bluuebunny/crossref_metadata_2025 huggingface dataset:
Reversibility of delirium in terminally ill patients and predictors of mortality
Authors: Leonard M , Raju B , Conroy M , Donnelly S , Trzepacz PT , Saunders J , Meagher D
Published: October 2008

In this study, factors related to reversibility and mortality in consecutive cases of Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) delirium [ n = 121] occurring in palliative care patients were evaluated. Delirium was assessed with the revised Delirium Rating Scale (DRS-R98) and Cognitive Test for Delirium (CTD). Patients were followed until recovery from delirium or death. In all, 33 patients (27%) recovered from delirium before death. Mean time until death was 39.7 ± 69.8 days in patients with reversible delirium [ n = 33] versus 16.8 ± 10.0 days in those with irreversible delirium [ n = 88; P < 0.01]. DRS-R98 and CTD scores were higher in irreversible delirium ( P < 0.001) with greater disturbances of sleep, language, long-term memory, attention, vigilance and visuospatial ability. Irreversible delirium was associated with greater disturbance of CTD attention and higher DRS-R98 visuospatial function. Survival time was predicted by CTD score ( P < 0.001), age ( P = 0.01) and organ failure ( P = 0.01). Delirium was not necessarily a harbinger of imminent death. Less reversible delirium involved greater impairment of attention, vigilance and visuospatial function. Survival time is related to age, severity of cognitive impairment and evidence of organ failure.
Measures of spiritual issues for palliative care patients: a literature review
Authors: Vivat B
Published: October 2008

Members of the Quality of Life Group (QLG) of the European Organisation for Research and Treatment of Cancer (EORTC) are developing a stand-alone functional measure of spiritual well being for palliative care patients, which will have both a clinical and a measurement application. This article discusses data from a literature review, conducted at two time points as part of the development process of this instrument. The review identified 29 existing measures of issues relating to patients’ spirituality or spiritual well being. Twenty two are stand-alone measures of which 15 can be categorised as substantive (investigating the substance of respondents’ beliefs) and seven as functional (exploring the function those beliefs serve). However, perhaps owing to the lack of consensus concerning spirituality or spiritual well being, the functional measures all have different (although sometimes overlapping) dimensions. In addition, they were all developed in a single cultural context (the United States), often with predominantly Christian participants, and most were not developed with palliative care patients. None is, therefore, entirely suitable for use with palliative care patients in the United Kingdom or continental Europe.
Clinician perceived good practice in end-of-life care for patients with COPD
Authors: Roberts CM , Seiger A , Buckingham RJ , Stone RA
Published: October 2008

Patients with chronic obstructive pulmonary disease (COPD) have significant end-of-life needs, but are much less likely than patients with cancer to access or receive appropriate palliative care. Little is known about the existing availability or quality of available services within the United Kingdom. We surveyed 100 NHS acute hospitals enquiring into the provision of care for patients with COPD and requesting examples of current good practice that might be used to set standards. Forty-two percent of hospitals had formal palliative care arrangements for patients with COPD, whereas 59% had plans to develop or further develop services. Analysis of qualitative data suggested four strands that highlighted good practice; teams, care pathways, service components and linkages. These data may help to inform the debate leading to the development of standards in end-of-life care for patients with COPD.
The management of constipation in palliative care: clinical practice recommendations
Authors: Larkin PJ , Sykes NP , Centeno C , Ellershaw JE , Elsner F , Eugene B , Gootjes JRG , Nabal M , Noguera A , Ripamonti C , Zucco F , Zuurmond WWA ,   On behalf of The European Consensus Group on Constipation in Palliative Care
Published: October 2008

Constipation is one of the most common problems in patients receiving palliative care and can cause extreme suffering and discomfort. The aims of this study are to raise awareness of constipation in palliative care, provide clear, practical guidance on management and encourage further research in the area. A pan-European working group of physicians and nurses with significant experience in the management of constipation in palliative care met to evaluate the published evidence and produce these clinical practice recommendations. Four potentially relevant publications were identified, highlighting a lack of clear, practical guidance on the assessment, diagnosis and management of constipation in palliative care patients. Given the limited data available, our recommendations are based on expert clinical opinion, relevant research findings from other settings and best practice from the countries represented. Palliative care patients are at a high risk of constipation, and while general principles of prevention should be followed, pharmacological treatment is often necessary. The combination of a softener and stimulant laxative is generally recommended, and the choice of laxatives should be made on an individual basis. The current evidence base is poor and further research is required on many aspects of the assessment, diagnosis and management of constipation in palliative care.
Improving Problem Solving of Elementary Students With Mild Disabilities
Authors: Glago Karen , Mastropieri Margo A. , Scruggs Thomas E.
Published: October 2008

