The seismic method in exploration geophysics consists of creating a mechanical disturbance at or close to the surface of the earth, and observing its effects at a number of chosen locations along the surface. The purpose of seismic data acquisition is to record these effects in such a manner that their relation with the initial disturbance can be interpreted as a guide to the earth's subsurface structure (Nettleton, 1940).The validity of data interpretation depends upon the fidelity of recording. A better seismic interpretation can result from correctly collected data using instrumentation which faithfully records the seismic signal. Subsequent computer processing cannot reconstruct information which is not contained in the recorded field data. Hence, the quality of field data recording must be at an optimum level, otherwise the result will be an inferior interpretation (Donnell,1957).A reflection seismic data acquisition system was assembled and put into operation. The basic instrument was a Texas Instruments DFS IV, obtained from marine vessel M/V Banksia, and commissioned for land application.The system was tested and evaluated. The instrument analog filter phase distortion was studied in detail. The study indicated that phase distortion can be a major cause of seismic misties. Without a knowledge of the particular recording instrumentation filter transfer function, data processing bureaux may not compensate for phase distortion effects adequately (Gray et al., 1968).Once testing was completed satisfactorily, the operational system was applied to several practical field situations of commercial standard. A series of noise studies was performed to evaluate not only source generated noise, but also to study the effect of different types of energy sources on seismic data. In addition, two multi-fold seismic lines wore recorded, both of which were considered superior ++ / to those previously produced by the industry, at each location (Jacia, pers. comm., 1984).Finally, a single fold three-dimensional areal seismic survey was performed over the Woodada gas field. The results of this survey will be released after processing has been completed by Allied Geophysical Laboratories (University of Houston), and are not contained in this thesis.Future areas for field application are discussed. Recommendations are made for further research work in the area of phase distortion; the examination of different energy sources; a review of receiver properties and horizontally travelling seismic waves; a bore-hole seismic study and finally, a fourth-dimensional recording technique involving the performance of an offset VSP survey at the same time as an areal 3-D seismic survey.Volume 1 describes the establishment of the acquisition system and its subsequent field application.Volume 2 contains the Appendix of instrument tests and their analysis.

The study area, Dhurnal oil field, is located 74 km southwest of Islamabad in the Potwar basin of Pakistan. Discovered in March 1984, the field was developed with four producing wells and three water injection wells. Three main limestone reservoirs of Eocene and Paleocene ages are present in this field. These limestone reservoirs are tectonically fractured and all the production is derived from these fractures. The overlying claystone formation of Miocene age provides vertical and lateral seal to the Paleocene and Permian carbonates. The field started production in May 1984, reaching a maximum rate of 19370 BOPD in November 1989. Currently Dhurnal‐1 (D-1) and Dhurnal‐6 (D-6) wells are producing 135 BOPD and 0.65 MMCF/D gas. The field has depleted after producing over 50 million Bbls of oil and 130 BCF of gas from naturally fractured low energy shelf carbonates of the Eocene, Paleocene and Permian reservoirs. Preliminary geological and geophysical data evaluation of Dhurnal field revealed the presence of an up-dip anticlinal structure between D-1 and D-6 wells, seen on new 2003 reprocessed data. However, this structural impression is not observed on old 1987 processed data. The aim of this research is to compare and evaluate old and new reprocessed data in order to identify possible factors affecting the structural configuration. For this purpose, a detailed interpretation of old and new reprocessed data is carried out and results clearly demonstrate that structural compartmentalization exists in Dhurnal field (based on 2003 data). Therefore, to further analyse the available data sets, processing sequences pertaining to both vintages have been examined. After great effort and detailed investigation, it is concluded that the major parameter giving rise to this data discrepancy is the velocity analysis done with different gridding intervals. The detailed and dense velocity analysis carried out on the data in 2003 was able to image the subtle anticlinal feature, which was missed on the 1987 processed seismic data due to sparse gridding. In addition to this, about 105 sq.km 3D seismic data recently (2009) acquired by Ocean Pakistan Limited (OPL) is also interpreted in this project to gain greater confidence on the results. The 3D geophysical interpretation confirmed the findings and aided in accurately mapping the remaining hydrocarbon potential of Dhurnal field.

