Browsing by Subject "Geophysics"
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Item Characterization of the Cana-Woodford Shale using fractal-based, stochastic inversion, Canadian County, Oklahoma(2016-05) Borgman, Barry Michael; Spikes, Kyle; Sen, Mrinal K; Wilson, Clark RThe past decade has seen a surge in unconventional hydrocarbon exploration and production, driven by advances in horizontal drilling and hydraulic fracturing. Even with such advances, reliable models of the subsurface are crucial in all phases of exploitation. This study focuses on the methods used for estimation of the elastic properties (density, velocity, and impedance), which play a key role in targeting reservoir zones ideal for hydraulic fracturing. Well-log data provides high-resolution vertical measurements of elastic properties, but a relatively shallow depth of investigation imposes spatial limitations. Seismic data provides broader horizontal coverage at lower cost, but sacrifices vertical resolution. Thin beds present in many unconventional reservoirs fall below seismic resolution. In addition, the band-limited nature of seismic data results in the absence of low-frequency content of the Earth model, as well as the high-frequency content present in well logs. Seismic inversion is a process that provides estimates of elastic properties given input seismic and well data. Stochastic inversion is a method that uses well-log data as a priori information, with an added aspect of randomness. The method generates many realizations using the same input model and takes an average of those realizations. We implement two separate stochastic inversion algorithms to estimate P-impedance in the Cana-Woodford Shale in west-central Oklahoma. First, we use a fractal-based, very fast simulated annealing algorithm that exploits the fractal characteristics found in well-log data to build a prior model. The method of very fast simulated annealing optimizes our elastic model by searching for the minimum misfit between observed and synthetic seismic traces. Next, we use a principal component analysis (PCA) based stochastic inversion algorithm to invert for impedance at all traces simultaneously. Comparison of the results with traditional deterministic inversion results shows improved vertical resolution while honoring the low-frequency content of the Earth model. The PCA-based inversion results also show improved lateral continuity of the elastic profile along our 2D line. The impedance profile from the PCA-based approach provides a better representation of the vertical and horizontal variability of the reservoir, allowing for improved targeting of frackable zones.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, PaulOverpressure, 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.Item The effects of pressure variations and chemical reactions on the elasticity of the Lower Tuscaloosa sandstone of the Cranfield Field, Mississippi(2011-08) Joy, Corey Anthony; Sen, Mrinal K.; Tatham, Robert; Spikes, KyleCompliance with current and evolving federal and commercial regulations require the monitoring of injected carbon dioxide for geological sequestration. The goal of this project is to provide geophysicists with tools to quantitatively interpret seismic data for the amount of carbon dioxide retained in subsurface reservoirs. Rock physics can be used to predict the effects on the seismic response of injecting carbon dioxide on the reservoir. However, classical rock physics models fail when chemical reactions alter the microstructure of the host rock. These chemically induced changes can stiffen or soften the rock frame by precipitation or dissolution, respectively, of minerals in the pore space. Increasing pore pressure is another effect of sequestering carbon dioxide. The amount of change in the microstructure due to chemical reactions and pressure variations depends on the reservoir into which the fluid is injected. Therefore, measuring velocities on site-specific subsurface core samples may provide the ability to differentiate between chemical reactions and pressure variations on the elastic properties of the reservoir rock. Core samples come from the Lower Tuscaloosa Sandstone of the Cranfield study area in Mississippi. The experiments consisted of injecting core plugs with carbon dioxide rich brine and measuring compressional and shear velocities at different effective pressures. The elastic moduli of the rock frame are calculated from the measured elastic wave propagation velocities at specific injected pore volumes and effective pressures. Injecting carbon dioxide rich brine into sandstone core samples, which are composed on average of 80% quartz and 20% clay minerals, resulted in softening of the rock frame due to the dissolution of iron bearing minerals. The moduli exponentially decreased with injected pore volumes and were linearly proportional