In regular marine acquisition configuration, shallow sources and shallow streamers are used. Because of this configuration the high-frequency content of the seismic is favored, which is needed for sufficient vertical resolution. If the receivers were deployed at a larger depth the low-frequency content would be favored. The low frequencies are needed for inversion, deep penetration and visualization. However, this configuration would attenuate the higher frequencies and would suffer from poor vertical resolution. The attenuation of either high or low frequencies is a result of the receiver ghost, which attenuates higher frequencies for the deep tow, and lower frequencies for the shallow tow. Over/under acquisition method allows the wavefield to be separated into upgoing and downgoing wavefields. The configuration consists of 2 receiver-cables in vertical alignment with each other. By detecting only the upgoing wavefield, we are removing the receiver ghost and hence the frequency bandwidth should be broadened. These cables are towed at 18 and 25 meters, respectively. Regular receiver-cables are normally towed at 7 to 9 meters. Because of the deeper tow, the noise levels should also be lowered and result in a better signal to noise ratio. Post-stack seismic inversion is the process where we analyze the stacked seismic traces and try to reconstruct the velocity structure, or the acoustic impedance, of the sub-surface covered by the seismic. Inversion is sensitive to various parameters and small improvements in the seismic would result in improvements in the inversion volume. In the inversion configuration of this thesis, we are using a background model based on a-priori information from one known well. The a-priori information is used as an initial guess for the inversion to follow. To keep the inversion volumes as data-driven as possible, the inversions were processed with a weight factor on the initial model as low as possible, to enhance the changes made by the differences in the seismic volumes. Quantification of the difference in the inversion volumes based on different acquisition methods for the input seismic resulted in various comparisons of the acoustic impedance volumes. Difference in vertical resolution has been investigated and identified to have a relative difference in favor of the single cable seismic. The differences were due to a change in the wavelet shape and width and also change in dominating frequency of the respective time interval. Comparing the inversion volumes, based on the acquisition method, to their respective average inversion volumes identified changes in the inversion volumes due to feathering of the receiver cables. Further tests illustrated that the feathering had a significant impact on the inversion volumes. Since the feathering causes the receiver cables to deviate from a straight line astern of the vessel, the seismic volume is slightly changed compared to a volume where there is no feathering. Experiments illustrated that the frequency spectra are different. However, the frequency spectrum is not broadened, but shifted and shortened prone to lower frequencies. Dominating frequency was hence lower for cable combination seismic volumes compared to single cable seismic volumes. This resulted also in difference in the seismic wavelet as previously explained. Results indicate a significant change in inversion volumes due to fold, acquisition direction and feathering. Changes caused by the cable combination method were not as first anticipated. Since the method is used with a deeper tow, we were anticipating a significant change in the signal to noise ratio, also considering that receiver ghost is removed when evaluating only the upgoing wave. Results indicate that there was no significant change in signal to noise ratio. However, significant changes in the ratio were found when using the split spread method versus the single direction method (both single cable and cable combination method). This work concludes that the largest impact on the inversion volume is found where we have identified poor alignment of feathering, different acquisition direction and increased fold. The cable combination method doesn’t have significant impact on the inversion volume. Identified changes are quantified in this work and will verify these results.