The oil and gas industry defines ultra-deep-water regions as areas in which water depths are greater than 1500 m. It is now well established that there are hydrocarbons in these regions. The reservoirs in these areas are generally located below basalt rocks or below salts. The focus of this thesis is to understand reflections, refractions, diffractions and scattering for acoustic lenses located below basalt rocks. The results of this study can potentially be used to understand the effect of tube waves on borehole seismic data in ultra-deep waters. Finite-difference modeling technique was used for this study. Finite-difference modeling allowed us to model refractions, reflections, diffractions and scattering; actually all events in surface seismic data, as well as borehole seismic data can be modeled. However, because of limited computational resources, this study will be based on a 2D finite difference instead of a 3D finite difference. This limitation implies that laterally infinite lenses were used to describe cylindrical boreholes. The four main characteristics of the geological constructs used here in simulating the ultra-deep-water regions were the size of the water column, the topography of the sea floor, the interfaces of basalt layers with the surroundings rocks, and the structure of heterogeneities inside the basalt layers. The average wavelength of wave propagation below the basalt layer is 125 m, which is very large compared to the size of a typical borehole (0.1 m). A lens with a thickness of 2.5 m, which corresponds to a dimension 50 times smaller than the average wavelength, sub-basalt was constructed. Also included were some lateral extensions in the construction of the lens to simulate wash-out zones. This study investigates the wave propagation below the basalt rocks and the effect of tube waves on borehole seismic data below the basalt layer by using these lenses instead of a cylindrical borehole. As the borehole geometry is different from that of the lens, the results are considered preliminary. Results suggest that tube waves are negligible in ultra-deep waters below basalt rocks because the wavelength of the seismic waves is large in comparison to the wash-out zone (192 times larger).

Shear-wave splitting and azimuthal variations of seismic attributes are two major anisotropic effects induced by vertically-aligned fractures. They both have influences on seismic data processing and interpretation, and provide information on fracture properties. Azimuthal variations in P-wave data have been intensively studied to improve imaging and obtain fracture parameters. However, azimuthal variations in PS-converted wave seismic data, particularly the velocity variation in PS-converted wave data, have not been well studied. Shear-wave splitting has been frequently used to estimate fracture directions and densities. However, its influence on the azimuthal variations of PS-converted wave data has also lacked a proper analysis. In this thesis, I analyse the anisotropic behaviour of PS-converted wave seismic data in the presence of azimuthal anisotropy, which includes the azimuthal variation of the PSconverted wave and PS-converted wave splitting. First, I demonstrate the robustness of PS-converted wave splitting for fracture characterisation. PS-converted wave seismic data is also influenced by the splitting effect due to its upgoing shear-wave leg. This important feature enables the application of shear-wave splitting analysis to PS-converted wave seismic data. I use synthetic data to show the necessity for separation of the split PS-converted waves. Then I apply the PS-converted wave splitting analysis to Sanhu 3D3C land seismic data. By separation of the fast and slow PS-converted waves and compensation for the time delays, the imaging quality has been improved. Dominant fracture properties obtained from the splitting analysis show a good correlation with the stress-field data. However, this work is accomplished by assuming only one set of vertical fractures in processing a given time window. In future work a specific layer-stripping algorithm could be constructed and applied. . Second, I study azimuthal variations of velocities in PS-converted wave seismic data. It involves two major parts: analysing azimuthal variations of NMO velocities to improve imaging, and examining the sensitivity of azimuthal variations to different fluid saturations. For a layer with HTI anisotropy induced by a set of vertical fractures, seismologists usually analyse the azimuthal behaviour exhibited on the radial and transverse components, on which PS-converted wave data are recorded. However, PS-converted waves also undergo shear-wave splitting, which complicates the azimuthal variations of PS-converted wave data. I demonstrate that it is essential to separate the fast P-SV1 wave from the slow P-SV2 wave, before applying any azimuthal analysis. I derive an equation describing the azimuthal variation in PSconverted wave NMO velocities, which shows the variation can be approximated into an ellipse. Based on this theory, I build a workflow to analyse the azimuthal variations of velocities in PS-converted wave data and apply this workflow to synthetic data. The imaging quality can be improved by using this workflow. Different fluid saturations in fractures have different influences on the azimuthal variations of both P-wave and PS-converted wave data. I perform a numerical study to understand how dry or water-saturated fractures control the azimuthal variations. Through theoretical and synthetic studies, I find that the azimuthal variation of velocities in PS-converted wave data is sensitive to different fluid saturations. By analysing the azimuthal variation, the fracture properties can also be estimated, but results are not as robust as those from PS-converted wave splitting analysis. I find that azimuthal variations of fast P-SV1 and slow P-SV2 waves show in-phase characteristics in dry fractures, but exhibit out-of-phase characteristics in water-saturated fractures. This important feature could open a new application for using PS-converted wave seismic data to distinguish oil-filled fractures from gas-filled fractures. In cases where multiple HTI layers are involved, I have developed a specific layer-stripping method to analyse both azimuthal variations and splitting effects of PS-converted waves. By applying this method to synthetic data, the fracture properties of each HTI layer can be estimated. The analysis of azimuthal variations in PS-converted wave velocities is applied to Daqing 3D3C land data. By using azimuthal velocity models in the PS-converted wave seismic data processing, the imaging quality is improved, especially in the anticline area where intensive fractures are likely to be developed. Furthermore, all fracture information obtained from analysis of azimuthal variations and splitting effects is compared with the stress-field data. The results from splitting analysis show a better correlation with the stress-field study. Finally, it is important to conclude that the analysis of PS-converted wave splitting is a robust method to estimate fracture directions and densities. However, it is not sensitive to different fluid saturations, which limits its application to fractured reservoir characterisation. Azimuthal variations of PS-converted wave seismic data can be analysed to improve imaging quality. Moreover their sensitivity to fluid saturations may provide a new way to discriminate between oil-filled and gas-filled fractures. However, the analysis of azimuthal variations is not as robust as the analysis of splitting effects, and it may require appropriate calibration with other fracture characterisation methods.

