The estimation of anisotropic parameters in the shallow subsurface becomes increasinglyudimportant for 4C seismic data processing in order to obtain accurateudimages in both time and depth domain. I focus on two approaches to evaluateudanisotropy in seismic data: using P-wave data and PS-converted (C-wave) data.udTo gain better insight into the accuracy and sensitivity of anisotropic parametersudto for instance layering and compaction gradients, I undertake numerical modellingudstudies and verify the results with full-wave modelling as well as findingsudfrom the real data from a 4C data set from the Alba field.udThe focus of this thesis is on vertical transverse isotropy (VTI) which widely occursudin marine sediments and cannot be neglected in seismic processing. P-waveuddata alone cannot constrain the vertical velocity and the depth scale of the earthudmodel for a VTI medium. Therefore, the joint inversion of non-hyperbolic P- andudconverted wave (C-wave) or S-wave data from long offsets has been suggested. Iudcarried out a detailed analysis of the resolution and accuracy of non-hyperbolicudmoveout inversion for P-, S- and C-waves for a single VTI layer in two parts.udFirst, I introduce the concept of the inherited error delta inh as a measure of the possibleudresolution of the moveout approximations for the different wave types. Theudrange of this error stays constant regardless of the magnitude of the anisotropicudparameter for each wave type. Second, I analyse the accuracy of non-hyperbolicudmoveout inversion. I find that for anisotropy parameter eta the error of estimationudfrom C-wave data is in most cases about half that from P-wave data. Inversion ofudnon-hyperbolic S-wave moveout data does not resolve the anisotropy parameteruddue to the presence of cusps in the data.udThe study is then extended to a multilayered medium considering only P- andudC-waves. The results confirm the findings from the single layer case. Furthermore,udI investigate phase effects on parameter estimation for P- and C-waves. Itudis suggested that eta estimated from C-wave data gives a better description of theudanisotropy found in a medium than the eta values picked from P-wave data.udTo verify the above findings near-surface effects are studied on the 4C data fromudthe Alba field and accompanied by a full-waveform modelling study. I find thatudthe picked eta values from P-wave data are distinctly larger than the eta values fromudC-wave data and also larger than the eta values from VSP data. The full-waveudmodelling study shows that picked eta values from P-wave data may account forudinfluence of structure such as velocity gradients in the near-surface and are influencedudby high velocity ratios and phase reversals.udFinally, I have carried out an integrated analysis of the Alba 4C data to demonstrateudhow seismic anisotropy can be estimated from 4C seismic data and howudsuch information can be used to improve subsurface imaging. The results are presentedudin two parts. The first part deals with non-hyperbolic moveout analysis forudestimating anisotropic parameters to gain improved stacked sections. The secondudpart describes migration model building and final imaging. The models are evaluatedudby comparison with VSP data results and with a synthetic modelling studyudfor three events of the overburden. The evaluation confirms that the anisotropyudparameter obtained from C-wave moveout corresponds better with the VSP dataudthan the values directly estimated from P-wave data.
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