Twenty-one 4th- and 5th-grade students with learning disabilities and emotional disabilities were assigned at random to a control condition or to an experimental condition in which they were taught, over a 9-week period, a five-step self-determination strategy for solving school- or home-related problems. Maintenance was assessed 3 weeks after the completion of the posttests. Results revealed that students in the experimental condition statistically outperformed students in the control condition in their abilities to learn a problem-solving strategy and to apply that strategy to scenarios. Experimental condition students also demonstrated their ability to generalize the use of that strategy to a classroom problem and retained their learning on a 3-week postintervention maintenance test. Results are discussed in terms of future research and implications for practice.
Circadian Phase Resetting in Response to Light-Dark and Dark-Light Transitions
Authors: Comas M. , Beersma D.G.M. , Hut R.A. , Daan S.
Published: October 2008

Phase shifting of circadian systems by light has been attributed both to parametric effects on angular velocity elicited by a tonic response to the luminance level and to nonparametric instantaneous shifts induced by a phasic response to the dark-light (D > L) and light-dark (L > D) transitions. Claims of nonparametric responses are partly based on “step-PRCs,” that is, phase response curves derived from such transitions. Step-PRCs in nocturnal mammals show mostly delays after lights-on and advances after lights-off, and therefore appear incompatible with phase delays generated by light around dusk and advances by light around dawn. We have pursued this paradox with 2 experimental protocols in mice. We first use the classic step-PRC protocol on wheel running activity, using the center of gravity as a phase marker to minimize the masking effects of light. The experiment was done for 3 different light intensities (1, 10, and 100 lux). D > L transitions evoke mostly delays and L > D transitions show no clear tendency to either delay or advance. Overall there is little or no circadian modulation. A 2nd protocol aimed to avoid the problem of masking by assessing phase before and after the light stimuli, both in DD. Light stimuli consisted of either a slow light intensity increase over 48 h followed by abruptly switching off the light, or an abrupt switch on followed by a slow decrease toward total darkness during 48 h. If the abrupt transitions were responsible for phase shifting, we expected large differences between the 2 stimuli. Both light stimuli yielded similar PRCs characterized by delays only with circadian modulation. The results can be adequately explained by a model in which all PRCs evoked by steps result in fact from tonic responses to the light following a step-up or preceding a step-down. In this model only the response reduction of tonic velocity change after the 1st hour is taken into account. The data obtained in both experiments are thus compatible with tonic velocity responses. Contrary to standard interpretation of step-PRCs, nonparametric responses to the transitions are unlikely since they would predict delays in response to lights-off, advances in response to lights-on, while the opposite was found. Although such responses cannot be fully excluded, parsimony does not require invocation of a role for transitions, since all the data can readily be explained by tonic velocity (parametric) effects, which must exist because of the dependence of τ on light intensity.
Maternal Control of the Fetal and Neonatal Rat Suprachiasmatic Nucleus
Authors: El-Hennamy Rehab , Matějů Kristýna , Bendová Zdena , Sosniyenko Serhiy , Sumová Alena
Published: October 2008

The molecular clockwork underlying the generation of circadian rhythmicity within the suprachiasmatic nucleus (SCN) develops gradually during ontogenesis. The authors' previous work has shown that rhythms in clock gene expression in the rat SCN are not detectable at embryonic day (E) 19, start to form at E20 and develop further via increasing amplitude until postnatal day (P) 10. The aim of the present work was to elucidate whether and how swiftly the immature fetal and neonatal molecular SCN clocks can be reset by maternal cues. Pregnant rats maintained under a light-dark (LD) regimen with 12 h of light and 12 h of darkness were exposed to a 6-h delay of the dark period and released into constant darkness at different stages of the fetal SCN development. Adult rats maintained under the same LD regimen were exposed to an identical shifting procedure. Daily rhythms in spontaneous c-fos, Avp, Per1, and Per2 expression were examined within the adult and newborn SCN by in situ hybridization. Exposure of adult rats to the shifting procedure induced a significant phase delay of locomotor activity within 3 days after the phase shift as well as a delay in the rhythms of c-fos and Avp expression within 3 days and Per1 and Per2 expression within 5 days. Exposure of pregnant rats to the shifting procedure at E18, but not at E20, delayed the rhythm in c-fos and Avp expression in the SCN of newborn pups at P0-1. The shifting procedure at E20 did, however, induce a phase delay of Per1 and Per2 expression rhythms at P3 and P6. Hence, 5 days were necessary for phase-shifting the pups' SCN clock by maternal cues, be it the interval between E18 and P0-1 or the interval between E20 and P3, while only 3 days were necessary for phase-shifting the maternal SCN by photic cues. These results demonstrate that the SCN clock is capable of significant phase shifts at fetal developmental stages when no or very faint molecular oscillations can be detected.
Disclosing Sexual Assault to Parents
Authors: Smith Sharon G. , Cook Sarah L.
Published: October 2008

