Processing Of Seismic Reflection Data Research Articles

Geophysical Characterization and Attenuation Correction Applied for Hydrate Bearing Sediments in Japan Sea

Estimation of rock properties from seismic data is important for exploration and production activities in the petroleum industry. Considering the compressional velocity—the speed of propagating body waves in formations—and the quality factor (Q)—a measure of the frequency-selective energy losses of waves propagating through formations—both properties are usually estimated from multichannel seismic data. Velocity is estimated during multichannel processing of seismic reflection data in either the time or depth domain. In marine seismic acquisition, Q can be estimated from the following sources: Vertical Seismic Profile (VSP) surveys, where sources are located near the sea surface and geophones are distributed at depth along a borehole; and multichannel reflection data, where sources are also located near the sea surface and receivers are distributed either at the sea surface (conventional seismic survey with streamers) or on the sea floor (use of nodes or Ocean Bottom Cables (OBC)). The aforementioned acquisition devices, VSP, conventional streamers, nodes, and OBCs are much more expensive than single-channel acquisition with one receiver per shot due to the cost of operation. There are numerous old and new datasets from academia and the oil industry that have been acquired with single-channel acquisition devices. However, there is a paucity of work addressing the estimation of velocity and Q from this type of equipment. We investigate the estimation of Q and velocity from single-channel seismic data. Using the windowed discrete Fourier transform for a single seismic trace, we calculate the peak and dominant frequency that changes with time. In the geologic environment, higher frequencies are attenuated at shallow depths (time), while lower frequencies remain at deeper positions. From the rate at which higher frequencies are attenuated with time, we estimate the effective quality factor (Qeff). However, when using Kirchhoff migration to process single-channel seismic data, events far from the vertical projection of the receiver contribute to the trace at a given time. Then, an underestimation of the effective quality factor occurs. To compensate for the effects of more distant events with lower-frequency content contaminating the shorter events, we propose a linear equation to correct the effective quality factor estimated from migrated seismic data. Effective Q and its correction are estimated in five single-channel seismic lines surveyed along the Joetsu Knoll, a SW-NE anticline structure on the eastern margin of the Sea of Japan. These results are linked to geomorphological and geological features and the velocity field. Joetsu Knoll is a known site of massive gas hydrates (GH), which occur in the first hundred metres of Neogene sediments and, together with gas chimneys, play an important role in seismic wave absorption. Qeff estimated from migrated seismic data maintains the spatial relationship between high and low Q regions. The region of low Q, which is below 124 and has an average value of 57, occurs near the anticlinal hinge and tends to coincide with the region in which the Bottom Simulating Reflector (BSR) resides. The coexistence of GH and free gas coincides with the very low P velocity gradient of 0.225 s−1. BSR occurrence, Qeff and the geometry of the Joetsu anticline testify to progressive gas hydrate depletion northward along the dome.

Impact of the Iquique Ridge on structure and deformation of the north Chilean subduction zone

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found.

Reflection imaging of complex geology in a crystalline environment using virtual-source seismology: case study from the Kylylahti polymetallic mine, Finland

Abstract. For the first time, we apply a full-scale 3D seismic virtual-source survey (VSS) for the purpose of near-mine mineral exploration. The data were acquired directly above the Kylylahti underground mine in Finland. Recorded ambient noise (AN) data are characterized using power spectral density (PSD) and beamforming. Data have the most energy at frequencies 25–90 Hz, and arrivals with velocities higher than 4 km s−1 have a wide range of azimuths. Based on the PSD and beamforming results, we created 10 d subset of AN recordings that were dominated by multi-azimuth high-velocity arrivals. We use an illumination diagnosis technique and location procedure to show that the AN recordings associated with high apparent velocities are related to body-wave events. Next, we produce 994 virtual-source gathers by applying seismic interferometry processing by cross-correlating AN at all receivers, resulting in full 3D VSS. We apply standard 3D time-domain reflection seismic data processing and imaging using both a selectively stacked subset and full passive data, and we validate the results against a pre-existing detailed geological information and 3D active-source survey data processed in the same way as the passive data. The resulting post-stack migrated sections show agreement of reflections between the passive and active data and indicate that VSS provides images where the active-source data are not available due to terrain restrictions. We conclude that while the all-noise approach provides some higher-quality reflections related to the inner geological contacts within the target formation and the general dipping trend of the formation, the selected subset is most efficient in resolving the base of formation.