to effective pressure. The bulk modulus and rigidity of the more quartz rich sample decreased by 13% and 6.5%, respectively, due to a combined effect of changing differential pressure from 35 MPa to 27 MPa and injecting CO₂-rich brine. For the more clay rich sample, the moduli decreased by even larger percentages (39.0% and 20.1%, respectively), which could have significant implications on time-lapse seismic data and subsequent estimations of injected CO₂ volumes.Item Fluid Characterization at the Cranfield CO₂ Injection Site : Quantitative Seismic Interpretation from Rock-Physics Modeling and Seismic Inversion(2014-12) Carter, Russell Wirkus; Spikes, KyleThis dissertation focuses on quantitatively interpreting the elastic properties of the Cranfield reservoir for CO₂ saturation. In this work, quantitative interpretation starts by examining the relationship between CO₂ saturation and the elastic properties of the reservoir. This relationship comes from a rock-physics model calibrated to measured well data. Seismic data can then be inverted using a model for CO₂ saturation and rock-property estimates. The location and saturation of injected CO₂ are important metrics for monitoring the long-term effectiveness of carbon capture utilization and storage. Non-uniform CO₂ saturation is a contributing factor to both lateral and time-lapse changes in the elastic properties of the Cranfield reservoir. In the Cranfield reservoir, CO₂ saturation and porosity can be estimated from the ratio of P-wave velocity (Vp) to S-wave velocity (Vs) and P-impedance (Ip), respectively. Lower values of Ip for a given rock matrix often correlate to higher porosity. Similarly, for a given area of the reservoir, lower Vp/Vs frequently can be associated with higher CO₂ saturation. If a constant porosity from the baseline to the time-lapse survey is assumed, changes in Ip over time can be attributed to changes in CO₂ saturation in lieu of using Vp/Vs. Decreases in Ip between the baseline and time-lapse survey can be attributed to increases in CO₂ saturation. With a rock-physics model calibrated to the reservoir, Ip and Is from a vertical seismic profile were correlated to statistical ranges of porosity and CO₂ saturations. To expand the lateral interpretation of reservoir porosity and CO₂ saturation, the time-variant changes in Ip between baseline and time-lapse surface seismic datasets were compared to changes in CO₂ saturation calculated from the rock-physics model. Characterizing the CO₂ saturation of the Tuscaloosa sandstones helped to establish a workflow for estimating reservoir properties and fluid saturation from multiple types of geophysical data. Additionally, this work helped establish an understanding for how CO₂ injected into a reservoir alters and changes the elastic properties of the reservoir and the degree to which those changes can be detected using geophysical methods.Item Geophysical investigations in the Nankai Trough and Sumatran subduction zones(2011-12) Martin, Kylara Margaret; Gulick, Sean P. S.The 2004 Sumatra-Andaman and the 2011 Tohoku-Oki earthquakes demonstrate the importance of understanding subduction zone earthquakes and the faults that produce them. Faults that produce earthquakes and/or tsunamis in these systems include plate boundary megathrusts, splay faults (out of sequence thrusts), and strike-slip faults from strain partitioning. Offshore Japan, IODP Exp. 314 collected logging while drilling (LWD) data across several seismically-imaged fault splays in the Nankai Trough accretionary prism. I combine LWD resistivity data with a model of fluid invasion to compare the permeabilities of sands. My results indicate that sands within faulted zones are 2-3 orders of magnitude more permeable than similar undisturbed sands. Therefore fault zones are likely to be fluid conduits within the accretionary wedge. Fluids can affect the physical and chemical properties of the faulted material, increasing pore pressures and effectively lubricating the faults. Fluids play an important role in fault slip, but hazard analysis also requires an understanding of fault geometry and slip direction. Both Japan and Sumatra exhibit strain partitioning, where oblique convergence between tectonic plates is partitioned between the megathrust and strike-slip faults proximal to the arc. Offshore Sumatra, I combine profiles from a 2D seismic survey (SUMUT) with previous bathymetry and active seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Along this fault I interpret transpressional flower structures that cut older thrust faults. These flower structures indicate that the modern West Andaman Fault is a right lateral strike-slip fault and thus helps to accommodate the translational component of strain in this highly oblique subduction zone. Offshore the Kii Peninsula, Japan, I analyze a trench-parallel depression that forms a notch in