant theme of high-resolution seismic imaging, I illustrate how the superior illumination due to the uniform radiation of diffraction carries the additional potential for drastically reduced acquisitions and discuss the possibility of a systematic extraction of inter-scatterer traveltimes from coda waves.

Recent Advances and Challenges of Waveform‐Based Seismic Location Methods at Multiple Scales

“…Kirchhoff migration is a weighted backprojection of wavefields onto an isochron surface (a surface with equal traveltime) that passes through the scattering object (Esmersoy & Miller, ). When the scattering object turns into a point or an edge, the diffraction theory works (e.g., Klem‐Musatov, ; B. Schwarz, ). The seismic source can be approximated by a point in most scenarios ranging from laboratory to global scales of seismic monitoring, because the source‐receiver distance usually exceeds more than five prevailing wavelengths (e.g., see discussion on dominant frequency of microseismic sources in Eisner et al, ).…”

“…Similar to PWS, kinematic summation trajectories (here wavefronts) are combined with the utilization of waveform coherence, which makes this approach a hybrid method. Also, in analogy to PWS, the concept of diffraction is naturally honored (B. Schwarz, ), and the same workflow has been successfully applied to controlled‐source back‐scattering (Bauer et al, , ). Owing to different instrumentation and the scale‐related differences in wavefield complexity, the array‐based detection of directional information has dominated in earthquake studies (Vespagrams, slowness‐back azimuth analysis; e.g., Rost & Thomas, ), whereas in exploration seismology, wavefront tilt is often neglected, and curvatures have traditionally been at the center of attention (controlled‐source velocity spectra; Taner & Koehler, ).…”

“…Kirchhoff migration is a weighted backprojection of wavefields onto an isochron surface (a surface with equal traveltime) that passes through the scattering object (Esmersoy & Miller, 1989). When the scattering object turns into a point or an edge, the diffraction theory works (e.g., Klem-Musatov, 1994;B. Schwarz, 2019).…”

Coherent diffraction imaging for enhanced fault and fracture network characterization

“…To provide some intuition, Fig. 1 illustrates some of the key properties of diffractions and how they can be of use for geophysical subsurface imaging (for more details, see Schwarz, 2019a). As arguably the first rigorous experimental evidence, Young's slit experiments concluded that when light hits a small enough opening in a screen, an intricate interference pattern appears on a second screen.…”

“…Likewise applied before migration, there are some techniques that make direct use of wave-field coherence for diffraction separation (Berkovitch et al, 2009;Dell and Gajewski, 2011;Bauer et al, 2016;Bakhtiari Rad et al, 2018). While these methods specifically target the diffracted wave field for extraction, recent developments have shown that a more surgical, amplitude-preserving separation can be achieved by assessing the coherence of reflections instead (Schwarz and Gajewski, 2017a;Schwarz, 2019b). Although other methods like plane-wave destruction can achieve a similar quality of extraction in many applications, the systematic and physically intuitive assessment of coherence can be carried out in any imaginable data configuration and allows for a seamless integration of data enhancement, wave-field separation, and imaging into a single framework.…”