IMPROVING PRE-SALT RESERVOIRS SEISMIC IMAGES WHEN CONSIDERING THE STRATIFIED EVAPORITES INSERTION IN THE INITIAL MODEL FOR THE VELOCITY UPDATING PROCESSES PRIOR TO THE SEISMIC MIGRATION

ABSTRACT. Structurally complex areas, such as the pre-salt section in the offshore Santos Basin, SE Brazil, is a challenge to represent the geology using seismic images. One of the main causes of the observed imaging problems is the evaporitic section and its considerations about velocities used for seismic migration purposes. Some authors consider set to this section an almost constant value (close to 4,500 m/s) which approximately represents the halite velocity, the most abundant mineral in this salt formation. Others, over these models, apply the tomographic inversion or FWI schemes giving to the velocity model the mathematical support to build confident seismic images. We believe in the importance to build starting velocity models reflecting the existing geological features prior to applying the tomographic/FWI updating. In this sense, we propose the insertion of the so-called stratifications within the evaporitic section using an adaptation of the model-based seismic inversion technique. Following this new velocity model including the stratification, we suggest tomographic iterations update, or FWI, to add to the geological constrains of the model the needed mathematical convergence. Finally, in this work, we performed the seismic migration with and without inserting these geological features in the initial velocity model and compared the results.Keywords: evaporitic section, stratifications, velocity model, seismic migration, seismic image.RESUMO. Em áreas estruturalmente complexas, como na seção pré-sal da Bacia offshore de Santos, região SE do Brasil, é um desafio representar a geologia utilizando imagens sísmicas. Uma das principais causas dos problemas observados está nas considerações sobre a seção evaporítica e suas velocidades com propósito de migração sísmica. Alguns autores consideram esta seção como tendo velocidades aproximadamente constantes (próximas de 4.500 m/s), o que representa aproximadamente o comportamento da halita, o mineral mais abundante nesta seção. Outros, sobre este modelo aplicam a atualização por inversão tomográfica ou FWI para dar ao modelo de velocidades o suporte matemático necessário para construir imagens sísmicas confiáveis. Nós acreditamos na importância de construir modelos iniciais de velocidades que reflitam as características geológicas existentes antes de aplicar esta atualização tomográfica/FWI mencionada. Neste sentido, propomos a inserção das denominadas estratificações dentro da seção evaporítica, utilizando uma adaptação da técnica de inversão sísmica model-based. Seguindo este novo modelo incluindo as estratificações, sugerimos a atualização por iterações tomográficas, ou FWI, para adicionar ao controle geológico do modelo a convergência matemática necessária. Finalmente, neste trabalho, nós realizamos a migração com e sem a inserção destas características geológicas no modelo inicial de velocidades e comparamos os resultados.Palavras-chave: seção evaporítica, estratificações, modelo de velocidade, migração sísmica, imagem sísmica.

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CSEM-regularized seismic velocity inversion: A multiscale, hierarchical workflow for subsalt imaging

Geophysics ◽

10.1190/geo2017-0454.1 ◽

2018 ◽

Vol 83 (5) ◽

pp. B241-B252 ◽

Cited By ~ 7

Author(s):

Daniele Colombo ◽

Diego Rovetta ◽

Ersan Turkoglu

Keyword(s):