the seafloor just landward of the megasplay fault system, along the seaward edge of the forearc Kumano Basin. Using a 12 km wide, 3D seismic volume, I observe vertical faults and faults which dip toward the central axis of the depression, forming apparent flower structures. The along-strike geometry of the vertical faults makes predominantly normal or thrust motion unlikely. I conclude, therefore, that this linear depression is the bathymetric expression of a transtensional fault system. While the obliquity of convergence in the Nankai Trough is small (~15 degrees), this Kumano Basin Edge Fault Zone could be due to partitioning of the plate convergent strain. The location of the West Andaman Fault and KBEFZ within the forearc may be controlled by the rheology contrast between active accretionary wedges and the more stable crust beneath forearc basins.Item Identification of Pore Structure and Clay Content from Seismic Data within an Argillaceous Sandstone Reservoir(2014-08-11) Schelstrate, RobertSandstone facies are good reservoirs for the accumulation of hydrocarbons in conventional exploration due to high porosity and permeability. Grain size variations within a sandstone reservoir can range from pebbles to shale, depending on the depositional environment. Increasing amounts of shale become a limiting factor in reservoir quality by creating baffles to fluid flow. Seismic inversion has been used to map reservoir properties such as lithology and porosity. Previous studies have established a relationship between acoustic velocity and porosity, but have not accounted for pore structure, and most methods require data that is not easily available in hydrocarbon exploration and production. Rock physics models have been used to differentiate pore structure of spherical quartz grains and elongated clay minerals. Other studies have developed applicable rock physics models for identifying clay content from experimental and well log data in a shaly sandstone reservoir. The purpose of this study was to correlate a rock physics-based petrophysical parameter with seismic attributes in order to map and predict the location of fluid baffles. The project entailed calculating the clay content within the target reservoir, utilizing the Hertz-Mindlin and Sun (HMS) rock physics model to wells logs within the Norne field, offshore Norway. The HMS model provided the ability to correlate clay content with acoustic impedance. A new variable was established that links acoustic impedance to the product of porosity and the pore structure parameter (?) from Sun?s rock physics model. This new variable allows pore structure to be identified using post-stack seismic inversion. At the well locations, the relationships for acoustic impedance (AI)-porosity (?) and AI-product (??) were developed using the following two equations : AI = A - B * ? and AI = A - B * (??) Upon completion of the petrophysical analysis, deterministic seismic inversion was performed, using well log and seismic data to build an inverse model to identify the spatial distribution of clay content within the reservoir. Deterministic seismic inversion generated a best-case reservoir model, which was used to predict zones of increased clay content within the argillaceous sandstone reservoir. Using the established AI-? and AI-?? relationships, the ? and ?? were calculated from the acoustic impedance volume, and were depicted spatially and vertically throughout the target formation. The acoustic impedance-product relationship provided a better method of identifying variations in pore structure than the traditional acoustic impedance-porosity relationship. Additionally, the results also showed an increase in resolution using the AI-?? relationship. Mapping levels of clay content and porosity using this method can aid in reservoir characterization, field development, and maximizing hydrocarbon production.Item Landscape archaeogeophysics : a study of magnetometer surveys from Etowah (9BW1), the George C. Davis site (41CE19), and the Hill Farm site (41BW169)(2009-12) Walker, Chester Phil; Wilson, Samuel Meredith, 1957-Archaeogeophysics, the use of eophysical mapping techniques to recover archaeological information, is being used with increasing success in North America. Archaeologists can often use geophysics as a tool for collecting data suitable for direct archaeological interpretation (Kvamme 2003). In some cases, geophysics can be used to map entire archaeological landscapes providing an image of the site that is not easily achievable through the use of traditional archaeological excavations. This dissertation uses archaeogeophysical data from three prehistoric sites to gain insights into their layout and community organization as well as explore the possibilities and potentials of using broad scale geophysical surveys in North American archaeological research.Item Mapping the Rivera and Cocos subduction zone(2013-12) Suhardja, Sandy Kurniawan; Grand, Stephen P.The crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear.Item Mechanics of lithospheric delamination in extensional settings.