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.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.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-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.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-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.Item MCMC algorithm, integrated 4D seismic reservoir characterization and uncertainty analysis in a Bayesian framework(2008-08) Hong, Tiancong, 1973-; Sen, Mrinal K.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.Item Residual migration velocity analysis in the plane wave domain : theory and applications(2001-05) Jiao, Junru, 1959-; Stoffa, Paul L., 1948-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.