# Browsing by Subject "Seismic reflection method"

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Item 3D seismic surface multiple attenuation: algorithms and analysis(2001-12) Alaslani, Abdulaziz Saleh, 1965-; Stoffa, Paul L.; Sen, Mrinal K.Show more The aim of seismic exploration is to provide a comprehensive description of subsurface geologic structure in terms of its reflectivity function at the boundaries between geological units. Seismic multiples are coherent noise that obscure primary events and considerably degrade the quality of seismic images in the target zones. In spite of the fact that many methods have been designed to suppress multiples, only a limited success has been achieved. I have developed two different approaches to address the problem of seismic multiples. The first approach attempts to suppress multiples in terms of decomposition of the measured seismic wavefields into its upgoing and downgoing waves. The separation process is accomplished by using some statistical characteristics of the data in the plane-wave p domain. The ratio of these two components yields the true reflectivity function free of multiples. Although encouraging results are obtained in the separation process, instability occurs during the wavefield division step. As a result, the effectiveness of this approach is limited. I have also investigated seismic multiples for 3D geology and proposed a new methodology in which 3D multiples are predicted and attenuated successfully. The departure of the predicted multiple arrival times from the observed multiple arrival times explains why demultiple algorithms that assume two-dimensional multiple reflections often fail. In this approach, I employed 3D ray tracing to predict the arrival times of the primary and its multiples in individual shot gathers generated from a three-dimensional reflector. A non-linear optimization method, called Very Fast Simulated Annealing (VFSA) is used to determine geometry of the subsurface reflector in 3D. This is achieved by applying a ray traced normal moveout (NMO) correction to seismic reflections with respect to the zero offset time. Based on the optimized NMO-corrected shot gathers, the autoconvolution of the seismic trace is employed to predict the multiple reflections, which are then scaled and subtracted from the original data. The application of this technique to real data demonstrates that the new method successfully suppresses many surface multiples, and is able to recover several deep primary events. This algorithm is robust and computationally very efficient.Show more Item Analyzing deep-water near seafloor geology with chirp sonar sub-bottom profiles : Green Canyon, Gulf of Mexico(2006-12) Hernandez, Jaime, 1968-; Fisher, W. L. (William Lawrence), 1932-Show more The study area is located on the continental slope in the Green Canyon deep-water area of the Gulf of Mexico. This area is being investigated by the Bureau of Economic Geology as part of several active gas hydrate studies across the area. The chirp sonar profiles used in my study were collected with an Autonomous Underwater Vehicle (AUV) utilizing a frequency-modulated seismic (sonar) source that emitted a 2 to 8 kHz sweep (chirp) frequency signal (wavelengths less than 2 meters). The recording time is limited to about 50 milliseconds, with time zero occurring at the altitude of the AUV about 50 meters above the seafloor. The signal images to about 40 meters below the seafloor, and profiles are as long as 5 km. An interpretation of deep-water, near sea-floor geology has been conducted using both chirp sonar profiles and multibeam bathymetry. Seismic reflections from within the shallow sediments are caused primarily by contrasts in density, rather than acoustic velocity. Reflections were successfully simulated using a model with a constant velocity of 1560 m/s and densities of 2.1 g/cc for sand, 1.4 g/cc for mud, and 1.7 g/cc for silty sand. The chirp sonar profiles imaged near-seafloor geology at nearly a meter scale and allowed for detailed interpretation. The interaction of soft sediment deformation, creep movement and neo-tectonic activity related to gas expulsion controlled the actual topography of the sea-floor. The geologic time represented in the 50 milliseconds of chirp data recorded with the AUV, which is about 40 meters of depth, is approximately 0.050 Ma. B.P., consistent with a depositional rate of 0.8 meters per 1000 years. Reflection patterns are interpreted to be related to fluctuations in sea level. High reflectivities (density contrasts) are interpreted to be deposited during the last sea level low stand, and low reflectivities are interpreted as sedimentation during the last high stand. Sediments deposited during the low stand are proximal while others are distal, deposited in deeper water environments. Shallow structures observed in the chirp sonar profiles are mainly related to gas mobilization mechanisms, which shaped the topography of the seafloor in conjunction with soft sediment deformation and creep movement. The geomorphic features are related in some cases to gas expulsion zones such as pockmarks and mud volcanoes. Other structures are related to soft sediment deformation and creep mobilization. Soft sediment deformation is confined to the deepest part of the minibasin, while the other features are not depth dependent. Highly deformed intervals at the bottom of the sequence seem to control subsequent sedimentation.Show more Item Fluid content effect on acoustic impedance and limits of direct detection capability : illustrated on an offshore prospect(1980-12) Catto, Antonio José; Backus, Milo M., 1932-Show more The presence of gas and oil in some sand formations decreases the seismic velocity and density to such an extent that anomalously large reflections coefficients are encountered at fluid contacts. Geerstma and Gassmann's theories are equivalent and provide a good way to study the physical properties that affect the elastic behavior of the porous rock. The fluid-contact reflectivity (gas-water, oil-water) can be well estimated based on the brine saturated velocity alone. A comparison between the estimated and observed fluid-contact