A generalized view on normal moveout

et al. 2020

Reviews of Geophysics

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132

Source locations provide fundamental information on earthquakes and lay the foundation for seismic monitoring at all scales. Seismic source location as a classical inverse problem has experienced significant methodological progress during the past century. Unlike the conventional traveltime‐based location methods that mainly utilize kinematic information, a new category of waveform‐based methods, including partial waveform stacking, time reverse imaging, wavefront tomography, and full waveform inversion, adapted from migration or stacking techniques in exploration seismology has emerged. Waveform‐based methods have shown promising results in characterizing weak seismic events at multiple scales, especially for abundant microearthquakes induced by hydraulic fracturing in unconventional and geothermal reservoirs or foreshock and aftershock activity potentially preceding tectonic earthquakes. This review presents a comprehensive summary of the current status of waveform‐based location methods, through elaboration of the methodological principles, categorization, and connections, as well as illustration of the applications to natural and induced/triggered seismicity, ranging from laboratory acoustic emission to field hydraulic fracturing‐induced seismicity, regional tectonic, and volcanic earthquakes. Taking into account recent developments in instrumentation and the increasing availability of more powerful computational resources, we highlight recent accomplishments and prevailing challenges of different waveform‐based location methods and what they promise to offer in the near future.

t \approx t_{0}^{'} + boldp ()(, boldx - x_{0}) + \frac{1}{2} {(x - {boldx}_{bold0})}^{T} boldH ()(, boldx - x_{0}),

where p is the horizontal slowness, x − x 0 is the lateral distance vector to the reference location, and

t_{0}^{'}

denotes the reference onset time.…”

Section: Methodologiesmentioning

confidence: 99%

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

Tan

et al. 2020

Reviews of Geophysics

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132

“…Equation 8in conjunction with Eq. 7and one of the positive-valued coherence measures can be used to set up an optimization problem to arrive at an approximate but fully data-driven reconstruction of the subsurface scatterer distribution (Fomel, 2007;Schwarz et al, 2014;Bonomi et al, 2014). In addition to considering a near-surface projection, wavefront slopes and curvatures can also be estimated using the assumption of an effective replacement medium, which, like in Eq.…”

Section: Imaging By Projectionmentioning

confidence: 99%

Coherent diffraction imaging for enhanced fault and fracture network characterization

Krawczyk

2020

Solid Earth

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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.

“…the poststack domain), the common-source or the common-receiver gather, the hyperbolic traveltime formula 5 reduces to formally equivalent expressions that relate to auxiliary one-way wave propagation and, consequently, can likewise be employed in passive seismic investigations (e.g. Schwarz et al, 2016;Schwarz and Gajewski, 2017a;Diekmann et al, 2018). So as a by-product, the use of equation 5 in conjunction with equation 4 allows for the automated extraction of first and second order attributes of the event's shape, which can be directly related to the slope and curvature of the emerging wavefront causing the observed trace-to-trace traveltime differences.…”

Section: Coherence and Wavefrontsmentioning

confidence: 99%

“…While these differ in the way the wavefront attributes appear in the expressions, they share the overall double-square-root shape and are equally accurate for the limiting diffraction case (Schwarz and Gajewski, 2017c). In fact, if parametrized consistently they were demonstrated to provide equivalent accuracy and can be considered largely equivalent descriptions (Schwarz and Gajewski, 2017a;. For an effective auxiliary medium they incorporate the conventional Kirchhoff time migration formula…”

Section: Diffraction Focusingmentioning

confidence: 99%

See 1 more Smart Citation

An introduction to seismic diffraction

2019

Advances in Geophysics

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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.