(2015-03-23) Jex, Jeffrey A. 1988-; Dunbar, John A., 1955-Delamination, the foundering of the lower crust and sub-crustal lithosphere, is one of the most important geodynamic processes that is still poorly understood. Geodynamic modeling has constrained conditions and likely outcomes of delamination in orogenically-thickened crust. In this study, I do the same for delamination in extensional settings by using finite element models of young passive margins. Delamination in these models may occur as melt beneath oceanic crust intrudes between the lower continental crust and sub-crustal lithosphere, driven by buoyancy. When sufficient melt is available and the lower crust is weak, the melt wedges between the lower crust and sub-crustal lithosphere, initiating delamination of the sub-crustal lithosphere. The speed of delamination is strongly dependent on weakness of the lower crust followed by the amount of melt present.Item Multiazimuth velocity analysis using velocity-independent seismic imaging(2011-05) Burnett, William Andrew, 1983-; Fomel, Sergey B.; Stoffa, Paul L., 1948-Multiazimuth seismic data contains information about how the Earth’s seismic response changes with azimuthal direction. Directional-dependence of the seismic response can be caused by anisotropy or heterogeneity, associated with subsurface features such as fractures, stresses, or structure. Characterizing azimuthal variations is done through velocity analysis, which provides a link between an acquired data set and its image, as well as between the image and subsurface geology. At the stage which conventional velocity analysis is applied, it is difficult to distinguish the geologic cause of observed azimuthal velocity variations. The inability to distinguish the similar effects of anisotropy and heterogeneity leads to positioning errors in the final image and velocity estimates. Regardless of the cause, azimuthally variable velocities require at least three parameters to characterize, as opposed to the conventional single-parameter isotropic velocity. The semblance scan is the conventional tool for seismic velocity analysis, but it was designed for the isotropic case. For multiple parameters, the semblance scan becomes computationally impractical. In order to help address the xiissues of geologic ambiguity and computational efficiency, I develop three methods for multiazimuth seismic velocity analysis based on “velocity-independent” imaging techniques. I call this approach, velocity analysis by velocity-independent imaging, where I reverse the conventional order of velocity estimation followed by image estimation. All three methods measure time-domain effective-velocity parameters. The first method, 3D azimuthally anisotropic velocity-independent NMO, replaces the explicit measurement of velocity with local slope detection. The second method, time-warping, uses local slope information to predict traveltime surfaces without any moveout assumption beforehand, and then fit them with a multiparameter velocity model. The third method, azimuthal velocity continuation, uses diffraction image focusing as a velocity analysis criterion, thereby performing imaging and velocity analysis simultaneously. The first two methods are superior to the semblance scan in terms of computational efficiency and their ability to handle multi-parameter models. The third method is similar to a single multi-parameter semblance scan in computational cost, but it helps handle the ambiguity between structural heterogeneity and anisotropy, which leads to better positioned images and velocity estimates.Item Paleomagnetism of Igenous Rocks from Shatsky Rise(2013-04-24) Pueringer, MargaretShatsky Rise is oceanic plateau in the northwest part of the Pacific Ocean, and the formation of Shatsky Rise is poorly known. To get a better understanding of the formation Integrated Ocean Drilling Program (IODP) Expedition 324 drilled five sites: Sites U1347 and U1348 on Tamu Massif, Site U1349 and U1350 on Ori Massif, and Site U1346. Paleomagnetic measurements of the basaltic flows recovered can give insight into the timing and paleolatitude of each site. Relating the change in principle component inclination over depth at each site to the paleosecular variation of the geomagnetic field can better constrain the timing of the eruptions. Measurements were carried out by different sources during IODP Expedition 324 and after. This study is an amalgamation of the results from Sites U1346, U1347, U1349, and U1350. Samples from each site were divided into half and demagnetized using alternating field (AF) demagnetization and thermal (TH) demagnetization. After the drilling overprint was removed most samples displayed univectorial decay in the orthogonal vector plot. AF demagnetized samples displayed a low median destructive field (MDF) behavior, <10 mT, and a moderate MDF behavior, >10-20 mT. Thermal demagnetized samples displayed three behaviors: a rapid decline in magnetic intensity after moderate temperature steps behavior, a linear decline in magnetic intensity behavior, and some samples displayed a small segment of self-reversal at 300?