# Browsing by Subject "Seismic waves"

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Item A ray path explanation for RG waves from earthquakes(Texas Tech University, 1983-05) Ismail, Mohamad Izham BShow more Not availableShow more Item The anisotropic seismic structure of the Earth's mantle : investigations using full waveform inversion(2002) Matzel, Eric M.; Grand, Stephen P.Show more I have developed a waveform inversion procedure to invert 3 component broadband seismic data for models of the anisotropic seismic structure of the Earth and applied the technique to an investigation of wave propagation through anisotropic media and earthquake data sampling the upper mantle beneath the East European platform. The procedure combines the conjugate-gradient and very fast simulated annealing methods and attempts to minimize a cross-correlation misfit function comparing data to synthetic seismograms. A series of inversion passes are performed over a range of frequency and time windows to progressively focus in on structural details. The intent is to obtain P and S velocity models that simultaneously match all components of the data (radial, vertical and tangential). The variables in the problem are the seismic velocities ([alpha] and [beta]) as a function of depth. When radial anisotropy is required this set is expanded to include the five variables that determine the seismic velocities in a radially anisotropic medium ([alpha subscript h, alpha subscript v, beta subscript h, beta subscript v, eta]). I investigate the propagation of seismic waves through radially anisotropic media, evaluate which elements of radial anisotropy are best resolved by seismic data and discuss strategies for identifying radial anisotropy in the Earth. S anisotropy, [beta]%, and the horizontal component of P velocity, [alpha subscript h], are typically well resolved by multicomponent seismic data. P anisotropy, [alpha]%, and [eta] are often poorly resolved and trade off with one another in terms of their effect on S[subscript V] arrivals. Erroneous structure will be mapped into models if anisotropy is neglected. The size of the erroneous structure will be proportional to the magnitude of anisotropy present and extend well below the anisotropic zone. The effects of anisotropy on P models produced with an isotropic assumption are most similar to the effects on isotropic S[subscript H] models. When comparing isotropic models, [alpha/beta subscript sh] is therefore often a better measure than [alpha/beta subscript sv] for characterizing mantle petrology. Isotropic S[subscript H], S[subscript V] and P models developed separately using the same data set can provide a good initial estimate of the presence, location and magnitude of anisotropy and those results can be used to create an initial model for an anisotropic inversion solving simultaneously for all 3 components of the data. Finally, I present models for the P and S velocity structure of the upper mantle beneath the East European platform including an analysis of radial anisotropy. The data are 3-component broadband seismograms from strike-slip earthquakes located near the edge of the platform and recorded in Russia and Europe. The timing, amplitude and interference characteristics of direct arrivals (S, P), multiply reflected arrivals (SS, PP), converted phases and surface waves provide very good radial resolution throughout the upper 400 km of the mantle. The platform is underlain by a radially anisotropic seismic mantle lid extending to a depth of 200 km with a largely isotropic mantle below. The model has a positive velocity gradient from 41 km to 100 km depth, and a relatively uniform velocity structure from 100 km to 200 km depth with high S[subscript H] and P[subscript H] velocities (4.77 km /s, 8.45 km/s). Shear anisotropy is uniform at 5% ([beta subscript H] > [beta subscript V]) from 41 to 200 km depth, drops to 2% from 200 to 250 km and is isotropic below that. The average shear velocity from 100 to 250 km is also uniform at 4.65 km/s and the drop in anisotropy is matched by a drop in [beta subscript H] to 4.70 km/s combined with an increase in [beta subscript V] to 4.60 km/s. Below 250 km there is a positive velocity gradient in both P and S velocity down to 410 km. P anisotropy is not well resolved, but P structure mimics the S[subscript H] velocity structure, suggesting that P is also anisotropic within the lid.Show more Item Characterizing Vs profiles by the SASW method and comparison with other seismic methods(2007) Lin, Yin-Cheng; Stokoe, Kenneth H.Show more The shear wave velocity (VS) profile has been used as an important parameter in characterizing geotechnical sites and performing earthquake designs. The SpectralAnalysis-of-Surface-Wave (SASW) method, one of the VS profiling methods, was developed in the early 1980s. This method is a non-intrusive test which uses Rayleigh waves, one