Without frank discussion of what sex is, women may not learn what sex is not and what experiences constitute sexual assault. This qualitative study explores the relation between parental discussion and messages about sex and women's decisions of whether to disclose sexual assault to parents. Participants were 18 women from diverse ethnic backgrounds. Data were analyzed using a grounded theory approach. Findings indicate that women more often disclosed sexual assault to parents who discussed sex with them in a frank and positive manner. In addition to the role of disclosure in recovery, implications for sex and parent education are discussed.
Frequency and Perceived Effectiveness of Strategies to Survive Abuse Employed by Battered Mexican-Origin Women
Authors: Brabeck Kalina M. , Guzmán Michele R.
Published: October 2008

This study documented the frequency and perceived effectiveness of battered Mexican-origin women's usage of formal and informal help and personal strategies to survive abuse. Semistructured interviews were conducted with 75 battered Mexican-origin women. Consistent with survivor theory, results indicated that participants sought help multiple times from several formal and informal help sources; some (i.e., shelter, family) were perceived more effective than others (i.e., lawyer, in-laws). Participants engaged in various personal strategies to survive abuse; some (i.e., using faith/religion) were rated more effective than others (i.e., placating the batterer). Responses to open-ended questions suggest why specific help sources and strategies were/were not effective and provide socioculturally specific suggestions for improving services. This study illuminates battered Mexican-origin women's strengths and barriers that impede their survival efforts. Contributions include focusing on a subset of battered Latinas and documenting the frequency and perceived effectiveness of a wide array of help sources and strategies.
Sex Education in Croatia
Authors: Bijelić Nataša
Published: October 2008

This article explores the influence of the Catholic church on educational policy, more specifically on sex education, in Croatia. It explores tensions between secular and religious discourses regarding the introduction of a sex education programme supported by the Catholic church into Croatian schools. The presence of the Catholic doctrine in the educational system provided the basis for the introduction of sex education with a religious framework, namely the GROZD sex education programme. The GROZD (Glas roditelja za djecu [Parents' Voice for Children]) programme triggered a public discussion that soon became a polarization between liberal (i.e. secular) and traditional (i.e. religious) discourses. In the discussion, the traditional/Catholic value system was used as a justification for the introduction of a sex education programme that was seen by opponents as harmful.
Young Women, Sexuality and Protestant Church Community
Authors: Sharma Sonya
Published: October 2008

Although Christianity's clout on sexuality has generally declined in Britain due to secularization, contemporary conservative Protestantism continues to encourage a conventional construction of sexuality — sex is only for the context of heterosexual marriage. Qualitative interviews with 26 heterosexual women and two lesbian women on how their Protestant church involvement impacted their sexuality revealed the pervasive discourse of a marital-confined sexuality and participants' sense of `accountability' to the group for carrying this out. Such accountability can result in a repressed sexuality that is oppressive. Alternatively, it can produce a `sense of community' that functions as a source of empowerment. The oppressive and empowering nature of accountability, however, can be difficult to untangle because of women's religious commitment and supportive friendships found in church life. Relying on participants' accounts, this article addresses the oppression and empowerment young women experience when they negotiate their gendered identities in relation to a marital-confined sexuality.
On Women, Egg Cells and Embryos
Authors: Kirejczyk Marta
Published: October 2008

In contrast to many countries, the political debates in the Netherlands on reproductive technologies and embryo research have paid particular attention to the issue of health risks to women. This article focuses on the question to what extent the discourse of gender has contributed to shaping the space for embryonic (stem cell) research in this country. The author argues that in the policy arena flexible conceptualizations of risks and burdens to women and of the identities of embryos have been crucial in drawing and maintaining a discursive boundary between the domains of medically assisted reproduction and embryo research. The author shows that feminist intervention challenging this discursive strategy has had an impact on the present regulation of research. However, the main weakness of this temporary and partial feminist success is the virtual absence of a public debate on the meaning and desirability of embryo research.
All's Fair in Love and War?
Authors: Blaney Aileen
Published: October 2008

This article investigates the textual strategies with which Maeve Murphy's Silent Grace addresses viewers in contemporary Northern Ireland. Borrowing Eric Santner's concept of `narrative fetishism', the analysis examines how the film's representation of the past obscures the historical realities experienced by female political prisoners in Armagh jail in the late 1970s and early 1980s. From this standpoint, its ethical relation to historical `truth' and responsibilities to its local audience are debated.
Thyroid function following hematopoietic cell transplantation in children: 30 years' experience
Authors: Sanders Jean E. , Hoffmeister Paul A. , Woolfrey Ann E. , Carpenter Paul A. , Storer Barry E. , Storb Rainer F. , Appelbaum Frederick R.
Published: October 2008