-350?. Using the Cox and Gordon (1984) method Sites U1346, U1347, and U1349 displayed very little variation in principal component inclinations over depth, implying a relatively rapid lava emplacement of 10^2-10^3 years. Site U1350 display more variation, implying a longer eruptive time frame of 10^4-10^5years. With the assumption of a normal polarity the paleolatitude estimates are -11.0? +22.2?/-21.4? for Site U1346, 11.3? 27.4?/-28.5? for Site U1347, -5.0? +20.8?/-20.6? for Site U1349 and 1.6? ?7.7 ? for Site U1350. The site paleolatitudes imply that Ori Massif (Sites U1350 and U1349) formed at the equator and Tamu Massif (Site U1347) and Shirshov Massif (Site U1346) formed slightly north and south of the equator respectively. All results are consistent with the interpretation that Shatsky Rise formed near the equator.Item Phase-space imaging of reflection seismic data(2014-08) Bashkardin, Vladimir; Fomel, Sergey B.; Stoffa, Paul L., 1948-Modern oil and gas exploration depends on a variety of geophysical prospect tools. One of them is reflection seismology that allows to obtain interwell information of sufficient resolution economically. This exploration method collects reflection seismic data on the surface of an area of prospect interest and then uses them to build seismic images of the subsurface. All imaging approaches can be divided into two groups: wave equation-based methods and integral schemes. Kirchhoff migration, which belongs to the second group, is an indispensable tool in seismic imaging due to its flexibility and relatively low computational cost. Unfortunately, the classic formulation of this method images only a part of the surface data, if so-called multipathing is present in it. That phenomenon occurs in complex geologic settings, such as subsalt areas, when seismic waves travel between a subsurface point and a surface location through more than one path. The quality of imaging with Kirchhoff migration in complex geological areas can be improved if multiple paths of ray propagation are included in the integral. Multiple arrivals can be naturally incorporated into the imaging operator if it is expressed as an integral over subsurface take-off angles. In this form, the migration operator involves escape functions that connect subsurface locations with surface seismic data values through escape traveltime and escape positions. These escape quantities are functions of phase space coordinates that are simply related to the subsurface reflection system. The angle-domain integral operator produces output scattering- and dip-angle image gathers, which represent a convenient domain for subsurface analysis. Escape functions for angle-domain imaging can be simply computed with initial-value ray tracing, a Lagrangian computational technique. However, the computational cost of such a bottom-up approach can be prohibitive in practice. The goal of this work was to construct a computationally efficient phase space imaging framework. I designed several approaches to computing escape functions directly in phase space for mapping surface seismic reflection data to the subsurface angle domain. Escape equations have been introduced previously to describe distribution of escape functions in the phase space. Initially, I employed these equations as a basis for building an Eulerian numerical scheme using finite-difference method in the 2-D case. I show its accuracy constraints and suggest a modification of the algorithm to overcome them. Next, I formulate a semi-Lagrangian approach to computing escape functions in 3-D. The second method relies on the fundamental property of continuity of these functions in the phase space. I define locally constrained escape functions and show that a global escape solution can be reconstructed from local solutions iteratively. I validate the accuracy of the proposed methods by imaging synthetic seismic data in several complex 2-D and 3-D models. I draw conclusions about efficiency by comparing the compute time of the imaging tests with the compute time of a well-optimized conventional initial-value ray tracing.Item The seismic response to fracture clustering : a finite element wave propagation study(2014-05) Becker, Lauren Elizabeth; Spikes, KyleCharacterizing natural and man-made fracture networks is fundamental to predicting the storage capacity and pathways for flow of both carbonate and shale reservoirs. The goal of this study is