Multiple geophysical and geological data sets were compiled, reprocessed, and interpreted using state-of-the-art signal processing and modeling algorithms to characterize the complex post-Paleozoic geology that overlies the southwestern projection of the Fluorspar Area Fault Complex (FAFC) in western Kentucky. Specific data included 21.5 km of SH-wave seismic reflection, 1.5 km of P-wave seismic reflection, 2 km of electrical resistivity, vertical seismic profiles, Vp and Vs sonic-suspension logs, and 930 lithologic borehole logs. The resultant model indicates three general northeast–southwest-oriented fault zones pass through the study area as southwestern extensions of parts of the FAFC. These fault zones form two significant subparallel grabens with ancillary substructures. The geometry of the interpreted fault zones indicates that they have undergone episodic tectonic deformation since their first formation. Evidence of thickening and steeply dipping reflectors within Tertiary and Quaternary sediment in the downthrown blocks indicate syndepositional movement. Subtle thickening and lack of steeply dipping intraformational reflectors in the Cretaceous suggest a more quiescent period, with sediment deposition unconformably draping and filling the earlier Paleozoic structural surface. There is also evidence that the Tertiary and early Quaternary reactivation was associated with an extensional to compressional regional stress reversal, as manifested by the antiformal folds seen in the hanging wall reflectors and the potential small-amplitude force folds in the Quaternary alluvium, as well as a clear displacement inversion along the Metropolis-loess seismic horizon in two high-resolution reflection images. A surface shear-wave splitting experiment proved to be an efficient and effective tool for characterizing shallow subsurface azimuthally anisotropic geologic inclusions in low-impedance water-saturated sediment environments. The measured azimuthal anisotropy across a well-constrained N60ºE-striking fault exhibited a natural coordinate system that had a fast direction coincident with the fault strike and an orthogonal slow direction. This is also one indicator that faults inactive during significant geologic intervals (i.e., Holocene) do not "heal". Integrated shear-wave velocity models and electrical resistivity tomography profiles across the fault zones exhibit lower shear-wave velocities and resistivities within the deformation zones compared with values outside the boundaries. This is additional evidence that the deformed sediment does not reconsolidate or heal, but that the sediment particle configuration remains more loosely packed, providing an increase in the overall porosity (i.e., hydraulic conductivity). This can wholly or in large part explain the anomalous contaminant plume migration path that is coincident with the deformed zones of the regional gravel groundwater aquifer.