Thyroid dysfunction is a known complication after hematopoietic cell transplantation (HCT) in children with reports involving relatively short follow-up and small patient numbers. This study involves 791 patients less than 18 years of age at HCT at the Fred Hutchinson Cancer Research Center with follow-up from 1969 through 2007. Thyroid dysfunction continued for 28 years after transplantation. Hypothyroidism was the most common abnormality with other abnormalities of hyperthyroidism and thyroiditis. Multivariate analysis showed that thyroid dysfunction was more likely if patients were less than 10 years of age (P < .001), but there was no difference between receiving a total body irradiation or busulfan based regimens (P = .48) compared with cyclophosphamide conditioning alone (P = .008). Thyroid tumors occurred at a median of 9.9 (4.5-22.3) years after HCT and included 13 with papillary carcinoma and 5 with benign adenomas. Children who receive a HCT should be monitored for thyroid abnormalities throughout life.
Seismic velocity model building in an area of complex geology, southern Alberta, Canada
Authors: Isaac J. Helen , Lawton Don C.
Published: October 2008

We developed velocity models to prestack depth migrate two seismic lines acquired in an area of complex mountainous geology in southern Alberta, Canada. Initial processing in the time domain was designed to attenuate noise and enhance the signal in the data. The prestack and poststack time-migrated sections were poorly focused, implying the velocity models would be inadequate for prestack depth migration. The velocity models for prestack depth migration, developed by flattening reflections on common image gathers, ineffectively imaged the complex geology. We developed our most effective velocity models by integrating the mapped surface geology and dips, well formation tops, geological cross sections, and seismic-velocity information into the interpretation of polygonal areas of constant velocity on several iterations of prestack depth-migrated seismic sections. The resulting depth-processed sections show a more geologically realistic geometry for the reflectors at depth and achieve better focusing than either the time-migrated sections or the depth sections migrated with velocity models derived by flattening reflections on offset gathers.
Comparison of prestack stereotomography and NIP wave tomography for velocity model building: Instances from the Messinian evaporites
Authors: Dümmong Stefan , Meier Kristina , Gajewski Dirk , Hübscher Christian
Published: October 2008

Velocity-model determination during seismic data processing is crucial for any kind of depth imaging. We compared two approaches of grid tomography: prestack stereotomography and normal-incidence-point (NIP) wave tomography. Whereas NIP wave tomography is based on wavefield attributes obtained during the common reflection surface stack and thus on the underlying hyperbolic second-order traveltime approximation, prestack stereotomography describes traveltimes by local slopes (i.e., linearly) in the prestack data domain. To analyze the impact of the different traveltime approximations and the different input-data domains on velocity model building, we applied two implementations of these techniques to two profiles of a field marine data set from the Levante Basin, eastern Mediterranean. Because ofthe presence of a thick, tabular mobile unit of the Messinian evaporites, strong vertical and lateral velocity contrasts had been expected. The velocity models revealed the reconstruction of high-velocity contrasts by grid tomographic methods is limited because of the smooth description of the velocity distribution. The lateral resolution of velocities obtained from prestack stereotomography appears to be better than those from NIP wave tomography, which is related to the difference in the approximation of traveltimes, the determination of input data, and the description of the velocity distribution. Other differences are caused mainly by different implementations of the inversion schemes. Nevertheless, both algorithms provide suitable models for high-quality depth imaging, whereas most of the reflections are fairly flat in CIGs.
Assessing estimated velocity-depth models: Finding error bars in tomographic inversion
Authors: Chiţu D. A. , Al-Ali M. N. , Verschuur D. J.
Published: October 2008

In conventional migration velocity analysis methods, a velocity model is estimated that results in flattened events in common-image gathers. However, after this process, no information is available on the accuracy of this velocity model. A statistical analysis of velocity-model parameters is very difficult because of the integrated nature of the process. In common-focus-point technology, velocity estimation is split into two processes: a first step to estimate one-way focusing operators from the seismic data and a second step to translate these one-way propagation operators into a velocity-depth model. Because the second step does not involve seismic data and uses a hands-off model parameterization, a statistical analysis of the inversion result becomes rather straightforward. We developed a methodology for obtaining a suite of possible solutions, from which statistical measures can be extracted. By varying initial settings, the inversion of one-way traveltimes provides a space of solutions. Rather than having a single estimated model, we can obtain an ensemble of models. By performing statistical analysis of this ensemble, the error bars of the estimated velocity model can be retrieved. The procedure was tested for a 2D synthetic and field data set, for which the latter compares favorably to a conventional two-way traveltime tomography approach. The information provided by such an analysis is important because it shows the reliability of the final estimated model and could provide feedback for acquisition geometry design. More or better data might be needed to obtain a model to which a smaller degree of ambiguity is associated.
Stereotomography
Authors: Lambaré Gilles
Published: October 2008