kind of surface wave, to explore the subsurface. The SASW method has been widely used in geotechnical earthquake engineering to profile soil and rock sites. All equipment required to conduct the SASW test is deployed on the ground surface and no boreholes are needed. In this study, the SASW method was used to measure shear wave velocity profiles in four different geographic regions. These four regions are: (1) Imperial Valley, CA, (2) Taiwan, (3) Hanford, WA and (4) Yucca Mountain, NV. The SASW tests performed at these locations were for different purposes. At the Imperial Valley and Taiwan sites, the SASW tests were carried out at the locations of strong motion recorders (SMR) to obtain VS profiles of the top 30 m (VS,30). At the Hanford and Yucca Mountain sites, deeper profiling (>300 m) was required to obtain VS values of the geotechnical structure around or beneath critical facilities associated with the handling, treatment and/or storage of high-level radioactive waste. The VS,30 values determined by the SASW method were used to classify the test sites based on the International Building Code (IBC-2006) provisions. Available downhole and suspension logging measurements at/near the SASW test sites were also used to determine VS,30. In addition, deeper VS profiles determined by the SASW, downhole and suspension logging methods were compared. By doing so, the consistency between the three seismic surveys methods and the reliability of the SASW method were studied. Finally, sensitivity studies of the SASW method were conducted to investigate: (1) the impact on the final VS profile of changing assumed parameters in the SASW data reduction process, and (2) the capability of the SASW method to detect relatively soft layers sandwiched between stiffer layers.Show more Item Characterizing Vs profiles by the SASW method and comparison with other seismic methods(2007-05) Lin, Yin-Cheng, 1972-; Stokoe, Kenneth H.Show more Item Complete anisotropic analysis of three component seismic data related to the marine environment and comparison to nine component land seismic data(2006) Gumble, Jason Ethan; Tatham, Robert H.Show more Item Computation of synthetic seismograms by the method of characteristics(Texas Tech University, 1979-08) Castro, Louis ReyesShow more Not availableShow more Item Deep water Gulf of Mexico pore pressure estimation utilizing P-SV waves from multicomponent seismic in Atlantis Field(2009-12) Kao, Jeffrey Chung-chen; Tatham, R. H. (Robert H.), 1943; Flemings, Peter B.; Krail, PaulShow more Overpressure, or abnormally low effective pressures, is hazardous in drilling operations and construction of sea-bottom facilities in deepwater environments. Estimation of the locations of overpressure can improve safety in these operations and significantly reduce overall project costs. Propagation velocities of both seismic P and S wave are sensitive to bulk elastic parameters and density of the sediments, which can be related to porosity, pore fluid content, lithology, and effective pressures. Overpressured areas can be analyzed using 4C seismic reflection data, which includes P-P and P-SV reflections. In this thesis, the effects on compressional (P) and shear (S) wave velocities are investigated to estimate the magnitude and location of excess pore pressure utilizing Eaton’s approach for pressure prediction (Eaton, 1969). Eaton’s (1969) method relates changes in pore pressure to changes in seismic P-wave velocity. The underlying assumption of this method utilizes the ratio of observed P-wave velocity obtained from areas of both normal and abnormal pressure. This velocity ratio evaluated through an empirically determined exponent is then related to the ratio of effective stress under normal and abnormal pressure conditions. Effective stress in a normal pressured condition is greater than the effective stress value in abnormally overpressured conditions. Due to an increased sensitivity of variations in effective pressure to seismic interval velocity, Ebrom et al. (2003) employ a modified Eaton equation to incorporate the S-wave velocity in pore pressure prediction. The data preparation and subsequent observations of seismic P and S wave velocity estimates in this thesis represent a preliminary analysis for pore pressure prediction. Six 2D receiver gathers in the regional dip direction are extracted from six individual ocean-bottom 4C seismic recording nodes for P-P and P-SV velocity analysis. The receiver gathers employed have minimal pre-processing procedures applied. The main processing steps applied were: water bottom mute, 2D rotation of horizontal components to SV and SH orientation, deconvolution, and frequency filtering. Most the processing was performed in Matlab with a volume of scripts designed by research scientists from the University of Texas, Bureau of Economic Geology. In this thesis, fluid pressure prediction is