Azimuthal anisotropy, assumed to be associated with vertical, aligned cracks, fractures, and subsurface stress regimes, causes vertically propagating shear waves to split into a fast component, with particle motion polarized parallel to fracture strike, and a slow component, with particle motion polarized perpendicular to fracture strike. Determining the polarization of each split shear wave and the time lag between them provides valuable insight regarding fracture azimuth and intensity. However, analysis of shear wave polarizations in seismic data is hampered by reflection-induced polarization distortion. Traditional polarization analysis methods are limited to zero offset and are not valid if implemented over the full range of offsets available in typical 3D seismic data sets. Recent proposals for normalizing amplitudes recorded at non-normal incidence to values recorded at normal incidence may provide an extension to correcting offset-dependent shear wave polarization distortion. Removing polarization distortion from shear wave reflections allows a larger range of offsets to be used when determining shear wave polarizations. Additional complexities arise, however, if fracture orientation changes with depth. Reflections from layers with different fracture orientations retain significant energy on off-diagonal components after initial rotations are applied. To properly analyze depth-variant azimuthal anisotropy, time lags associated with each interval of constant anisotropy are removed and additional iterative rotations applied to subsequent offset-normalized reflections. Synthetic data is used to evaluate the success of these methods, which depends largely on the accuracy of AVA approximations used in the correction. The polarization correction effectively removes SV polarity reversals but may be limited in corrections to SH polarizations at very far offsets. After the polarization correction is applied, energy calculations including incidence angles up to 20° more effectively compensates individual SV and SH reflection components, allowing for more faithful polarization information identification of the isotropy plane and the symmetry axis. The polarization correction also localizes diagonal component energy maxima and off-diagonal component energy minima closer to the true orientation of the principal axes when a range of incidence angles up to 20° is used. / text

The estimation of anisotropic parameters in the shallow subsurface becomes increasingly important for 4C seismic data processing in order to obtain accurate images in both time and depth domain. I focus on two approaches to evaluate anisotropy in seismic data: using P-wave data and PS-converted (C-wave) data. To gain better insight into the accuracy and sensitivity of anisotropic parameters to for instance layering and compaction gradients, I undertake numerical modelling studies and verify the results with full-wave modelling as well as findings from the real data from a 4C data set from the Alba field. The focus of this thesis is on vertical transverse isotropy (VTI) which widely occurs in marine sediments and cannot be neglected in seismic processing. P-wave data alone cannot constrain the vertical velocity and the depth scale of the earth model for a VTI medium. Therefore, the joint inversion of non-hyperbolic P- and converted wave (C-wave) or S-wave data from long offsets has been suggested. I carried out a detailed analysis of the resolution and accuracy of non-hyperbolic moveout inversion for P-, S- and C-waves for a single VTI layer in two parts. First, I introduce the concept of the inherited error delta inh as a measure of the possible resolution of the moveout approximations for the different wave types. The range of this error stays constant regardless of the magnitude of the anisotropic parameter for each wave type. Second, I analyse the accuracy of non-hyperbolic moveout inversion. I find that for anisotropy parameter eta the error of estimation from C-wave data is in most cases about half that from P-wave data. Inversion of non-hyperbolic S-wave moveout data does not resolve the anisotropy parameter due to the presence of cusps in the data. The study is then extended to a multilayered medium considering only P- and C-waves. The results confirm the findings from the single layer case. Furthermore, I investigate phase effects on parameter estimation for P- and C-waves. It is suggested that eta estimated from C-wave data gives a better description of the anisotropy found in a medium than the eta values picked from P-wave data. To verify the above findings near-surface effects are studied on the 4C data from the Alba field and accompanied by a full-waveform modelling study. I find that the picked eta values from P-wave data are distinctly larger than the eta values from C-wave data and also larger than the eta values from VSP data. The full-wave modelling study shows that picked eta values from P-wave data may account for influence of structure such as velocity gradients in the near-surface and are influenced by high velocity ratios and phase reversals. Finally, I have carried out an integrated analysis of the Alba 4C data to demonstrate how seismic anisotropy can be estimated from 4C seismic data and how such information can be used to improve subsurface imaging. The results are presented in two parts. The first part deals with non-hyperbolic moveout analysis for estimating anisotropic parameters to gain improved stacked sections. The second part describes migration model building and final imaging. The models are evaluated by comparison with VSP data results and with a synthetic modelling study for three events of the overburden. The evaluation confirms that the anisotropy parameter obtained from C-wave moveout corresponds better with the VSP data than the values directly estimated from P-wave data.