Seismic Velocity ◽

Fundamental Problem ◽

Velocity Model ◽

Seismic Inversion ◽

Velocity Fields ◽

Coupling Mechanism ◽

Correct Result ◽

Velocity Analysis ◽

Velocity Inversion ◽

Velocity Models

Seismic imaging in salt geology is complicated by highly contrasted velocity fields and irregular salt geometries, which cause complex seismic wavefield scattering. Although the imaging challenges can be addressed by advanced imaging algorithms, a fundamental problem remains in the determination of robust velocity fields in high-noise conditions. Conventional migration velocity analysis is often ineffective, and even the most advanced methods for depth-domain velocity analysis, such as full-waveform inversion, require starting from a good initial estimate of the velocity model to converge to a correct result. Nonseismic methods, such as electromagnetics, can help guide the generation of robust velocity models to be used for further processing. Using the multiphysics data acquired in the deepwater section of the Red Sea, we apply a controlled-source electromagnetic (CSEM) resistivity-regularized seismic velocity inversion for enhancing the velocity model in a complex area dominated by nappe-style salt tectonics. The integration is achieved by a rigorous approach of multiscaled inversions looping over model dimensions (1D first, followed by 3D), variable offsets and increasing frequencies, data-driven and interpretation-supported approaches, leading to a hierarchical inversion guided by a parameter sensitivity analysis. The final step of the integration consists of the inversion of seismic traveltimes subject to CSEM model constraints in which a common-structure coupling mechanism is used. Minimization is performed over the seismic data residuals and cross-gradient objective functions without inverting for the resistivity model, which is used as a reference for the seismic inversion (hierarchical approach). Results are demonstrated through depth imaging in which the velocity model derived through CSEM-regularized hierarchical inversion outperforms the results of a seismic-only derived velocity model.

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Illumination compensation for image-domain wavefield tomography

Geophysics ◽

10.1190/geo2012-0278.1 ◽

2013 ◽

Vol 78 (5) ◽

pp. U65-U76 ◽

Cited By ~ 27

Author(s):

Tongning Yang ◽

Jeffrey Shragge ◽

Paul Sava

Keyword(s):

Model Building ◽

Velocity Model ◽

Model Errors ◽

Image Domain ◽

Illumination Compensation ◽

Illumination Problem ◽

Velocity Models ◽

Velocity Model Building ◽

Penalty Operator ◽

Seismic Images

Image-domain wavefield tomography is a velocity model building technique using seismic images as the input and seismic wavefields as the information carrier. However, the method suffers from the uneven illumination problem when it applies a penalty operator to highlighting image inaccuracies due to the velocity model error. The uneven illumination caused by complex geology such as salt or by incomplete data creates defocusing in common-image gathers even when the migration velocity model is correct. This additional defocusing violates the wavefield tomography assumption stating that the migrated images are perfectly focused in the case of the correct model. Therefore, defocusing rising from illumination mixes with defocusing rising from the model errors and degrades the model reconstruction. We addressed this problem by incorporating the illumination effects into the penalty operator such that only the defocusing by model errors was used for model construction. This was done by first characterizing the illumination defocusing in gathers by illumination analysis. Then an illumination-based penalty was constructed that does not penalize the illumination defocusing. This method improved the robustness and effectiveness of image-domain wavefield tomography applied in areas characterized by poor illumination. Our tests on synthetic examples demonstrated that velocity models were more accurately reconstructed by our method using the illumination compensation, leading to a more accurate model and better subsurface images than those in the conventional approach without illumination compensation.

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Improving seismic image using the common-horizon panel

Geophysics ◽

10.1190/geo2018-0656.1 ◽

2019 ◽

Vol 84 (5) ◽

pp. S449-S458

Author(s):

Lu Liu

Keyword(s):

Time Lag ◽

Velocity Model ◽

Migration Velocity ◽

Quantitative Information ◽

Time Shift ◽

Velocity Models ◽

Seismic Image ◽

The Common ◽

Seismic Images ◽

Zero Time

Generating high-quality seismic images requires accurate velocity models. However, velocity errors are predictably brought into the models. To mitigate the influences of velocity errors, we have used the common-horizon panel (CHP) for migration velocity analysis. CHP provides quantitative information to adjust mispositioned interfaces or correct deformed wavefields, which leads to improved image quality. It is generated by extrapolating seismic gathers to a selected target horizon and applying the time-shift imaging condition. Compared with the commonly used common-image gathers, the events in CHPs are more trackable because geologic interfaces are typically continuous in space. For a correct velocity model, the panel indicates a flat event at zero time lag, whereas in the case of an erroneous velocity model, the event becomes kinematically oscillating. This distinguishing difference provides a practical criterion to verify whether the migration velocity model is correct and to estimate the velocity or wavefield errors based on how much the event deviates from zero time lag. Tests on synthetic and field data sets have shown that the seismic images are improved by using the proposed CHP technique.