Ground-roll extraction using the Karhunen-Loeve transform in the time-frequency domain

Ground-roll suppression is critical for seismic reflection data processing. Many standard methods, i.e., f-k fan filtering, fail when spatially aliased surface wave interference is present in the data. Spatial aliasing is a common problem; receiver spacing is often not dense enough to extract wavenumbers of low-velocity surface waves. It has long been known that the Karhunen-Loeve (KL) transform can be used to suppress aliased ground roll. However, the ground roll should be flattened before suppression, which is challenging due to the dispersion of surface wave velocities. I propose to solve this problem via the time-frequency domain. I apply the S-transform, which was previously shown to perform well in the multichannel analysis of surface waves. A simple complex-valued constant phase shift is a suitable model of surface wave propagation in common-frequency S-transform gathers. Therefore, it is easy to flatten the corresponding S-transform narrow-band frequency surface wave packet and extract it from the data by principal component analysis of the corresponding complex-valued data-covariance matrix. As the result, our S-transform KL (SKL) method filters the aliased ground roll without damaging the reflection amplitudes. The advantages of SKL filtering have been confirmed by synthetic- and field-data processing.

Multichannel Seismic Reflection Data Processing: Multiple Attenuation of Line 250 at the Coast of Southern California Using Hyperbolic Radon Transform

Multichannel seismic reflection data processing of line 250 at the Coast of Southern California has been done by using Echos®. The purpose of this research is to separate multiple reflections from the primary reflection. Multiple reflections yield dramatic effect especially on marine seismic survey. Multiple reflections in marine seismic data is the noise caused by waves that trapped in a layer of sea water or rock layers. One of geophysics exploration problem is removing multiple reflections from primary reflection. Multiple reflections often destructively interfere with the primary reflection making interpretation difficult, so it’s needed to get seismic section free from multiple reflections. There are some methods to attenuate the multiple reflections, one of them is Hyperbolic Radon transform. Hyperbolic Radon transform method works by transforming the data from time-offset domain into the Radon domain. Primary reflection and multiple reflections can be separated into Radon domain. In the end of this research, seismic section before and after multiple attenuation using hyperbolic Radon transform were compared. Based on the result of processing, it can be concluded that hyperbolic Radon transform is effective to attenuate multiple reflections at 0,24-l,25s TWT. But, it can eliminate some of data and makes seismic section seen unclear.

Identification of accretionary wedge at Sumatra subduction zone using 2D seismic reflection

This research is focusing on structure at Nias Islands subduction zone. Subduction zone Sumatra is the collision zone of Eurasia continental plate and Indo-Australia oceanic plate. Prior to the establishment of the subduction zone there is an event of the making of continental crust in which in this research area is marked by Wharton Fossil Ridge. An odd feature of the Sunda subduction zone is the non-volcanic forearc ridge that comes up raised sea level assemble a lot of islands betwixt the trench and the mainland. The forearc ridge is the cap of a chunky sequence of sediments and sections of sea floor, known as the accretionary wedge, that are folded, faulted and plastered into the upper plate. Accretionary wedge structure which has the melange rock characteristics because of the plates’ subduction, The accretionary wedge could be intrepred using seismic reflection data with characteristic reflector doesn’t formed or visible. The islands offshore southwestern Sumatra might have a akin origin. The most appropriate study of these is Nias, which its southwest coastline is 90 km form the subduction deformation front and 20 km above the dipping plate interface. Southwest of Nias, the oceanic seafloor is dipping at a homogeneous angle 6° to 7° against Nias for a horizontal distance of 25 km beneath the accretionary wedge, where it attaints a depth of just over 10 km. In this research it has been done 2D seismic reflection data processing and the interpretation of the Wharton fossil ridge establishment, subduction zone, and accretionary wedge.

