Abstract
Abstract. Faults and fractures represent unique features of the solid Earth and are especially pervasive in the shallow crust. Aside from directly relating to crustal dynamics and the systematic assessment of associated risk, fault and fracture networks enable the efficient migration of fluids and therefore have a direct impact on concrete topics relevant to society, including climate-change-mitigating measures like CO2 sequestration or geothermal exploration and production. Due to their small-scale complexity, fault zones and fracture networks are typically poorly resolved, and their presence can often only be inferred indirectly in seismic and ground-penetrating radar (GPR) subsurface reconstructions. We suggest a largely data-driven framework for the direct imaging of these features by making use of the faint and still often underexplored diffracted portion of the wave field. Finding inspiration in the fields of optics and visual perception, we introduce two different conceptual pathways for coherent diffraction imaging and discuss respective advantages and disadvantages in different contexts of application. At the heart of both of these strategies lies the assessment of data coherence, for which a range of quantitative measures is introduced. To illustrate the versatility and effectiveness of the approach for high-resolution geophysical imaging, several seismic and GPR field data examples are presented, in which the diffracted wave field sheds new light on crustal features like fluvial channels, erosional surfaces, and intricate fault and fracture networks on land and in the marine environment.
Highlights
Crustal faults and fracture systems are of significant importance for the structural interpretation of geophysical images
The fourth and final example consists of a groundpenetrating radar (GPR) line that was acquired by the US Geological Survey in the frame of a multidisciplinary effort to study the impact of seasonal tropical storms on coastal change as part of the Barrier Island and Estuarine Wetland Physical Change Assessment (Zaremba et al, 2016)
Similar to the single-channel seismic example, owing to the fact that emitting and receiving antennae typically coincide, reflections in GPR data are predominantly sensitive to vertical structural changes, whereas lateral information stemming from channels and other dynamically important erosional structures is largely encoded in the diffracted wave field
Summary
Crustal faults and fracture systems are of significant importance for the structural interpretation of geophysical images. Crystalline rock environments, which are of special interest for geothermal exploration and production, are known to be brittle and scarred by intricate fracture networks, whose successful identification and characterization have an immediate impact on the desired transition to sustainable energies Despite their importance, pronounced direct geophysical images of crustal faults, in particular when temporarily inactive, remain largely elusive, owing in large part to their sub-wavelength structural complexity and the seemingly diffuse and complex wave fields that are typically associated with them. The suitability of seismic diffractions as a direct fault indicator was already explored in the 1950s and was further investigated in the following 2 decades (Krey, 1952; Kunz, 1960; Trorey, 1970; Berryhill, 1977) While these studies were mostly concerned with the accurate numerical modelling of the diffraction response, the first imaging attempts, despite their novelty, largely suffered from inadequate data quality (Landa et al, 1987; Kanasevich and Phadke, 1988). Concluding community-spanning seismic and electromagnetic examples suggest that coherent diffraction imaging leads to overall highly resolved subsurface reconstructions, and directly and reliably highlights small-scale erosional features, faults, and fractures
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