This study analyzes 2D seismic sections of extensional growth-fault basins, covering two tectonic realms; (i) Carboniferous rifting in Central Spitsbergen, and (ii) shallow Triassic extensional basins of the SE Svalbard region. The study of the Carboniferous Billefjorden Trough in Sassenfjorden-Tempelfjorden and from Reindalen, focus on the rift infill with associated wedge and lenticular shaped depocenter geometries. The two fundamental geometries are identified by either variable fault truncation of the wedge-shaped basin fill (fault onlap relationship) or fault-tip monoclines with associated basinward offset of the related lenticular basins. The interpretation of lines from Eastern Svalbard focus on a series of Triassic, shallow basins (< 200 m; up to 150 ms deep) of which most are bound by listric faults that sole out in underlying shale successions. These observations are correlated with similar faulting with basins in cliffs of Edgeøya. The offshore Triassic faulting of Eastern Svalbard represent a first assessment, as such analysis has not been carried out before. This study goes deeper into details on the evaporite-dominated Carboniferous Billefjorden Trough than those presented by Bælum and Braathen (2012). Some new information and characterization of the basin infill link seismic facies analysis of Carboniferous rifting to reflector belts that can be correlated with the pre-rift Billefjorden Group, the syn-rift successions of the Hultberget, Ebbadalen and Minkinfjellet formations, and the immediate post-rift (or late syn-rift) Wordiekammen Formation. The Billefjorden Trough is the result of a complex basin evolution history, and published results of outcrop studies in the northern Billefjorden area shows a basin that changes basin depocenter geometry, from a lenticular shape to a wedge shape and then back to a lenticular shape. Similar patterns are recognized by the seismic facies analysis. In a conceptual framework, the Billefjorden Trough differs from the rift basins described in Prosser (1993), in that the basin is significantly influence of fault-propagation folding, probably controlled by thick basin-center successions of low-shear strength evaporites. Encountered geometries are more similar to those of rift basins of the Gulf of Suez. Eastern Svalbard offers world class examples of extensional fault-growth basins in mountain slopes of Edgeøya and partly Hopen. Similar faulting is encountered in the lines interpreted from the Eastern Svalbard dataset, where the intricacy of faulting and their associated shallow basins of Triassic age offer complex geometries but also challenging interpretation work because of limited seismic resolution. Revealed internal geometries include rollovers and drag-folds, offering some general geometrical similarities with the much larger Carboniferous rifts. However, the depositional systems are very different. For the Triassic, regional clinoform progradation in a northerly direction interacted with the faulting, indicating that the coastal or deltafront migration at times were arrested by the faulting. This arrest is suggested by vertically stacked sequences, before the fault systems are bypassed by renewed clinoform progradation.

Abstract Pre stack inversion of seismic data consists of numerous difficulties. Two of the problems of greatest concern are the problems of non-uniqueness and non-linearity of the inversion. There may exist several solutions to any given inversion problem, and to be able to choose the correct solution we are dependent on a priori information. This thesis will explain how a priori information can be implemented with the seismic data using Bayesian modeling and fractal based initial methods in order to obtain the most likely solution for the inversion. This thesis will also explain the theory behind global optimization routines, such as the random walk Monte Carlo, the Metropolis algorithm and Simulated Annealing. A Simulated Annealing routine has been made, and this is used to solve optimization problems. The routine is analyzed for its capability of finding global optimums and the requirements for its success. It is then implemented to simulate the inversion of a seismic dataset. The solutions of the inverted data is then analyzed and compared to the actual solution. This is done for an uncontaminated dataset, and for a dataset containing noise. The work has shown that Simulated Annealing can be a good method for finding a global optimum, but that the global optimization routine is unable to produce good results without good constraints and a good initial model, due to the problem of non-uniqueness.

Proper correlation between formation mechanical properties and acoustic data is essential for acquiring field rock mechanical data for analysis, and it has thereby a great significance to oilfield development.This thesis presents results from a correlation study between formation mechanical properties and acoustic wave velocities from a set of unpublished rock mechanical experiments on sandstone samples from the Norwegian shelf. The core samples from the Norwegian shelf were subjected to triaxial compression tests performed at various confining pressures with simultaneous measurements of acoustic velocities. Correlations between formation compressive strength, elastic stiffness and Poisson's ratio and compressional and shear transit time have been established.The results obtained in this study confirm that the stress level and the stress configuration affect the acoustic velocities, and this should be accounted for when using generalized empirical correlations to estimate formation strength, elastic stiffness and Poisson's ratio from acoustic logs in field studies. The empirical correlations established through this work are found to match reasonable well with other published relations. By acoustic logs from field studies, it is found that the empirical correlations overestimate the formation strength and the elastic stiffness.