estimated utilizing several 4C multicomponent ocean-bottom nodes in the Atlantis Field in deepwater Gulf of Mexico. Velocity analysis is performed through a ray tracing approach utilizing P-P and P-SV registration. A modified Eaton’s Algorithm is then used for pore pressure prediction using both P and S wave velocity values. I was able to successfully observe both compressional and shear wave velocities to sediment depths of approximately 800 m below the seafloor. Using Hamilton (1972, 1976) and Eberhart-Phillips et al. (1989) regressions as background depth dependent velocity values and well-log derived background effective pressure values from deepwater Gulf of Mexico, I am able to solve for predicted effective pressure for the study area. The results show that the Atlantis subsurface study area experiences a degree of overpressure.Show more Item Digital signal processing for seismic inverse problem(Texas Tech University, 1983-05) Yuan, Kai-chih JohnShow more Not availableShow more Item Discussion of RG propagation in terms of ray paths of P and SV(Texas Tech University, 1984-08) Finlay, Corey DaneShow more Not availableShow more Item Dispersion of short period Rayleigh waves in the Atlantic Ocean(Texas Tech University, 1981-12) Harrison, Ben SShow more Eighteen earthquakes, recorded at Bermuda, from in front of and behind the West Indian Island Arc and near the Mid-Atlantic Ridge were analyzed. Short-period Rayleigh waves propagating from earthquakes in front of Puerto Rico and the Virgin Islands have apparent velocities which indicate that the energy travels from the epicenters as a P-phase to the steep wall of the Puerto Rico trench where it is transformed to Rayleigh first shear mode. Short-period Rayleigh waves from the Windward Passage exhibit slower apparent velocities indicating that they travel from epicenter through the Bahamian Platform in the fundamental Rayleigh mode where they are transformed to travel through oceanic structure in the first shear mode. Earthquakes from the Mid-Atlantic Ridge between 22Â°N and 31Â°N latitude produced only Rayleigh waves with periods of 16 seconds or greater. These waves are in fundamental Rayleigh mode in an environment of high attenuation of short period energy. Earthquakes in the Windward and Leeward Islands behind the West Indian Arc produce only Rayleigh waves with periods of 17 seconds or greater which are also propagated in the fundamental mode, and short-period energy is strongly attenuated before transformation to the first shear mode becomes possible.Show more Item Parameter inversion for seismic anisotropy(2006) Kumar, Chandan; Ferguson, Robert J.; Sen, Mrinal K.Show more An important objective of reflection seismology is to transform a seismic dataset into a reflectivity image of the subsurface. Reflectivity imaging involves removal of propagation effects by creating an inverse propagation operator using an estimated macro velocity model. The accuracy of a reflectivity image depends on the accuracy of the velocity model used. Hence, the estimation of a good velocity model is crucial. Flat sedimentary layers, though laterally homogeneous over small-scale, often exhibit a simple kind of anisotropy called vertical transverse isotropy (VTI). Negligence of anisotropy in migration may result in significant deviation of an imaged reflector from its true position. Our ability to estimate anisotropic parameters using limited aperture, limited bandwidth seismic data is restricted. Several recent developments in data acquisition including more intense acquisition (e.g., 3-D), broader bandwidths, and new survey methodologies (e.g., Ocean Bottom Seismics and vertical cables) have improved our ability in dealing with anisotropy. In light of these recent developments the objective of my research is three-fold - (i) finding better algorithms for anisotropy estimation that makes use of enhanced data acquisition, more powerful computers and graphical interfaces, (ii) understanding and re-evaluating our limitations/capabilities of anisotropy estimation in light of the new algorithms, and (iii) using the results of analysis as an aid to constrain anisotropy estimates. The Common Focus Point (CFP) technology defines a recent method of prestack migration velocity analysis which has several advantages over other popular methods. One remarkable advantage is that the imaging error is given directly in time allowing easier model updates through inversion. The CFP technique has been used in recent past for estimating isotropic heterogeneous velocity model for geologically complex subsurface. For my research, I extend this method to anisotropic parameter inversion by suitably implementing the space-time domain version of CFP method for picking the traveltime error resulting from the incorrect guess model. I successfully implement this approach to estimate anisotropy of the shale thrust-sheets in the