131,344 research outputs found
Virtual plane-wave imaging via Marchenko redatuming
No full textMarchenko redatuming is a novel scheme used to retrieve up- and downgoing Green's functions in an unknown medium.Marchenko equations are based on reciprocity theorems and are derived on the assumption of the existence of functions exhibiting space-time focusing properties once injected in the subsurface. In contrast to interferometry but similarly to standard migration methods, Marchenko redatuming only requires an estimate of the direct wave from the virtual source (or to the virtual receiver), illumination from only one side of the medium and no physical sources (or receivers) inside the medium. In this contribution we consider a different time-focusing condition within the frame of Marchenko redatuming that leads to the retrieval of virtual plane-wave responses. As a result, it allows multiple-free imaging using only a 1-D sampling of the targeted model at a fraction of the computational cost of standard Marchenko schemes. The potential of the new method is demonstrated on 2-D synthetic models.Applied Geophysics and PetrophysicsImPhys/Acoustical Wavefield Imagin
On the Marchenko equation for multicomponent single-sided reflection data
No full textRecent work on the Marchenko equation has shown that the scalar 3-D Green’s function for a virtual source in the subsurface can be retrieved from the single-sided reflection response at the surface and an estimate of the direct arrival. Here, we discuss the first steps towards extending this result to multicomponent data. After introducing a unified multicomponent 3-D Green’s function representation, we analyse its 1-D version for elastodynamic waves in more detail. It follows that the main additional requirement is that the multicomponent direct arrival, needed to initiate the iterative solution of the Marchenko equation, includes the forward-scattered field. Under this and other conditions, the multicomponent Green’s function can be retrieved from single-sided reflection data, and this is demonstrated with a 1-D numerical example.ImPhys/Imaging PhysicsApplied Science
Towards understanding the impact of the evanescent elastodynamic mode coupling in Marchenko equation-based demultiple methods
No full textMarchenko equation-based methods promise data-driven, true-amplitude internal multiple elimination. The method is exact in 1-D acoustic media, however it needs to be expanded to account for the presence of 2- and 3-D elastodynamic wave-field phenomena, such as compressional (P) to shear (S) mode conversions, total reflections or evanescent waves. Mastering high waveform-fidelity methods such as this, could further advance amplitude vs offset analysis and lead to improved reservoir characterization. This method-expansion may comprise of re-evaluating the underlying assumptions and/or appending the scheme with additional constraints (e.g. minimum phase). To do that, one may need to better understand the construction of the Marchenko equation solutions, the so-called focusing functions, in a mathematically simple and numerically stable fashion. The latter could be a challenge at large angles of incidence where the elastodynamic effects and evanescent waves start playing a dominant role. We demonstrate that the elastodynamic focusing functions are the bridge between the Marchenko equation theory and the transfer matrix formalism. Using the latter, we show how we can try to gain further insights into how time-reversal (correlations) behaves when either of the elastic modes becomes evanescent. We also show how this construction allows us to shed light on into the mathematical properties of elastodynamic inverse transmissions, which takes us a step closer towards understanding the elastodynamic minimum phase reconstruction.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geophysics and Petrophysic
Investigating the robustness of Green’s function retrieval via Marchenko focusing and Seismic Interferometry
No full textSeismic interferometry and Marchenko focusing are alternative techniques to retrieve the Green’s function between a virtual source in the subsurface and receivers at the surface. Seismic interferometry requires the presence of a receiver in the subsurface at the position of the virtual source, while Marchenko focusing utilizes only the reflection measurements at the Earth’s surface and an estimate of the direct wave from the virtual source to the acquisition level. I find that, for both methodologies, limited recording aperture of the acquisition array is a strong limiting factor when trying to retrieve events caused by interactions with curved scattering objects in the subsurface. It is also established that, given the dense sampling of the wave field, applying the Marchenko focusing scheme provides a more accurate retrieval of the Green’s functions in such scenarios. However, if the source and receiver arrays are subsampled, Marchenko focusing provides less robust retrieval of the accurate subsurface fields. Marchenko focusing also has more severe requirements on the reflection response. When an erroneous scaling of the amplitudes and/or a constant phase-shift is introduced the method fails to retrieve accurate subsurface wave fields without artifacts caused by internal multiples. I propose a workflow to calibrate the