Effects of lateral heterogeneity on time-domain processing parameters

SUMMARY Time-domain processing of seismic reflection data has always been an important engine that is routinely utilized to produce seismic images and to expeditiously construct subsurface models. The conventional procedure involves analysing parameters related to the derivatives of reflection traveltime with respect to offset including normal moveout (NMO) velocities (second-order derivatives) and quartic coefficients (fourth-order derivatives). In this study, we propose to go beyond the typical assumption of 1-D laterally homogeneous medium when relating those ‘processing’ parameters to the subsurface medium parameters and take into account the additional influences from lateral heterogeneity including curved interfaces and smoothly variable velocities. We fill in the theoretical gap from previous studies and develop a general framework for such connection in layered anisotropic media. We show that in general, the influences of lateral heterogeneity get accumulated from all layers via a recursive relationship according to the Fermat’s principle and can be approximately quantified in terms of the lateral derivatives of the layer interface surfaces and velocities. Based on the same general principle, we show that our approach can also be used to study the lateral heterogeneity effects on diffraction traveltime and its second-order derivative related to time-migration velocity. In this paper, we explicitly specify expressions for NMO and time-migration velocities with the influences from both types of heterogeneity suitable for 2-D data sets and also discuss possible extensions of the proposed theory to 3-D data sets and to parameters related to higher-order traveltime derivatives. Using numerical examples, we demonstrate that the proposed theory can lead to more accurate reflection and diffraction traveltime predictions in comparison with those obtained based on the 1-D assumption. Both the proposed theoretical framework and its numerical testing for forward traveltime computation presented in this study aid in understanding the effects from lateral heterogeneity on time-processing parameters and also serve as an important basis for designing an efficient technique to separate those influences in important processes such as Dix inversion for a more accurate subsurface model in the future.

Automatic velocity analysis using high-resolution hyperbolic Radon transform

Velocity analysis is crucial in reflection seismic data processing and imaging. Velocity picking is widely used in the industry for building the initial velocity model. When the size of the seismic data becomes extremely large, we cannot afford the corresponding human endeavor that is required by the velocity picking. In such situations, an automatic velocity-picking algorithm is highly demanded. We have developed a novel automatic velocity-analysis algorithm that is based on the high-resolution hyperbolic Radon transform. We formulate the automatic velocity-analysis problem as a constrained optimization problem. To solve the optimization problem with a hard constraint on the sparsity and distribution of the velocity spectrum, we relax it to a more familiar L1-regularized optimization problem in two steps. We use the iterative preconditioned least-squares method to solve the L1-regularized problem, and then we apply the hard constraint of the target optimization during the iterative inversion. Using synthetic and field-data examples, we determine the successful performance of our algorithm.

Near-surface Seismic Velocity Model Building from First Arrival Travel-times - A Case Study from an Onshore, Niger Delta Field

Seismic refraction was used to image the near-surface, using a3D seismic data acquired from an onshore Niger Delta field. The objective of the study was to build a near-surface model of the earth in terms of sub-weathering layer thicknesses and seismic wave velocities and highlight the near-surface seismic wave velocity variation in the region. A velocity-depth model consisting of the weathering layer and three consolidated (sub-weathering) layers was first estimated. The first-breaks were picked and used as input for this study. The models were generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general. Results support confidence in the reliability and robustness of the refraction inversion method. The method of analysis and resolution for 3-layer models adopted in this study may be used in resolving such cases observed during processing of seismic reflection data from the Niger-Delta Basin.