to determine the seismic response specifically to networks of fractures clustered closely together through the analysis of seismic wavefield scatter, directional phase velocities, and amplitude attenuation. To achieve this goal, finite element modeling techniques are implemented to allow for the meshing of discontinuous fracture interfaces and, therefore, provide the most accurate calculation of seismic events from these irregular surfaces. The work presented here focuses on the center layer of an isotropic model that is populated with two main phases of fracture network alteration: a single large-scale cluster and multiple smaller-scale clusters. Phase 1 first confirms that the seismic response of a single idealized vertically fractured cluster is distinct crosscutting energy within a seismogram. Further investigation shows that, as fracture spacing within the cluster decreases, the depth at which crosscutting energy appears exponentially increases, placing it well below the true location of the cluster. This relationship holds until 28% of the fractures are moved from their uniformly spaced locations to random locations within the cluster. The vertical thickness of the cluster has little effect on the location or strength or the crosscutting signature. Phase 2 shows that, although clusters of more randomly spaced fractures mask crosscutting energy, a marked decrease in amplitude coinciding with a bend in the wavefront produces a heterogeneous anisotropic seismic response. This amplitude decay and heterogeneous anisotropy is visible until cluster spacing drops below one half of the wavelength or the ratio of fractured material to matrix material within a cluster drops below 37%. Therefore, the location of an individual fracture cluster can be determined from the location of amplitude decay, heterogeneous anisotropy, and crosscutting energy. Furthermore, the density of the cluster can be determined from the degree of amplitude decay, the angle of heterogeneous anisotropy, and the depth of cross-cutting energy. These relationships, constrained by limits on their detectability, can aid fracture network interpretation of real seismic data.Item Time reversal and plane-wave decomposition in seismic interferometry, inversion and imaging(2012-12) Tao, Yi, active 2012; Sen, Mrinal K.This thesis concerns the study of time reversal and plane-wave decomposition in various geophysical applications. Time reversal is a key step in seismic interferometry, reverse time migration and full waveform inversion. The plane-wave transform, also known as the tau-p transform or slant-stack, can separate waves based on their ray parameters or their emergence angles at the surface. I propose a new approach to retrieve virtual full-wave seismic responses from crosscorrelating recorded seismic data in the plane-wave domain. Unlike a traditional approach where the correlogram is obtained from crosscorrelating recorded data, which contains the full range of ray parameters, this method directly chooses common ray parameters to cancel overlapping ray paths. Thus, it can sometime avoid spurious arrivals when the acquisition requirement of seismic interferometry is not strictly met. I demonstrate the method with synthetic examples and an ocean bottom seismometer data example. I show a multi-scale application of plane-wave based full waveform inversion (FWI) with the aid of frequency domain forward modeling. FWI uses the two-way wave-equation to produce high-resolution velocity models for seismic imaging. This technique is implemented by an adjoint-state approach, which viii involves a time-reversal propagation of the residual wavefield at receivers, similar to seismic interferometry. With a plane-wave transformed gather, we can decompose the data by ray parameters and iteratively update the velocity model with selected ray parameters. This encoding approach can significantly reduce the number of shots and receivers required in gradient and Hessian calculations. Borrowing the idea of minimizing different data residual norms in FWI, I study the effect of different scaling methods to the receiver wavefield in the reverse time migration. I show that this type of scaling is able to significantly suppress outliers compared to conventional algorithms. I also show that scaling by its absolute norm generally produces better results than other approaches. I propose a robust stochastic time-lapse seismic inversion strategy with an application of monitoring Cranfield CO2 injection site. This workflow involves two steps. The first step is the baseline inversion using a hybrid starting model that combines a fractal prior and the low-frequency prior from well log data. The second step is to use a double-difference inversion scheme to focus on the local areas where time-lapse changes have occurred. Synthetic data and field data show the effectiveness of this method.