reflection response, prior to Green’s function retrieval via Marchenko focusing, using additional information in the form of a VSP dataset. First a virtual VSP dataset is estimated via Marchenko focusing, to subsequently compare its upgoing component to the upgoing part of the recorded VSP. Thereby, making it possible to correct for a constant phase shift applied to the reflection data. By identifying the minimum residual energy between the virtual VSP and the recorded VSP wave fields, an erroneous scaling of the reflection response can also be corrected. This workflow leads to a more robust Marchenko focusing approach where the reflection response can be redatumed to a target zone in the subsurface: the resulting ghost-free gathers, and ultimately images of the subsurface, show more illumination and improved resolution, leading to better delineation of thin stratigraphy as well as faulted structures.Civil Engineering and GeosciencesGeoscience & EngineeringMaster in Applied Geophysics - IDEA Leagu
Marchenko Methods in a Three-dimensional World
No full textMarchenko methods are a suite of geophysical techniques that convert seismograms of energy created by surface sources and measured by surface receivers into seismograms that would have been recorded by a virtual receiver at an arbitrary point inside the subsurface—an operation called redatuming. In principle these redatumed seismograms contain all contributions from direct, primary (singly-reflected) and multiply-reflected waves that would have been recorded by a real subsurface receiver, without requiring prior information about interfaces that generated the reflections. The potential of these methods for seismic imaging and redatuming has been demonstrated extensively in previous literature, but only using 1-D and 2-D Marchenko methods. There remain aspects of the methods that are poorly understood when applied in a 3-D world, so we investigate the application of Marchenko methods to 3-D data, subsurface structures and wavefields. We first show that for waves propagating in three dimensions, Marchenko methods can be applied to seismic data collected using both linear (so-called 2-D seismic) and areal (3-D seismic) acquisition arrays. However, for 2-D acquisition arrays the Marchenko workflow requires additional dimensionality correction factors to obtain accurate solutions, even in a subsurface that only varies with depth. Without these correction factors phase errors occur in redatumed Marchenko estimates; these errors propagate through the Marchenko algorithm and create depth errors in the Marchenko images. Furthermore, applying Marchenko methods to fully 3-D seismic wavefields recorded by linear (2-D seismic) arrays that contain out-of-plane reflections deteriorates surface-to-subsurface Green’s function estimates with spurious energy and resulting images are less accurate than those created using ‘conventional’ imaging methods. The application of fully 3-D Marchenko methods using data recorded on areal arrays solves both of the above problems, creating accurately redatumed wavefields and images with reduced artefact contamination. However, it appears that source–receiver spacing at most of λA/4 is required for accurate results using existing Marchenko methods, where λA is the dominant wavelength and in many real 3-D seismic acquisition scenarios this is impractical
Adaptation of the iterative Marchenko scheme for imperfectly sampled data
No full textThe Marchenko method retrieves the responses to virtual sources in the Earth's subsurface from reflection data at the surface, accounting for all orders of multiple reflections. The method is based on two integral representations for focusing- A nd Green's functions. In discretized form, these integrals are represented by finite summations over the acquisition geometry. Consequently, the method requires ideal geometries of regularly sampled and colocated sources and receivers. Recently new representations were derived, which handle imperfectly sampled data. These new representations use point spread functions (PSFs) that reconstruct results as if they were acquired using a perfect geometry. Here, the iterative Marchenko scheme is adapted, using these new representations, to account for imperfect sampling. This new methodology is tested on a 2-D numerical data example. The results show clear improvement of the proposed scheme over the standard iterative scheme. By removing the requirement for perfect geometries, the Marchenko method can be more widely applied to field data. Applied Geophysics and PetrophysicsImPhys/Medical Imagin
Iphidozercon altaicus Gwiazdowicz & Marchenko 2012, sp. n.