Uncertainty analysis of depth predictions from seismic reflection data using Bayesian statistics

Estimating the depths of target horizons from seismic reflection data is an important task in exploration geophysics. To constrain these depths we need a reliable and accurate velocity model. Here, we build an optimum 2D seismic reflection data processing flow focused on pre – stack deghosting filters and velocity model building and apply Bayesian methods, including Gaussian process emulation and Bayesian History Matching (BHM), to estimate the uncertainties of the depths of key horizons near the borehole DSDP-258 located in the Mentelle Basin, south west of Australia, and compare the results with the drilled core from that well. Following this strategy, the tie between the modelled and observed depths from DSDP-258 core was in accordance with the ± 2σ posterior credibility intervals and predictions for depths to key horizons were made for the two new drill sites, adjacent the existing borehole of the area. The probabilistic analysis allowed us to generate multiple realizations of pre–stack depth migrated images, these can be directly used to better constrain interpretation and identify potential risk at drill sites. The method will be applied to constrain the drilling targets for the upcoming International Ocean Discovery Program (IODP), leg 369.

SEISGAMA: A Free C# Based Seismic Data Processing Software Platform

Seismic reflection is one of the most popular methods in geophysical prospecting. Nevertheless, obtaining high resolution and accurate results requires a sophisticated processing stage. There are many open-source seismic reflection data processing software programs available; however, they often use a high-level programming language that decreases its overall performance, lacks intuitive user-interfaces, and is limited to a small set of tasks. These shortcomings reveal the need to develop new software using a programming language that is natively supported by Windows® operating systems, which uses a relatively medium-level programming language (such as C#) and can be enhanced by an intuitive user interface. SEISGAMA was designed to address this need and employs a modular concept, where each processing group is combined into one module to ensure continuous and easy development and documentation. SEISGAMA can perform basic seismic reflection processes. This ability is very useful, especially for educational purposes or during a quality control process (in the acquisition stage). Those processes can be easily carried out by users via specific menus on SEISGAMA’s main user interface. SEISGAMA has been tested, and its results have been verified using available theoretical frameworks and by comparison to similar commercial software.

A high-resolution weighted AB semblance for dealing with amplitude-variation-with-offset phenomenon

Velocity analysis is an essential step in seismic reflection data processing. The conventional and fastest method to estimate how velocity changes with increasing depth is to calculate semblance coefficients. Traditional semblance has two problems: low time and velocity resolution and an inability to handle amplitude variation-with-offset (AVO) phenomenon. Although a method known as the AB semblance can arrive at peak velocities in the areas with an AVO anomaly, it has a lower velocity resolution than conventional semblance. We have developed a weighted AB semblance method that can handle both problems simultaneously. We have developed two new weighting functions to weight the AB semblance to enhance the resolution of velocity spectra in the time and velocity directions. In this way, we increase the time and velocity resolution while eliminating the AVO problem. The first weighting function is defined based on the ratio between the first and the second singular values of the time window to improve the resolution of velocity spectra in velocity direction. The second weighting function is based on the position of the seismic wavelet in the time window, thus enhancing the resolution of velocity spectra in time direction. We use synthetic and field data examples to show the superior performance of our approach over the traditional one.

The Application of S‐transform Spectrum Decomposition Technique in Extraction of Weak Seismic Signals