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Velocity rays for heterogeneous anisotropic media: Theory and implementation

Geophysics ◽

10.1190/1.2227525 ◽

2006 ◽

Vol 71 (5) ◽

pp. T117-T127 ◽

Cited By ~ 15

Author(s):

Einar Iversen

Keyword(s):

Model Updating ◽

Seismic Velocity ◽

Velocity Model ◽

Media Theory ◽

First Order ◽

Velocity Models ◽

Model Independent ◽

Upper Level ◽

The Time Domain ◽

Seismic Images

The surface of equal two-way time referred to as the isochron is a fundamental concept in seismic imaging. The shape of an isochron depends on the source and receiver locations, on the wave type, and on the parameters constituting the seismic velocity model. A perturbation of a parameter of the velocity model forces the isochron points to move along trajectories called velocity rays, with the selected model parameter as the variable along the rays. Based on earlier work describing first-order approximations to velocity rays, I develop a general theory for velocity rays valid for 3D heterogeneous and anisotropic velocity models. By this theory, velocity rays can be obtained in a way similar to the way conventional rays are computed by numeric integration of a system of ordinary differential equations (ODEs). The process is organized with ODE solvers on two levels, where the upper level is model independent. The lower level includes conventional one-way kinematic and dynamic tracing of source and receiver rays, as well as calculation of ray perturbation quantities. Accurate velocity rays are expected to be useful for perturbation of reflectors mapped from the time domain to the depth domain, for remigration of seismic images in the depth domain, and for velocity model updating.

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GEOLOGICAL CHARACTERIZATION OF EVAPORITIC SECTIONS AND ITS IMPACTS ON SEISMIC IMAGES: SANTOS BASIN, OFFSHORE BRAZIL

Brazilian Journal of Geophysics ◽

10.22564/rbgf.v37i1.1989 ◽

2019 ◽

Vol 37 (1) ◽

pp. 55

Author(s):

Alexandre Rodrigo Maul ◽

Marco Antonio Cetale Santos ◽

Cleverson Guizan Silva ◽

Josué Sá da Fonseca ◽

María de Los Ángeles González Farias ◽

...

Keyword(s):

Reservoir Characterization ◽

Seismic Imaging ◽

Low Frequency ◽

Seismic Inversion ◽

Velocity Models ◽

Exploration And Production ◽

Seismic Characterization ◽

Well Data ◽

Seismic Images

ABSTRACT. The pre-salt reservoirs in Santos Basin are known for being overlaid by thick evaporitic layers, which degrade the quality of seismic imaging and, hence, impacts reservoir studies. Better seismic characterization of this section can then improve decision making in E&P (Exploration and Production) projects. Seismic inversion - particularly with adequate low-frequency initial models – is currently the best approach to build good velocity models, leading to increased seismic resolution, more reliable amplitude response, and to attributes that can be quantitatively connected to well data. We discuss here a few considerations about inverting seismic data for the evaporitic section, and address procedures to improve reservoir characterization when using this methodology. The results show that we can obtain more realistic seismic images, better predicting both the reservoir positioning and its amplitude. Keywords: evaporitic section, seismic imaging, seismic inversion, reservoir characterization, seismic resolution.RESUMO. Os reservatórios do pré-sal da Bacia de Santos são conhecidos por estarem abaixo de uma espessa camada de evaporitos, que degradam a qualidade das imagens sísmicas e impactam os estudos de reservatórios. Melhores caracterizações desta seção podem, então, melhorar o processo de tomada de decisão em projetos de E&P (Exploração e Produção). Inversão sísmica – particularmente com modelos de baixa frequência inicial adequados – é correntemente a melhor abordagem para se construir modelos de velocidades, auxiliando no aumento de resolução sísmica, obtendo-se respostas de amplitude mais coerentes, e tendo seus atributos quantitativamente conectados com as informações de dados de poços. Aqui discutiremos algumas considerações sobre inversões sísmicas para seção evaporítica, e indicaremos procedimentos para melhorar a caracterização de reservatórios quando utilizando esta metodologia. Os resultados mostram que podemos obter imagens sísmicas mais realistas, com melhores predições tanto em termos de posicionamento quanto de sua amplitude.Palavras-chave: seção evaporítica, imagem sísmica, inversão sísmica, caracterização de reservatórios, resolução sísmica.