Stereotomography was proposed [Formula: see text] ago for estimating velocity macromodels from seismic reflection data. Initially, the goal was to retain the advantages of standard traveltime tomography while providing an alternative to difficult interpretive traveltime picking. Stereotomography relies on the concept of locally coherent events characterized by their local slopes in the prestack data cube. Currently, stereotomography has been developed in two and three dimensions, and precious experience has been gained. The expected advantages have been demonstrated fully (in particular, the efficiency and reliability of the semiautomatic stereotomographic picking strategies), and further studies have increased the method’s potential and flexibility. For example, stereotomographic picking can now be done in either the prestack or poststack domain, in either the time (migrated or unmigrated) or depth domain. It appears that the theoretical frame of stereotomography can reconcile, very satisfactorily and efficiently, most methods proposed for velocity-macromodel estimation for depth imaging. Moreover, an extension of the method to full-waveform inversion already exists and opens the way for very interesting developments.
3D prestack plane-wave, full-waveform inversion
Authors: Vigh Denes , Starr E. William
Published: October 2008

Prestack depth migration has been used for decades to derive velocity distributions in depth. Numerous tools and methodologies have been developed to reach this goal. Exploration in geologically more complex areas exceeds the abilities of existing methods. New data-acquisition and data-processing methods are required to answer these new challenges effectively. The recently introduced wide-azimuth data acquisition method offers better illumination and noise attenuation as well as an opportunity to more accurately determine velocities for imaging. One of the most advanced tools for depth imaging is full-waveform inversion. Prestack seismic full-waveform inversion is very challenging because of the nonlinearity and nonuniqueness of the solution. Combined with multiple iterations of forward modeling and residual wavefield back propagation, the method is computer intensive, especially for 3D projects. We studied a time-domain, plane-wave implementation of 3D waveform inversion. We found that plane-wave gathers are an attractive input to waveform inversion with dramatically reduced computer run times compared to traditional shot-gather approaches. The study was conducted on two synthetic data sets — Marmousi2 and SMAART Pluto 1.5 — and a field data set. The results showed that a velocity field can be reconstructed well using a multiscale time-domain implementation of waveform inversion. Although the time-domain solution does not take advantage of wavenumber redundancy, the method is feasible on current computer architectures for 3D surveys. The inverted velocity volume produces a quality image for exploration geologists by using numerous iterations of waveform inversion.
Residual depth moveout and velocity determination for parametric media
Authors: Schneider Joerg
Published: October 2008

For the case when prestack depth migration has been performed with inaccurate velocities, it can be shown that second-order approximations can be used to predict the residual depth moveout of common-image and angle-domain common-image gathers. Corrections may be made in terms of offset and common-reflection angles that can be estimated from the migrated data using moveout analysis schemes. The residual moveout determined can be used to improve the signal-to-noise ratio of the migrated data and to determine the variation of parameters of well-known formation-dependent velocity laws away from borehole positions. Although the method is developed for small offsets, reflection angles, and velocity differences, realistic examples may be used to demonstrate that satisfactory approximations to the residual moveout curve can be obtained over a relatively wide range of offsets or reflection angles and for moderate velocity differences. A zero-offset inversion was applied to the estimated residual radii of curvature. This inversion was performed layerwise. For each layer, the velocity was assumed to vary according to a known depth-varying and a laterally varying component to be determined. Iterative applications of the suggested approach have rendered satisfactory results for several representative velocity models. For each application, appropriate initial models were provided. Applications to initial models with large velocity differences and to seismic data with large offsets have to be investigated.
Use of refraction, reflection, and wave-equation-based tomography for imaging beneath shallow gas: A Trinidad field data example
Authors: Kabir Nurul , Albertin Uwe , Zhou Min , Nagassar Vishal , Kjos Einar , Whitaker Phillip , Ford Alan
Published: October 2008

Shallow localized gas pockets cause challenging problems in seismic imaging because of sags and wipe-out zones they produce on imaged reflectors deep in the section. In addition, the presence of shallow gas generates strong surface-related and interbed multiples, making velocity updating very difficult. When localized gas pockets are very shallow, we have limited information to build a near-surface velocity model using ray-based reflection tomography alone. Diving-wave refraction tomography successfully builds a starting model for the very shallow part. Usual ray-based reflection tomography using single-parameter hyperbolic moveout might need many iterations to update the deeper part of the velocity model. In addition, the method generates a low-velocity anomaly in the deeper part of the model. We present an innovative method for building the final velocity model by combining refraction, reflection, and wave-equation-based tomography. Wave-equation-based tomography effectively generates a detailed update of a shallow velocity field, resolving the gas-sag problem. When applied as the last step, following the refraction and reflection tomography, it resolves the gas-sag problem but fails to remove the low-velocity anomaly generated by the reflection tomography in the deeper part of the model. To improve the methodology, we update the shallow velocity field using refraction tomography followed by wave-equation tomography before updating the deeper part of the model. This step avoids generating the low-velocity anomaly. Refraction and wave-equation-based tomography followed by reflection tomography generates a simpler velocity model, giving better focusing in the deeper part of the image. We illustrate how the methodology successfully improves resolution of gas anomalies and significantly reduces gas sag from an imaged section in the Greater Cassia area, Trinidad.
Multiscale migration tomography to constrain depth-imaging artifacts
Authors: Cao Hongmei , Xie Jing , Kim Young , Zhou Hua-wei
Published: October 2008