[pt] Dados sísmicos, utilizados principalmente na indústria de petróleo, costumam apresentar dimensões de dezenas de gigabytes e em alguns casos, centenas. Este trabalho apresenta propostas de manipulação destes dados que ajudem a contornar problemas enfrentados por aplicativos de processamento e interpretação sísmica ao trabalhar com arquivos deste porte. As propostas se baseiam em reorganização e compressão. O conhecimento do formato de utilização dos dados permite reestruturar seu armazenamento diminuindo o tempo gasto com a transferência entre o disco e a memória em até 90%. A compressão é utilizada para diminuir o espaço necessário para armazenamento. Para dados desta natureza os melhores resultados, em taxa de redução, são das técnicas de compressão com perda, entre elas as compressões por agrupamento. Neste trabalho apresentamos um algoritmo que minimiza o erro médio do agrupamento uma vez que o número de grupos tenha sido determinado. Em qualquer método desta categoria o grau de erro e a taxa de compressão obtidos dependem do número de grupos. Os dados sísmicos possuem uma coerência espacial que pode ser aproveitada para melhorar a compressão dos mesmos. Combinando-se agrupamento e o aproveitamento da coerência espacial conseguimos comprimir os dados com taxas variando de 7% a 25% dependendo do erro associado. Um novo formato é proposto utilizando a reorganização e a compressão em conjunto. / [en] Seismic data, used mainly in the petroleum industry, commonly present sizes of tens of gigabyte, and, in some cases, hundreds. This work presents propositions for manipulating these data in order to help overcoming the problems that application for seismic processing and interpretation face while dealing with file of such magnitude. The propositions are based on reorganization and compression. The knowledge of the format in which the data will be used allows us to restructure storage reducing disc- memory transference time up to 90%. Compression is used to save storage space. For data of such nature, best results in terms of compression rates come from techniques associated to information loss, being clustering one of them. In this work we present an algorithm for minimizing the cost of clustering a set of data for a pre-determined number of clusters. Seismic data have space coherence that can be used to improve their compression. Combining clustering with the use of space coherence we were able to compress sets of data with rates from 7% to 25% depending on the error associated. A new file format is proposed using reorganization and compression together.

>Magister Scientiae - MSc / The Orange Basin provides exceptional 3-D structures of folds and faults generated during soft-sediment slumping and deformation which is progressive in nature. 3-D seismic and structural evaluation techniques have been used to understand the geometric architecture of the gravity collapse structures. The location of the seismic surveyed area is approximately 370 km northwest of the Port of Saldanha. The interpretation of gravitational tectonics indicate significant amount of deformation that is not accounted for in the imaged thrust belt structure. The Study area covers 8200 square kilometre (km²) of the total 130 000 km² area of the Orange Basin offshore South Africa. The south parts of the Study area are largely featureless towards the shelf area. The north has chaotic seismic facies as the result of an increase in thrust faults in seismic facies 2. Episodic gravitational collapse system of the Orange Basin margin characterizes the late Cretaceous post-rift evolution. This Study area shows that implications of stress field and thrust faulting to the thickness change by gravity collapse systems are not only the result of geological processes such as rapid sedimentation, margin uplift and subsidence, but also could have occurred as the result of the possible meteorite impact. These processes caused gravitational potential energy contrast and created gravity collapse features that are observed between 3000-4500ms TWT intervals in the seismic data. / Shell Exploration & Production Company

Accurate earthquake prediction is a difficult, unsolved problem that is central to the ambitions of many geoscientists. Understanding why earthquakes occur requires a profound understanding of many interrelated processes; our planet functions as a massive, complex system. Scientific visualization can be applied to such problems to improve understanding and reveal relationships between data. There are several challenges inherent to visualizing seismic data: working with large, high-resolution 3D and 4D data sets in a myriad of formats, integrating and rendering multiple models in the same space, and the need for real-time interactivity and intuitive interfaces. This work describes a product of the collaboration between computer science and geophysics. TerraVis is a real-time system that incorporates advanced visualization techniques for seismic data. The software can process and efficiently render digital elevation models, earthquake catalogs, fault slip distributions, moment tensor solutions, and scalar fields in the same space. In addition, the software takes advantage of stereoscopic viewing and head tracking for immersion and improved depth perception. During reconstruction efforts after the devastating 2010 earthquake in Haiti, TerraVis was demonstrated as a tool for assessing the risk of future earthquakes.