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Seismic Activity in the Central Adriatic Offshore of Italy: A Review of the 1987 ML 5 Porto San Giorgio Earthquake

Seismological Research Letters ◽

10.1785/0220190048 ◽

2019 ◽

Author(s):

Elvira Battimelli ◽

Guido Maria Adinolfi ◽

Ortensia Amoroso ◽

Paolo Capuano

Keyword(s):

Probabilistic Approach ◽

Active Faults ◽

Velocity Model ◽

Gas Production ◽

Seismic Sequence ◽

Velocity Models ◽

Geological Features ◽

Seismogenic Faults ◽

Intensity Field ◽

Hypocentral Location

ABSTRACT On 3 July 1987, a seismic sequence, with a mainshock of ML 5, took place in the offshore Adriatic, close to the coast of Porto San Giorgio (PSG), Italy. We present an accurate relocation of the PSG seismic sequence using a nonlinear probabilistic approach (Lomax et al., 2000). The trade‐off between the hypocentral location and the velocity model was exhaustively explored using six different velocity models available for the area provided by previous studies. Through numerous tests performed by relocating the mainshock, we selected the two best velocity models providing two different depths (2.0 and 18.0 km). To resolve this intrinsic ambiguity, we developed a technique that uses the macroseismic intensity field data based on a grid search of the magnitude–depth space. The results show that the mainshock has a depth of 5.7 km and a magnitude (ML) equal to 5; moreover, the relocated seismic sequence (∼30 events) developed in the upper portion of the crust (at a depth less than 15 km), thus activating thrust faults, which is typical of the main geological features that characterize the outer Apennines thrust belt and the Adriatic foreland folds. Because the Adriatic Sea hosts several hydrocarbon (mainly gas) production fields located near active faults, with some of them in the area of this study, analyzing the instrumental seismicity is necessary to better understand the seismicity generated by these seismogenic faults and improve the assessment of the area’s seismic hazards.

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First-arrival traveltime inversion of seismic diving waves observed on undulant surface

Geophysical Journal International ◽

10.1093/gji/ggab025 ◽

2021 ◽

Vol 225 (2) ◽

pp. 1020-1031

Author(s):

Huachen Yang ◽

Jianzhong Zhang ◽

Kai Ren ◽

Changbo Wang

Keyword(s):

Turning Points ◽

Analytical Formula ◽

Velocity Model ◽

Western China ◽

Inversion Method ◽

Mountain Area ◽

Traveltime Inversion ◽

Static Correction ◽

Velocity Models ◽

First Arrival

SUMMARY A non-iterative first-arrival traveltime inversion method (NFTI) is proposed for building smooth velocity models using seismic diving waves observed on irregular surface. The new ray and traveltime equations of diving waves propagating in smooth media with undulant observation surface are deduced. According to the proposed ray and traveltime equations, an analytical formula for determining the location of the diving-wave turning points is then derived. Taking the influence of rough topography on first-arrival traveltimes into account, the new equations for calculating the velocities at turning points are established. Based on these equations, a method is proposed to construct subsurface velocity models from the observation surface downward to the bottom using the first-arrival traveltimes in common offset gathers. Tests on smooth velocity models with rugged topography verify the validity of the established equations, and the superiority of the proposed NFTI. The limitation of the proposed method is shown by an abruptly-varying velocity model example. Finally, the NFTI is applied to solve the static correction problem of the field seismic data acquired in a mountain area in the western China. The results confirm the effectivity of the proposed NFTI.