reflectivities on seismic and well log data from an Offshore prospect showed a remarkable agreement.Show more Item MCMC algorithm, integrated 4D seismic reservoir characterization and uncertainty analysis in a Bayesian framework(2008-08) Hong, Tiancong, 1973-; Sen, Mrinal K.Show more One of the important goals in petroleum exploration and production is to make quantitative estimates of a reservoir’s properties from all available but indirectly related surface data, which constitutes an inverse problem. Due to the inherent non-uniqueness of most inverse procedures, a deterministic solution may be impossible, and it makes more sense to formulate the inverse problem in a statistical Bayesian framework and to fully solve it by constructing the Posterior Probability Density (PPD) function using Markov Chain Monte Carlo (MCMC) algorithms. The derived PPD is the complete solution of an inverse problem and describes all the consistent models for the given data. Therefore, the estimated PPD not only leads to the most likely model or solution but also provides a theoretically correct way to quantify corresponding uncertainty. However, for many realistic applications, MCMC can be computationally expensive due to the strong nonlinearity and high dimensionality of the problem. In this research, to address the fundamental issues of efficiency and accuracy in parameter estimation and uncertainty quantification, I have incorporated some new developments and designed a new multiscale MCMC algorithm. The new algorithm is justified using an analytical example, and its performance is evaluated using a nonlinear pre-stack seismic waveform inversion application. I also find that the new technique of multi-scaling is particularly attractive in addressing model parameterization issues especially for the seismic waveform inversion. To derive an accurate reservoir model and therefore to obtain a reliable reservoir performance prediction with as little uncertainty as possible, I propose a workflow to integrate 4D seismic and well production data in a Bayesian framework. This challenging 4D seismic history matching problem is solved using the new multi-scale MCMC algorithm for reasonably accurate reservoir characterization and uncertainty analysis within an acceptable time period. To take advantage of the benefits from both the fine scale and the coarse scale, a 3D reservoir model is parameterized into two different scales. It is demonstrated that the coarse-scale model works like a regularization operator to make the derived fine-scale reservoir model smooth and more realistic. The derived best-fitting static petrophysical model is further used to image the evolution of a reservoir’s dynamic features such as pore pressure and fluid saturation, which provide a direct indication of the internal dynamic fluid flow.Show more Item One and two dimensional velocity inversion in the domain of intercept time and ray parameter : an example in the Nankai Trough(1989) Wood, Warren Theodore, 1962-; Stoffa, Paul L., 1948-Show more The intercept time equation in τ-p (Diebold and Stoffa, 1981) [mathematical equation] is used as the basis for exact 1D and 2D τ-p velocity analysis. Intercept time curves for an initial model are superimposed on the τ-p seismic data. Model parameters such as layer velocity, thickness, and dip are adjusted until the intercept time curves are coincident with the reflections in the data. Normal moveout in the domain of τ and p is applied during the analysis so as to check the picks of the reflections. When a reflection has been moved out correctly, it has been properly modeled, (i.e. the velocity, thickness, and dip of the layer have been determined). Once all the reflections have been imaged, the analysis is complete. One attractive feature of this method is that all of the calculations can be done quickly, so the analysis can be done interactively on a computer with a graphics screen. This velocity analysis method was then applied to long offset seismic data collected in the Nankai Trough. Eight expanding spread profiles (ESPs) and five split spread profiles (SSPs) were collected in two different areas and analyzed to accurately determine sediment velocities in the trough and on the accretionary wedge. The results are a series of 1D earth models in the ESP area and 2D models in the SSP area. The analysis of the ESP data clearly shows a low velocity zone associated with a bottom simulating reflector but does not show evidence of a large (200 - 300 m/sec) velocity reversal at the decollement as expected. The analysis of the SSPs in an area about 100 km away, however, does point to a trend of decreasing velocity just above the decollement with increasing distance under the wedge.Show more Item Post mid-Cretaceous seismic stratigraphy and depositional history, deep Gulf of Mexico(1995-05) Feng, Jianhua, 1953-; Buffler, Richard T.Show more A detailed seismic stratigraphic analysis of post mid-Cretaceous sedimentary rocks has resulted in the definition of eighteen post mid-Cretaceous seismic sequences in the deep Gulf of Mexico basin, based mainly on a pronounced depositional cyclicity. Each seismic sequence is interpreted to consist of distal lowstand systems tracts overlain by a regional condensed section, which is interpreted to be the basinal equivalent of transgressive and highstand systems tracts present on the adjacent shelf margins. True sequence boundaries, therefore, represent correlative conformities directly overlying the condensed sections. A chronostratigraphic framework established in this study allows for the first time a detailed comparison of deep basin stratigraphy with that on the peripheral Gulf regions. This comparison reveals genetic relationships between deep water sediments and Cenozoic shelf margin depocenters. Major depositional and tectonic events on the surrounding shelf margins control the development of deep basin sequences and determine the temporal and spatial distribution of deep water sediments. Late Cretaceous to middle Eocene sequences reflect the influences of the Laramide orogeny and mountain building in the southern Cordilleran. Tremendous amounts of sediments were derived mainly