When There Is No Offset: A Demonstration of Seismic Diffraction Imaging and Depth‐Velocity Model Building in the Southern Aegean Sea

Abstract

A vast majority of marine geological research is based on academic seismic data collected with single-channel systems or short-offset multichannel seismic cables, which often lack reflection moveout for conventional velocity analysis. Consequently, our understanding of Earth processes often relies on seismic time sections, which hampers quantitative analysis in terms of depth, formation thicknesses, or dip angles of faults. In order to overcome these limitations, we present a robust diffraction extraction scheme that models and adaptively subtracts the reflected wavefield from the data. We use diffractions to estimate insightful wavefront attributes and perform wavefront tomography to obtain laterally resolved seismic velocity information in depth. Using diffraction focusing as a quality control tool, we perform an interpretation-driven refinement to derive a geologically plausible depth-velocity model. In a final step, we perform depth migration to arrive at a spatial reconstruction of the shallow crust. Further, we focus the diffracted wavefield to demonstrate how these diffraction images can be used as physics-guided attribute maps to support the identification of faults and unconformities. We demonstrate the potential of this processing scheme by its application to a seismic line from the Santorini Amorgos Tectonic Zone, located on the Hellenic Volcanic Arc, which is notorious for its catastrophic volcanic eruptions, earthquakes, and tsunamis. The resulting depth image allows a refined fault pattern delineation and, for the first time, a quantitative analysis of the basin stratigraphy. We conclude that diffraction-based data analysis has a high potential, especially when the acquisition geometry of seismic data does not allow conventional velocity analysis. Plain Language Summary The active seismic method is a standard tool for studying and imaging the Earth's lithosphere. Proper imaging of complex geological targets requires seismic data of excellent quality, which are typically only acquired with expensive industrial surveys. Academic surveys, however, are often restricted to marine seismic equipment with limited illumination, which compromises imaging and interpretation. While most of the contemporary processing and interpretational routines are tailored to the reflected wavefield, recent research suggests that the often overlooked diffracted wavefield might help to overcome the gap between academic and industrial seismic imaging. Wave diffraction is the response of the seismic wavefield to small-scale subsurface structures and allows to estimate velocities even from single-channel seismic data. In this study, we use an academic seismic profile from the southern Aegean Sea and extract a rich diffracted wavefield from the data. We utilize these diffractions to estimate a velocity model that permits a reconstruction of the subsurface in depth and specifically highlight discontinuous features related to past dynamic processes. Such depth images allow us to reliably measure thicknesses and fault angles. We con...

Seismic Diffraction Imaging to Characterize Mass‐Transport Complexes: Examples From the Gulf of Cadiz, South West Iberian Margin

“…Future studies should concentrate on mitigating the effect of out‐of‐plane diffractions for focusing migration velocity analysis from 2D seismic profiles (e.g., Preine et al., 2020). This could be achieved by weighting the focusing analysis toward continuous diffraction generating structures such as faults, or deeper diffractors that are less biased by not being exactly in‐plane.…”

“…Identical migrations are performed for the full‐wavefield and diffraction images so that the geometry of both images is comparable (Figure 3). The diffraction images in this study are presented as the energy (squared envelope) of the diffraction image (as in, e.g., Preine et al., 2020).…”

“…A classic application for diffraction imaging is to derive migration velocity fields by focusing analysis of the diffracted wavefield (e.g., Decker et al., 2017; Fomel et al., 2007; Preine et al., 2020). Under the correct migration velocity, diffractions will collapse (focus) to a point at their apex.…”

“…The illumination of seismic reflectors depends on the local dip of the reflector and the geometry of the receiver array. Diffractions, however, are 3D phenomena, fully illuminated from all angles even by single-channel, zero-offset data (Figure 1a and Preine et al, 2020). This means that 2D diffraction images will suffer more strongly from out-of-plane energy than corresponding 2D reflection images.…”

“…Diffraction imaging works by separating the reflected and diffracted wavefields and migrating only the diffracted component, producing an image of these small-scale heterogeneities (Klem-Musatov et al, 2016;Schwarz, 2019b). Contrary to reflections, the radiation pattern of diffractions is independent of the dip (Figure 1), meaning that they can be fully illuminated even by short-offset or zero-offset receiver arrays (Preine et al, 2020). Combined with the general smaller scale of diffractors compared to reflectors, this radial spreading means that for a given seismic source the recorded diffracted wavefield tends to be significantly weaker and have higher frequency content than the reflected wavefield.…”