No full textIphidozercon altaicus sp. n. (Figs 1–4) <p>Description. Female (N = 3). Dorsum (Fig. 1). Dorsal shield oval, length 375–380 µm, width 230–250 µm distinct foveate sculpture throughout. 18 pairs of setae on podonotal part of shield and 14 pairs of setae on opisthonotal part of shield. All setae fine, smooth and pointed, length of 25–30 µm, except j1 (10 µm, inserted ventrally), and two antero-lateral setae s1, s2 (15 µm).</p> <p>Venter (Fig. 2) Tritosternum with trapezoidal base (25 µm) and finely pilose laciniae (35 µm). Sternal shield rectangular, 70 × 55 µm, setae st1–st3 smooth and pointed, length 10 µm. Metasternal setae st4 (10 µm) on soft membrane. Genital shield small and narrow (55 µm), spatulate posteriorly. Genital setae st5 (15 µm) outside the shield. Anal shield relatively large 60 µm long, 70 µm wide with para-anal setae (15 µm) and post-anal seta (20 µm). Narrow cribrum below post-anal seta. Sternal, genital and anal shields are unornamented. Peritremes ending anteriorly to coxae I, stigmata at level of coxae IV. Peritremal shields wide, with weak posterior lineate ornamentation. Opisthogastric integument behind coxae IV with one pair of oval metapodal plates, a pair of smaller plates near posterior ends of peritrematal shields. Opisthogastric setae JV1–JV5, ZV1–ZV2 15 µm long, others (R2–R4) approximately 20 µm.</p> <p>Gnathosoma. Hypostome with robust horn-like corniculi and four pairs of setae. Anterior seta h1 longest (30 µm), internal seta h3 (20 µm), palp coxal seta h4 (25 µm) shorter, external seta h2 (10 µm) shortest. Seven transverse rows of hypostomal denticles present, numbers of denticles per row (anterior to posterior) 12, 15, 17, 15, 17, 16, 13 (Fig. 3a). Chelicera typical of genus, fixed digit with three teeth, movable digit with two teeth (Fig. 3b), other details of chelicerae not visible in available specimens. Epistome with central prong longest, lateral prongs shorter, with denticulate outer margins (Fig. 3c).</p> <p>Legs and palps. Lengths of legs: I – 230 µm, II – 200 µm, III – 180 µm, IV – 210 µm. Setation of genua I–II–III–IV: 12–10–7–7 (Fig. 4a); tibiae 12–9–7–7 (Fig. 4b). Tarsus II to IV each with the dorsoproximal setae ad2 and pd2 short and straight (Fig 4c). Palp apotele 2-tined.</p> <p> Material examined: Holotype: Female. Russia, North-East of Altai Mountains, Teletskoe lake region, environs of Obogo village, in litter of <i>Betula pubescens – Populus tremula</i> forest, (51°30’48’’ N, 87°18’7’’ E, 900 m a.s.l.), 6 August 2007, leg. I.I. MARCHENKO. Paratypes: 2 females, North-East of Altai Mountains, Teletskoe lake region, environs of Obogo village, in litter of <i>Abies sibirica – Pinus sibirica</i> forest, (51°30’48’’N, 87°18’7’’E, 900 m a.s.l.), 6 August 2007, leg. I. I. MARCHENKO.</p> <p>Etymology. The name of this species reflects the fact that it was collected in the Altai Mountains.</p> <p> Differential diagnosis. <i>Iphidozercon altaicus</i> sp. n. is similar to <i>Iphidozercon foveatus</i> GWIAZDOWICZ et HALLIDAY, 2008. Both species have foveate sculpture on the dorsal shield and similar lengths of dorsal setae. The length of peritreme and the shape of genital shield is similar in both species. Nevertheless, many differences have been detected, such as shapes of the peritremal and anal shields. In <i>I. foveatus</i> the anal shield is narrow, while in <i>I. altaicus</i> it is wider than long. In <i>I. foveatus</i> the peritremal shield is wide, with tiny denticles on the internal side and in <i>I. altaicus</i> the shield is narrower and without denticles. In <i>I. foveatus</i> five pairs of smaller platelets bearing pores are located on the ventral side, in <i>I. altaicus</i> there are no such platelets. In <i>I. foveatus</i> the epistome has a central elongated prong ending in three denticles, but in <i>I. altaicus</i> the prong ends in spikes. In <i>I. foveatus</i> the movable digit has three teeth, but in <i>I. altaicus</i> it has two teeth.</p>Published as part of <i>Gwiazdowicz, D. J. & Marchenko, I. I., 2012, Two New Species Of Iphidozercon (Acari: Ascidae) With A Key To Females, pp. 41-52 in Acta Zoologica Academiae Scientiarum Hungaricae 58 (1)</i> on pages 42-44, DOI: <a href="http://zenodo.org/record/5732065">10.5281/zenodo.5732065</a>
The role of ubiquitination in the direct mitochondrial death program of p53
No full textp53 ubiquitination at C-terminal lysines by MDM2 and other E3 ligases had been considered a straightforward negative regulation of p53 with only one function, that is marking the protein for proteasomal degradation. In this review, we will focus on the recently uncovered activating role of ubiquitination in the transcription-independent direct mitochondrial death program of p53
The role of ubiquitination in the direct mitochondrial death program of p53
No full textp53 ubiquitination at C-terminal lysines by MDM2 and other E3 ligases had been considered a straightforward negative regulation of p53 with only one function, that is marking the protein for proteasomal degradation. In this review, we will focus on the recently uncovered activating role of ubiquitination in the transcription-independent direct mitochondrial death program of p53
Three-dimensional Marchenko equation for Green's function retrieval “beyond seismic interferometry”
No full textIn recent work we showed with heuristic arguments that the Green's response to a virtual source in the subsurface can be obtained from reflection data at the surface. This method is called “Green's function retrieval beyond seismic interferometry”, because, unlike in seismic interferometry, no receiver is needed at the position of the virtual source. Here we present a formal derivation of Green's function retrieval beyond seismic interferometry, based on a 3-D extension of the Marchenko equation. We illustrate the theory with a numerical example and indicate the potential applications in seismic imaging and AVA analysis.Geoscience & EngineeringCivil Engineering and Geoscience
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