from the west and were deposited in the western deep basin. A backstripping study reveals an anomalous tectonic subsidence event (2.5km) in the western deep basin as compared to normal thermal subsidence of oceanic crust. It suggests that the Laramide orogeny possibly modified the western deep basin into a foreland basin. Late Cenozoic sequences reflect a major reorganization of source areas and the development of the Mississippi River drainage system. Sediments were delivered through the Mississippi River drainage system and progressivly filled the northern part of the deep basin. Isochron, seismic facies, and depositional systems maps for the eighteen sequences reveal the detailed infill and depositional history of the deep basin. Each sequence contains several submarine fan or fan lobes along the deep basin margins, while the remaining area is dominated by basinal mud-rich turbidites and sediment starvation. The persistent outbuilding of Cenozoic deltaic shelf margins brought a progressive change in depositional systems in the deep basin, from distal to more proximal part of fan systems.Show more Item Pre-stack inversion for porosity estimation from seismic data in an oil field, Eastern Saudi Arabia(2008) AlMuhaidib, Abdulaziz Mohammad; Sen, Mrinal K.Show more The main objective of seismic inversion is to obtain earth model parameters from seismic reflection data. In other words, it is the process of determining what physical characteristics of rocks and fluids (i.e., P-impedance, shear impedance, and density) could have produced the seismic record. The aim of this study is to obtain reservoir properties, such as porosity both at the well locations and in the inter-well regions from seismic data and incorporated well logs. The target is a Jurassic carbonate reservoir from an oil field located to the East of Saudi Arabia. The purpose was to investigate the reliability of inferring the elastic properties (Zp, Zs, ρ) from seismic data in this field, and to build a geologic framework for flow simulation for better reservoir production forecasting and management. The seismic data were processed with special attention to preserving the true reflection amplitudes, and were time migrated before stack. Residual moveout from multiples after NMO, however, is almost horizontal at near offset, and constructively add to the stacked amplitude. Therefore, we applied a pre-stack inversion technique on the seismic data, after careful processing, including removal of residual internal multiples. Such an inversion incorporates all of the offsets to obtain an optimum acoustic impedance model. We also investigated the stability of inverting shear impedance and density in the field of study. The seismic inversion results were overall very good and stable for P-impedance. The match between borehole log and seismic impedance profiles was excellent for the high-contrast events and variable for the low contrast in acoustic impedance, depending on the location within the field. Inverted shear impedance results were less stable compared to P-Impedance, while density was totally unstable and has not been resolved. In general, areas of poor inversion coincided with the zones of poor quality seismic data. The borehole log data showed a good impedance-porosity relationship. The Raymer-Hunt-Gardner impedance-porosity empirical relation fits the borehole data very well. Thus, I used the Raymer-Hunt-Gardner relation, with coefficients for this field derived from the log data, to convert inverted acoustic impedance into a porosity model for the field. Based on the new quantitative seismic reservoir characterization, I was able to identify additional areas of potentially good reservoir qualityShow more Item Residual migration velocity analysis in the plane wave domain : theory and applications(2001-05) Jiao, Junru, 1959-; Stoffa, Paul L., 1948-Show more This dissertation addresses velocity depth model building using residual migration velocity analysis in the plane wave domain. The criterion used for residual migration velocity analysis is that the results of migration with the correct velocity-depth model should reveal the same geologic structure on common image gathers (CIG’s). That is, the events on the CIG are horizontally aligned since they represent the image of the same subsurface position obtained at different angles. Use of an incorrect velocity-depth model in migration causes misalignment of events in a CIG, i.e., the events on the CIG exhibit residuals. By analyzing the residuals on the CIG, we can derive the depth and the velocity corrections and thus obtain a corrected velocity depth model. I first discuss the kinematics of seismic wave propagation and explore prestack depth migration in the plane wave (τ, p) domain. Then, I derive the exact one-, two-, and three-dimensional residual migration equations in the depth-p domain after pre-stack depth migration. To perform interval velocity analysis, a suite of velocity corrections is tested to do residual migration but only one gives the best image. The combination of this velocity correction and the original migration velocity improves the velocity model. The two main advantages of the new method are that it derives interval velocities directly and is computationally very efficient because only a top down residual migration is needed instead of top-down pre-stack depth migration. Next, I apply the new method to both the synthetic and real seismic data. The synthetic data examples show that the 2D method gives a better residual migration result than the 1D method when strong dips are present but the 1D equation also works well for 2D models when the dip angles are small. After getting a new velocity depth model, one can use the new model to perform a complete residual migration which gives much better CIG’s and stacked sections than those without residual migration. Alternatively, we can also use the new model to migrate the data again and then repeat the residual velocity analysis for another iteration. The number of iterations depends on the initial model and the precision required. In the field data example, a reasonable model was obtained after only four iterations.Show more