Multiscale tomography (MST) can constrain depth-imaging artifacts that appear in the velocity field inverted using conventional single-scale tomography (SST). Depth-imaging artifacts are poorly constrained velocity results that cause defocusing and/or incorrect structures in the migrated stack. The quality of an inverted velocity distribution from a tomographic approach depends heavily on ray coverage. In areas of poor ray coverage with a small ray count or parallel rays, the velocity distribution can be nonunique because of the lack of constraints. In the case of large residual moveouts from prestack migration, which most likely could happen in the presence of a complex geologic setting, serious depth-imaging artifacts can appear in the velocity field inverted using SST. To solve this problem, we tested the ability of MST to constrain artifacts in a case study. MST parameterizes the study area into overlapping submodels of different grid sizes. The inverted velocity-perturbation value for a specific location is the superposition of velocity-perturbation values from those submodels. The submodel with a large grid size provides a stable regional value that helps constrain local artifacts from submodels of smaller grid sizes. Artifacts using SST are suppressed significantly by MST. Migrated stack and common-image gathers indicated improvements in the velocity field from MST compared with the initial velocity field and that from SST.
Numeric implementation of wave-equation migration velocity analysis operators
Authors: Sava Paul , Vlad Ioan
Published: October 2008

Wave-equation migration velocity analysis (MVA) is a technique similar to wave-equation tomography because it is designed to update velocity models using information derived from full seismic wavefields. On the other hand, wave-equation MVA is similar to conventional, traveltime-based MVA because it derives the information used for model updates from properties of migrated images, e.g., focusing and moveout. The main motivation for using wave-equation MVA is derived from its consistency with the corresponding wave-equation migration, which makes this technique robust and capable of handling multipathing characterizing media with large and sharp velocity contrasts. The wave-equation MVA operators are constructed using linearizations of conventional wavefield extrapolation operators, assuming small perturbations relative to the background velocity model. Similar to typical wavefield extrapolation operators, the wave-equation MVA operators can be implemented in the mixed space-wavenumber domain using approximations of differentorders of accuracy. As for wave-equation migration, wave-equation MVA can be formulated in different imaging frameworks, depending on the type of data used and image optimization criteria. Examples of imaging frameworks correspond to zero-offset migration (designed for imaging based on focusing properties of the image), survey-sinking migration (designed for imaging based on moveout analysis using narrow-azimuth data), and shot-record migration (also designed for imaging based on moveout analysis, but using wide-azimuth data). The wave-equation MVA operators formulated for the various imaging frameworks are similar because they share elements derived from linearizations of the single square-root equation. Such operators represent the core of iterative velocity estimation based on diffraction focusing or semblance analysis, and their applicability in practice requires efficient and accurate implementation. This tutorial concentrates strictly on the numeric implementation of those operators and not on their use for iterative migration velocity analysis.
A comparison between the behavior of objective functions for waveform inversion in the frequency and Laplace domains
Authors: Shin Changsoo , Ha Wansoo
Published: October 2008

In the frequency domain, gradient-based local-optimization methods of waveform inversions have been unsuccessful at inverting subsurface parameters without an accurate starting model. Such methods could not correct automatically for poor starting models because multiple local minima made it difficult to approach the true global minimum. In this study, we compared the behavior of objective functions in the frequency and Laplace domains. Wavefields in the Laplace domain correspond to the zero-frequency component of a damped wavefield; thus, the Laplace-domain waveform inversion can image smooth velocity models. Objective functions in the Laplace-domain inversion have a smoother surface and fewer local minima than in the frequency-domain inversion. We applied the waveform inversion to a 2D slice of the acoustic SEG/EAGE salt model in the Laplace domain and recovered smooth velocity models from inaccurate initial velocity conditions. We also successfully imaged velocities of the salt, SEG overthrust, and Institut Francais du Petrole Marmousi models with the frequency-domain inversion method by using the inverted velocity model of the Laplace-domain inversion as the initial model.
Unified imaging of multichannel seismic data: Combining traveltime inversion and prestack depth migration
Authors: Jaiswal Priyank , Zelt Colin A.
Published: October 2008