AbstractIn processing of deep seismic reflection data, when the frequency band difference between the weak useful signal and noise both from the deep subsurface is very small and hard to distinguish, the usefulness of traditional method of filtering will be limited. To solve this problem, we apply different spectral decomposition methods respectively to experimental data and real data and compare the results from these methods. Our purpose is to find an effective way to protect weak signals during processing deep seismic reflection data.The spectral decomposition method is based on the discrete Fourier transform, which uses the signal frequency‐amplitude spectrum and other information to generate a high‐resolution seismic image. Typically, it is used to identify the lateral distribution of medium properties, solve spectrum changes within complex media and local phase instability and other issues, such as locating faults and small‐scale complex fractures. S‐transform as a new time‐frequency analysis method, which is a generalization of STFT developed by Stockwell in 1994, has the ability to automatically adjust the resolution. This method has been widely applied to exploration seismic, MT and other geophysical datasets in recent years. It has become one of the effective methods in noise suppressing during geophysical data processing. Comparing with conventional oil reflection seismic data, in order to probe deep structure, deep seismic reflection employs a large quantity of explosives, long observing systems, leading to a phenomenon that valid signals from the deep and noise are mixed together both in time domain and frequency domain. Considering these characteristics of deep reflection data, this paper combines spectral decomposition with S transform technology. First we design a simple pulse function experimental data to confirm the validity of the S transform method. Then we illustrate the effectiveness of spectral decomposition which is influenced by the choice of frequency analysis methods, especially the transform window function which determines the resolving power of the method. On this basis, S transform spectrum decomposition is applied to a single channel of deep reflection seismic data and the stacked profile, then the application results of traditional transform spectral decomposition and S transform spectral decomposition are compared.Comparison of single channel data shows that compared with traditional spectral decomposition, the S transform spectral decomposition method is able to automatically adjust the resolution, accurately calibrate frequency component of weak signals at different times in deep reflection seismic data. Application to stacked profile data shows that the results obtained by the S transform spectral decomposition and those from other spectral decomposition method are largely consistent, while the results of S transform spectral decomposition clearly depict the characteristics of low‐frequency details which are superimposed by noise in original stacked profile. At the same time, it improves the resolution and enhances the phase axis continuity on the stacked profile. Comparison also clearly indicates that the phase axis on the resultant profile obtained by Gabor transform spectral decomposition is more broken, which is caused by fixed‐length window function used by Gabor transform decomposition, the length of the window function parameters can only be selected before the start of processing and is set to a certain value, while the S transform spectral decomposition method chooses the variable length of the window function according to signal change. It can automatically adjust the frequency characteristics of the signal by the local window length to better characterize the details of each frequency range. Such an effect is very obvious in deep reflection seismic imaging.Our results show that the key of the spectral decomposition technique is to select the transform window function. The S transform spectral decomposition technology used in real deep reflection seismic data processing can effectively protect the weak low‐frequency signals. It can effectively improve the signal to noise ratio and the resolution of weak reflection signals from the deep subsurface, while depicting the characteristics of low‐frequency details on the stacked section and ultimately obtaining better imaging results.

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

AbstractWe present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

Enhancement of Seismic Stacking Energy with Crossdip Correction for Crooked Survey Lines

In seismic reflection data processing, the crossdip correction effectively focuses the stacking energy near the sharp bends of a crooked survey line. Additionally, approximate 3-D information on the reflector (e.g., true crossdip angle and lateral continuity) are locally investigated as a by-product of the crossdip correction procedure. Improvement of the signal-to-noise ratio and estimation of reflector crossdip attitude are tested, in terms of both common midpoint bin direction and processing-line type, using synthetic seismic reflection data. To effectively image the reflection energy near bends in seismic survey lines, straight-line binning is preferred to slalom-line binning.

Resolving complex structure in near-surface refraction seismology with common-offset gathers

Common-offset gathers (COG) have been usefully employed in the processing of seismic reflection data (Fulton and Darr, 1984). COG methods have also been employed to a lesser extent with the processing of seismic refraction data (Coppens, 1985), but their wider application has yet to be fully exploited. This study describes novel COG adaptations of the generalized reciprocal method (GRM) (Palmer, 1981, 2010b). Initially, the COG GRM methods (Palmer, 2012) were developed as simple and convenient methods for the quality assurance of the large volumes of refraction data, which are characteristic of statics computations for high-fold CMP reflection data processing. However, their application to a variety of sets of data has demonstrated a number of additional unanticipated benefits.

Effects of anisotropy on time-depth relation in transversely isotropic medium with a vertical axis of symmetry

It is known that rock anisotropy can significantly influence the phase and energy velocities of an elastic wave, as well as its reflection/transmission (R/T) coefficients. As a result, it can distort the velocity analysis of seismic-reflection data. In this work we present a velocity analysis for seismic-reflection data based on the available anisotropic rock parameters. We analyzed the created errors on time-depth relation of the seismic-reflection data in neglecting rock anisotropy and/or neglecting the difference between energy velocity and phase velocity, including the case of wide-angle reflection. The calculated results show that the effect of rock anisotropy on time-depth relation of seismic-reflection data is dependent not only on the values of anisotropic parameters, but also on the space arrangement of both source and receiver-array. For all studied cases (weak, moderate or strong anisotropy), we found that the effect of rock anisotropy on time-depth relation could not be neglected. Nevertheless, for the case of weak anisotropy, the energy velocity may be replaceable by the phase velocity to obtain a very good approximation on time-depth relation. Consequently, the seismic-reflection data processing algorithm for numerical computations can be simplified.