Canadian Foothills model. This model has targets of interest below transversely isotropic shale thrust-sheets and estimating anisotropic parameters is important for proper depth imaging of target zone. The synthetic P-mode data used for the analysis has been generated using a finite difference algorithm. In order to quantitatively assess the advantages or limitations of CFP domain velocity analysis in estimating the anisotropic parameters, I have performed sensitivity analysis under different experimental settings like different observation apertures, layer thicknesses, tilt in symmetry axis, picking error and the use of mode converted data. The results quantitatively establish the advantage of joint inversion of P-P and P-Sv over the conventional inversion of solely P-P data in constraining the depth and anisotropic parameters. Also the constraint on parameter estimation improves with increase in angle of tilt of symmetry axis with respect to the reflector. The CFP domain analysis also has advantages in dealing with mode converted P-Sv data as separate focusing at the receiver side and source side allows it to deal with one wave type at a time. In complex media, this helps avoid the cumbersome calculation of common conversion point (CCP) location. The CFP analysis has been applied for estimating anisotropic parameters as well as layer thickness by joint inversion of P-P and P-Sv synthetic data for the geology encountered in Blackfoot field.Show more Item Propagation of elastic waves in plates duplicating seismic problems(Texas Tech University, 1961-05) Rice, G. WesleyShow more Not availableShow more Item Propagation of high frequency P and S waves under oceanic structures(Texas Tech University, 1993-12) Cakir, OzcanShow more The high frequency Po and So seismic waves observed at long distances under deep oceanic basins are investigated by means of numerical simulation. These waves are propagated particularly along the ray paths in relatively homogeneous and tectonicly stable regions of oceanic crust and upper mantle, and in continental shield areas. Propagation is terminated by structures such as midocean ridges, deep ocean trenches, and continental mountain chains. When expected variability in crust and upper mantle velocity depth structure is disrupted by such structural discontinuities, Po and So waves seize to travel. The seismic wave guide of crust and upper mantle may be continuous for hundreds of kilometers, as is the case in oceanic basins and continental shields. For these seismic ray paths, the wave guide is so efficient that waves recorded at large distances (as large as 3000 km or more) could include frequencies as high as 35 — 40 Hz. The long coda duration of 1 — 2 minutes is typical in these phases. The apparent velocity varies around 8.3 km/sec. for Po and 4.7 km/sec. for So. A special name, Po and So, has been given to these high frequency waves because of their predominance along oceanic paths.Show more Item Propagation of seismic waves in oceanic crust(Texas Tech University, 1985-08) Shih, Xiao RungShow more Not availableShow 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 Item Seismic modeling and imaging with Fourier method : numerical analyses and parallel implementation strategies(2009-12) Chu, Chunlei, 1977-; Stoffa, Paul L., 1948-Show more Our knowledge of elastic wave propagation in general heterogeneous media with complex geological structures comes principally from numerical simulations. In this dissertation, I demonstrate through rigorous theoretical analyses and comprehensive numerical experiments that the Fourier method is a suitable method of choice for large scale 3D seismic modeling and imaging problems, due to its high accuracy and computational efficiency. The most attractive feature of the Fourier method is its ability to produce highly accurate solutions on relatively coarser grids, compared with other numerical methods for solving wave equations. To further advance the Fourier method, I identify two aspects of the method to focus on in this work, i.e., its implementation on modern clusters of computers and efficient high-order time stepping schemes. I propose two new parallel algorithms to improve the efficiency of the Fourier method on distributed memory systems using MPI. The first algorithm employs non-blocking all-to-all communications to optimize the conventional parallel Fourier modeling workflows by overlapping communication with computation. With a carefully designed communication-computation overlapping mechanism, a large amount of communication overhead can be concealed when implementing different kinds of wave equations. The second algorithm combines the advantages of both the Fourier method and the finite difference method by using convolutional high-order finite difference operators to evaluate the spatial derivatives in the decomposed direction. The high-order convolutional