Imaging 2D multichannel land seismic data can be accomplished effectively by a combination of traveltime inversion and prestack depth migration (PSDM), referred to as unified imaging. Unified imaging begins by inverting the direct-arrival times to estimate a velocity model that is used in static corrections and stacking velocity analysis. The interval velocity model (from stacking velocities) is used for PSDM. The stacked data and the PSDM image are interpreted for common horizons, and the corresponding wide-aperture reflections are identified in the shot gathers. Using the interval velocity model, the stack interpretations are inverted as zero-offset reflections to constrain the corresponding interfaces in depth; the interval velocity model remains stationary. We define a coefficient of congruence [Formula: see text] that measures the discrepancy between horizons from the PSDM image andtheir counterparts from the zero-offset inversion. A value of unity for [Formula: see text] implies that the interpreted and inverted horizons are consistent to within the interpretational uncertainties, and the unified imaging is said to have converged. For [Formula: see text] greater than unity, the interval velocity model and the horizon depths are updated by jointly inverting the direct arrivals with the zero-offset and wide-aperture reflections. The updated interval velocity model is used again for both PSDM and a zero-offset inversion. Interpretations of the new PSDM image are the updated horizon depths. The unified imaging is applied to seismic data from the Naga Thrust and Fold Belt in India. Wide-aperture and zero-offset data from three geologically significant horizons are used. Three runs of joint inversion and PSDM are required in a cyclic manner for [Formula: see text] to converge to unity. A joint interpretation of the final velocity model and depth image reveals the presence of a triangle zone that could be promising for exploration.
Automated velocity model building with wavepath tomography
Authors: Fliedner Moritz M. , Bevc Dimitri
Published: October 2008

Wavefield-continuation migration generally is recognized as superior to Kirchhoff methods in complex velocity models, such as below rugose salt bodies. It accounts for multipathing, sharp velocity contrasts, and the limited bandwidth of seismic wave propagation. Wavepath tomography builds the velocity model in a way that is consistent with the wavefield-migration operator. Traveltime residuals are back-projected along a wavepath instead of rays. The actual wavefield-continuation operator is used to represent the wave propagation between surface source/receiver pairs and subsurface reflection points. A wavepath is obtained by multiplying impulse responses from a surface location and a reflection point. The inversion matrix is kept to a manageable size by restricting the wavepath to the first Fresnel zone. The considerable expense of computing a single wavepath kernel in comparison to ray tomography is partially offset by the smaller number of back projections necessary to sample the velocity model adequately. We have used wavepath tomography to build subsalt velocity models using 2D synthetic data. Wavepath tomography was implemented in 3D.
Resolving near-seabed velocity anomalies: Deep water offshore eastern India
Authors: Fruehn Juergen , Jones Ian F. , Valler Victoria , Sangvai Pranaya , Biswal Ajoy , Mathur Mohit
Published: October 2008

Imaging in deep-water environments poses a specific set of challenges, both in data preconditioning and velocity model building. These challenges include scattered, complex 3D multiples, aliased noise, and low-velocity shallow anomalies associated with channel fills and gas hydrates. We describe an approach to tackling such problems for data from deep water off the east coast of India, concentrating our attention on iterative velocity model building, and more specifically the resolution of near-surface and other velocity anomalies. In the region under investigation, the velocity field is complicated by narrow buried canyons ([Formula: see text] wide) filled with low-velocity sediments, which give rise to severe pull-down effects; possible free-gas accumulation below an extensive gas-hydrate cap, causing dimming of the image below (perhaps as a result of absorption); and thin-channel bodies with low-velocity fill. Hybrid gridded tomography using a conjugate gradient solver (with [Formula: see text] vertical cell size) was applied to resolve small-scale velocity anomalies (with thicknesses of about [Formula: see text]). Manual picking of narrow-channel features was used to define bodies too small for the tomography to resolve. Prestack depth migration, using a velocity model built with a combination of these techniques, could resolve pull-down and other image distortion effects in the final image. The resulting velocity field shows high-resolution detail useful in identifying anomalous geobodies of potential exploration interest.
Tomographic inversion by matrix transformation
Authors: Liu Jonathan , Bear Lorie , Krebs Jerry , Montelli Raffaella , Palacharla Gopal
Published: October 2008

We have developed a new method to build seismic velocity models for complex structures. In our approach, we use a spatially nonuniform parameterization of the velocity model in tomography and a uniform grid representation of the same velocity model in ray tracing to generate the linear system of tomographic equations. Subsequently, a matrix transformation is applied to the system of equations to produce a new linear system of tomographic equations using nonuniform parameterization. In this way, we improved the stability of tomographic inversion without adding computing costs. We tested the effectiveness of our process on a 3D synthetic data example.
Wave tracing: Ray tracing for the propagation of band-limited signals: Part 1 — Theory
Authors: Bube Kenneth P. , Washbourne John K.
Published: October 2008

Many seismic imaging techniques require computing traveltimes and travel paths. Methods to compute raypaths are usually based on high-frequency approximations. In situations such as head waves, these raypaths minimize traveltime but are not paths along which most of the energy travels. We have developed a new approach to computing raypaths, using a modification of ray bending that we call wave tracing; it computes raypaths and traveltimes that are more consistent with the paths and times for the band-limited signals in real data than the paths and times obtained using high-frequency approximations. Wave tracing shortens the raypath while keeping the raypath within the Fresnel zone for a characteristic frequency of the signal.
Automatic wave-equation migration velocity inversion using multiobjective evolutionary algorithms
Authors: Singh Vijay Pratap , Duquet Bertrand , Léger Michel , Schoenauer Marc
Published: October 2008