An investigation of the effects of the choice of stacking velocities on residual statics for hardrock reflection seismic processing

Crystalline rocks are typically deformed, folded and altered and contain interfaces that have varying dips. These interfaces are often short in length and sometimes can be represented as diffractors. Imaging of reflections from these interfaces, which are often of low acoustic impedance contrast, poses a challenge in reflection seismic data processing. An important step in the imaging process is the calculation of residual statics. In this study we investigate the effects of the choice of stacking velocities on the estimation of surface-consistent residual statics in the crystalline environment on a synthetic data set and two real data examples, one 2D data set and a 3D one. On the synthetic data set, residual statics were estimated using three different approaches. In the first approach (normal move out or NMO approach), residual statics were estimated using an NMO velocity function that focuses on steeply dipping reflections and uses velocities ranging from 5000 to 9000 m/s. In the second approach (dip move out or DMO approach), residual statics were estimated using the true media velocity or using an updated velocity function after the implementation of DMO corrections. For the third approach, single reflections were focused on for the residual statics estimation. Analysis of the resulting stacked sections, as well as the estimated residual statics, shows that when short steeply dipping reflections are present, the DMO approach allows better imaging of diffractions and gently dipping reflections. High-stacking velocities in the NMO approach produce more coherent noise, which negatively influences the estimation of residual statics. The best estimation of residual statics can be obtained if the calculation focuses in a narrow window along sub-horizontal or gently dipping reflections. Such reflections will not always be present in real data and the synthetic test shows that focusing on steeply dipping reflections within a narrow window, in the absence of horizontal or gently dipping reflections, provides superior results compared with a wide search window. Application of these approaches on real seismic data acquired over crystalline rocks demonstrates that an improper estimation of surface-consistent residual static corrections can result in the loss of short gently dipping reflections and diffractions that may be an indication of mineral deposits.

Seismic reflection data processing in active volcanic areas: an application to Campi Flegrei and Somma Vesuvius offshore (Southern Italy)

The Campanian volcanism develops near the sea. Therefore, the geophysical study of the marine environment is a key to a better understanding of the tectonic evolution and the origin of volcanism in the area. An abundance of high quality seismic data in the marine sector, where little direct information is available, is critical to the study of Campanian volcanism. This paper concerns the reprocessing of a seismic reflection dataset acquired in Naples Bay and processed during 1973. Even though the overall data quality was high for that time, of course their acquisition technological limits have been overcome by the new processing. Our reprocessing aimed at: 1) reduction of random noise in the data; 2) removal of unwanted coherent events; 3) reduction of spatial aliasing by means of trace interpolation on Commod Shot Point (CSP) gathering; 4) improvement of resolution of the seismic wavelet with spiking deconvolution algorithms and finally 5) reposition of reflectors in their correct locations in the space-TWT domain by means of dip moveout and post-stack time migration. A comparison between the new and old data shows that the new sections are characterized by a much higher S/N ratio. Diffraction hyperbole has been collapsed. Reverberations, ghosts and multiples have been removed or greatly attenuated, especially between the reflectors of interest, allowing us to follow them with more detail and with greater continuity. Furthermore, data resolution has been boosted by the reprocessing, allowing the interpreter to evaluate reflector position and continuity in greater detail. The reinterpretation phase of such lines, that is already in an advanced stage, will therefore allow us to gain new insights into the structural setting of the bay, with the aim of exploring the connection between tectonics and volcanism.

CRS-based depth model building and imaging of 3D seismic data from the Gulf of Mexico Coast

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.