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Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

Acta Geophysica ◽

10.1007/s11600-021-00594-6 ◽

2021 ◽

Author(s):

Francesca D’Ajello Caracciolo ◽

Rodolfo Console

Keyword(s):

Central Europe ◽

Velocity Model ◽

Moho Depth ◽

Seismic Sequence ◽

Tectonic Structures ◽

Study Region ◽

Waveform Data ◽

Seismic Stations ◽

Velocity Models ◽

Master Event

AbstractA set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained.

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Interferometric imaging condition for wave-equation migration

Geophysics ◽

10.1190/1.2838043 ◽

2008 ◽

Vol 73 (2) ◽

pp. S47-S61 ◽

Cited By ~ 14

Author(s):

Paul Sava ◽

Oleg Poliannikov

Keyword(s):

Large Scale ◽

Wigner Distribution ◽

Random Noise ◽

Velocity Model ◽

Distribution Functions ◽

Small Scale ◽

Interferometric Imaging ◽

Imaging Data ◽

Imaging Condition ◽

Velocity Models

The fidelity of depth seismic imaging depends on the accuracy of the velocity models used for wavefield reconstruction. Models can be decomposed in two components, corresponding to large-scale and small-scale variations. In practice, the large-scale velocity model component can be estimated with high accuracy using repeated migration/tomography cycles, but the small-scale component cannot. When the earth has significant small-scale velocity components, wavefield reconstruction does not completely describe the recorded data, and migrated images are perturbed by artifacts. There are two possible ways to address this problem: (1) improve wavefield reconstruction by estimating more accurate velocity models and image using conventional techniques (e.g., wavefield crosscorrelation) or (2) reconstruct wavefields with conventional methods using the known background velocity model but improve the imaging condition to alleviate the artifacts caused by the imprecise reconstruction. Wedescribe the unknown component of the velocity model as a random function with local spatial correlations. Imaging data perturbed by such random variations is characterized by statistical instability, i.e., various wavefield components image at wrong locations that depend on the actual realization of the random model. Statistical stability can be achieved by preprocessing the reconstructed wavefields prior to the imaging condition. We use Wigner distribution functions to attenuate the random noise present in the reconstructed wavefields, parameterized as a function of image coordinates. Wavefield filtering using Wigner distribution functions and conventional imaging can be lumped together into a new form of imaging condition that we call an interferometric imaging condition because of its similarity to concepts from recent work on interferometry. The interferometric imaging condition can be formulated both for zero-offset and for multioffset data, leading to robust, efficient imaging procedures that effectively attenuate imaging artifacts caused by unknown velocity models.

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Kinematic interpretation in the prestack depth‐migrated domain

Geophysics ◽

10.1190/1.1444224 ◽

1997 ◽

Vol 62 (4) ◽

pp. 1226-1237 ◽

Cited By ~ 8

Author(s):

Irina Apostoiu‐Marin ◽

Andreas Ehinger

Keyword(s):

Signal To Noise Ratio ◽

Velocity Model ◽

Point Of View ◽

Prestack Depth Migration ◽

Depth Migration ◽

Velocity Models ◽

Time Domain Data ◽

And Migration ◽

Point To Point ◽

Homogeneous Media

Prestack depth migration can be used in the velocity model estimation process if one succeeds in interpreting depth events obtained with erroneous velocity models. The interpretational difficulty arises from the fact that migration with erroneous velocity does not yield the geologically correct reflector geometries and that individual migrated images suffer from poor signal‐to‐noise ratio. Moreover, migrated events may be of considerable complexity and thus hard to identify. In this paper, we examine the influence of wrong velocity models on the output of prestack depth migration in the case of straight reflector and point diffractor data in homogeneous media. To avoid obscuring migration results by artifacts (“smiles”), we use a geometrical technique for modeling and migration yielding a point‐to‐point map from time‐domain data to depth‐domain data. We discover that strong deformation of migrated events may occur even in situations of simple structures and small velocity errors. From a kinematical point of view, we compare the results of common‐shot and common‐offset migration. and we find that common‐offset migration with erroneous velocity models yields less severe image distortion than common‐shot migration. However, for any kind of migration, it is important to use the entire cube of migrated data to consistently interpret in the prestack depth‐migrated domain.

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