To solve nonlinear seismic velocity inversion problems, we have developed an automatic velocity-estimation technique based on the stochastic method called multiobjective evolutionary algorithms (MOEA). Semblance and differential semblance are used as objective functions. To cope with the high computational cost, we customized MOEA, added domain knowledge (velocity increases with depth, slowly varies along layers, and so forth), which improves the conditioning of the problem and accelerates convergence. This approach is robust because it can cope with large velocity errors. Computational cost of this algorithm is at least two orders of magnitude faster than other stochastic methods and comparable to that of direct gradient methods.
Regularization in slope tomography
Authors: Costa Jessé C. , da Silva Francisco J. , Gomes Ellen N. , Schleicher Jörg , Melo Luiz André , Amazonas Daniela
Published: October 2008

Seismic imaging in depth is limited by the accuracy of velocity model estimation. Slope tomography uses the slowness components and traveltimes of picked reflection or diffraction events for velocity model building. The unavoidable data incompleteness requires additional information to assure stability to inversion. One natural constraint for ray-based tomography is a smooth velocity model. We propose a new, reflection-angle-based kind of smoothness constraint as regularization in slope tomography and have compared its effects to three other, more conventional constraints. The effects of these constraints were evaluated through angle-domain common-image gathers, computed with wave-equation migration using the estimated velocity model. We found that the smoothness constraints have a distinct effect on the velocity model but a weaker effect on the migrated data. In numerical tests on synthetic data, the new constraint leads to geologically more consistent models.
CRS-based depth model building and imaging of 3D seismic data from the Gulf of Mexico Coast
Authors: Pruessmann Juergen , Frehers Sven , Ballesteros Rodolfo , Caballero Alfredo , Clemente Gerardo
Published: October 2008

A seismic depth-imaging project starts from an initial depth model of interval velocities. From time processing of reflection seismic data, a set of stacking parameters or kinematic data attributes usually is available for an initial model building at little effort. Two methods for initial model building from time-processing attributes are compared in this case study, using 3D seismic land data from the coast of the Gulf of Mexico. Conventional normal moveout (NMO)/dip moveout (DMO) time processing performs one-parametric stacking using stacking velocity as the parameter. The stacking velocity field can be converted into a depth model by the well-known vertical Dix inversion, which is very fast and robust but degrades with increasing dip. Common-reflection surface (CRS) time processing, on the contrary, isbased on the multiparametric CRS stacking approach, providing several volumes of CRS-stacking attributes that include the wavefield dip, or horizontal slowness. Inversion of CRS attributes by CRS tomography incorporates this dip information in depth model building. In this case study, CRS or normal-incidence point (NIP) wave tomography is presented as a model-building link between high-resolution CRS time processing and subsequent depth processing. The CRS tomography model shows a better adaptation to the dipping subsurface structures than the Dix model and a good fit to well data. The smooth tomography model is well suited for further use in poststack and prestack depth migrations. It provides a good starting point for iterative model enhancement and salt-body definition in prestack depth migration.
Resolution of slowness and reflectors in crosswell tomography with transmission and reflection traveltimes
Authors: Bube Kenneth P. , Langan Robert T.
Published: October 2008

We sometimes encounter situations in seismic imaging in which knowing the position of key reflectors between wells would be very useful. In many crosswell data sets, both transmission and reflection traveltimes for selected reflectors can be picked. We investigated the possibility that transmission-plus-reflection crosswell traveltime tomography can determine the position of these reflectors with a high level of accuracy, thereby providing an independent way of verifying (and perhaps improving) the position of these reflectors obtained from crosswell reflection imaging. We studied the effect of combining reflection traveltimes for selected reflectors with transmission traveltimes on the resolution of the interwell slowness field and depth determination of selected reflectors. We found that theoretically, the position of reflectors is determined uniquely from transmission and reflection traveltimes in a linearized continuum formulation ofcrosswell tomography. We also computed diagonal elements of the resolu-tion matrix for two crosswell geometries based on field experiments conducted in a west Texas oil field to see what effect noise has on the accuracy of our determination of reflector depths. These computational results indicate that reflector positions are indeed very well determined for these geometries, with expected errors of [Formula: see text] of the well spacing when noise in traveltimes is [Formula: see text]. Because reflector-position parameters are so well determined, including reflection traveltimes does not degrade the resolution of the slowness field as a result of introducing additional reflector-depth parameters. Actually, the resolution of the slowness field, particularly near reflectors, improves by including reflection traveltimes, in spite of the fact that we must solve for these additional depth parameters. The improvement in slowness resolution should provide velocity models that can yield more accurate reflection images.