finite difference method guarantees a satisfactory accuracy and provides the flexibility of using non-blocking point-to-point communications for efficient interprocessor data exchange and the possibility of overlapping communication and computation. As a result, this hybrid method achieves an optimized balance between numerical accuracy and computational efficiency. To improve the overall accuracy of time domain Fourier simulations, I propose a family of new high-order time stepping schemes, based on a novel algorithm for designing time integration operators, to reduce temporal derivative discretization errors in a cost-effective fashion. I explore the pseudo-analytical method and propose high-order formulations to further improve its accuracy and ability to deal with spatial heterogeneities. I also extend the pseudo-analytical method to solve the variable-density acoustic and elastic wave equations. I thoroughly examine the finite difference method by conducting complete numerical dispersion and stability analyses. I comprehensively compare the finite difference method with the Fourier method and provide a series of detailed benchmarking tests of these two methods under a number of different simulation configurations. The Fourier method outperforms the finite difference method, in terms of both accuracy and efficiency, for both the theoretical studies and the numerical experiments, which provides solid evidence that the Fourier method is a superior scheme for large scale seismic modeling and imaging problems.Show more Item Spatial delineation, fluid-lithology characterization, and petrophysical modeling of deepwater Gulf of Mexico reservoirs through joint AVA deterministic and stochastic inversion of 3D partially-stacked seismic amplitude data and well logs(2006) Contreras, Arturo Javier; Fisher, William L.; Torres-Verdin, CarlosShow more This dissertation describes a novel Amplitude-versus-Angle (AVA) inversion methodology to quantitatively integrate pre-stack seismic data, well logs, geologic data, and geostatistical information. Deterministic and stochastic inversion algorithms are used to characterize flow units of deepwater reservoirs located in the central Gulf of Mexico. A detailed fluid/lithology sensitivity analysis was conducted to assess the nature of AVA effects in the study area. Standard AVA analysis indicates that the shale/sand interface represented by the top of the hydrocarbon-bearing turbidite deposits generate typical Class III AVA responses. Layer-dependent Biot-Gassmann analysis shows significant sensitivity of the P-wave velocity and density to fluid substitution, indicating that presence of light saturating fluids clearly affects the elastic response of sands. Accordingly, AVA deterministic and stochastic inversions, which combine the advantages of AVA analysis with those of inversion, have provided quantitative information about the lateral continuity of the turbidite reservoirs based on the interpretation of inverted acoustic properties and fluid-sensitive modulus attributes (PImpedance, S-Impedance, density, and LambdaRho, in the case of deterministic inversion; and P-velocity, S-velocity, density, and lithotype (sand-shale) distributions, in the case of stochastic inversion). The quantitative use of rock/fluid information through AVA seismic data, coupled with the implementation of co-simulation via lithotype-dependent multidimensional joint probability distributions of acoustic/petrophysical properties, provides accurate 3D models of petrophysical properties such as porosity, permeability, and water saturation. Pre-stack stochastic inversion provides more realistic and higher-resolution results than those obtained from analogous deterministic techniques. Furthermore, 3D petrophysical models can be more accurately co-simulated from AVA stochastic inversion results. By combining AVA sensitivity analysis techniques with pre-stack stochastic inversion, geologic data, and awareness of inversion pitfalls, it is possible to substantially reduce the risk in exploration and development of conventional and non-conventional reservoirs. From the final integration of deterministic and stochastic inversion results with depositional models and analogous examples, the M-series reservoirs have been interpreted as stacked terminal turbidite lobes within an overall fan complex (the Miocene MCAVLU Submarine Fan System); this interpretation is consistent with previous core data interpretations and regional stratigraphic/depositional studies.Show more Item Synthetic seismograms and character modeling: an aid to the determination of the earth's structure from Rayleigh waves(Texas Tech University, 1980-05) Crider, Richard LShow more Not availableShow more Item Upper mantle seismic structure beneath the central Rio Grande rift and beneath eastern Mexico and their implications(2006-05) Gao, Wei, 1960-; Grand, Stephen P.Show more