In this Master Thesis a method for estimating seabed p-wave velocities from normal mode seismic data is developed. This is done through forward modeling using two dimensional finite difference modeling to generate synthetic data based on a given parallel two layered laterally varying seabed velocity model and a constant two layered density model, with a common fixed water depth. A semblance inversion technique is developed in MATLAB using the period equation eqref{eq:period} and the resulting velocity profiles is plotted against the exact velocity model to check the validity of the estimates. The same method is extended to estimations of seabed densities. For analysis of the robustness of the method, analysis with added pseudo random noise is preformed.The results shows a good performance of the semblance method to reproduce the model velocity parameters. The introduction of noise is handled well and decent results are obtained for significantly low signal to noise ratios.It suggests that the semblance method is applicable to use for determination of other parameters influencing the normal mode response signal.

The Alvheim reservoir is a turbidite reservoir, which means that the complex deposition makes it a difficult reservoir to perform predictions regarding reservoir content on. In the preceding project work (Eggen 2012) AVO analyses were performed on the twelve modelled scenarios that can be present in a turbidite reservoir. These modelled scenarios were to be compared with the analyses performed on the real data in this master’s thesis to see if the modelled scenarios can help to predict what answers to expect from the analyses performed on the real data. One post-stack data set consisting of Near and Far stacks covering the whole Alvheim field including all three hydrocarbon discoveries, and one pre-stack data set focusing on the oil discovery named Kneler were available for this thesis in addition to well logs from well 25/4-7. Naturally, it was the Kneler oil discovery that was focused on, and on the gathers from the pre-stack data the top reservoir could be identified by a clear AVO effect. Different AVO analyses were performed on this AVO effect and the results were compared with the results obtained from the project work. In addition to performing AVO analyses on the data it was interesting to see if it was possible to see how the reservoir changed when moving away from the well location on the seismic data. To increase the signal to noise ratio, super gathers around the well location were created in addition to super gathers at some distance away from the well to see if there were changes that were noticeable on the seismic.The AVO analysis was performed on the top oil sand (top reservoir) in the Heimdal Member located in the Lista Formation. An AVO crossplot was created from both data sets, where the area around the Kneler discovery was picked by hand on the post-stack data set to match the area that was plotted from the pre-stack data. The crossplot created from the post-stack data showed the best deviation from the background trend out of the two, and the anomaly could be classified as a class III AVO anomaly. It was also performed an AVO gradient analysis on the AVO effect on a pre-stack seismic gather and on a synthetic seismic gather created with a normal Ricker wavelet and velocities taken from well 25/4-7. Both AVO curves from these analyses had a negative intercept and a negative gradient, which also could classify them as a class III AVO anomaly. It was known in advance that the upper part of the reservoir consisted of unconsolidated interbedded sand-shale and it was expected that the results would match the results obtained from the modelled scenario of the unconsolidated interbedded sand-shale. However, this was not completely the case and the results from the analyses of the real data turned out to match the analyses for the modelled unconsolidated massive sandstone. Even if the analyses from this master’s thesis do not match the expected analyses performed in the preceding project work, they can be said to be correct. The error in comparison is due to the fact that the analyses in this master’s thesis are performed on the top of a section of unconsolidated interbedded sand-shale, but the top layer is actually a layer of unconsolidated massive sandstone. This means that when making assumptions it should not be taken for granted that the real data will match the modelled data, especially not if there are uncertainties related to the assumptions the modelling is based on.

In the work of this Master's thesis, the anisotropy parameters, epsilon and delta, for the reservoir and the cap rock on the Troll Field have been estimated. This was done using well logs from 35 wells, including the P-wave sonic log and the inclination angle log of the wellbore. The velocity from the sonic log and the inclination angle were applied to a second order polynomial equation, which includes the anisotropy parameters.The Matlab software was utilized to perform the calculations and to generate plots necessary to estimate the parameters. To obtain more reliable results, different filters were applied to the data set for both the reservoir and the cap rock. The filters consisted of different intervals of porosity, acoustic impedance and depth, both individually and combined in different ways. In advance of the filtering, histograms were made for porosity, acoustic impedance and depth to look at the distribution of each, in order to find the range the different parameters could be filtered for.This process resulted in the following estimations of the anisotropy parameters for the reservoir: epsilon = -0.08 and delta = -0.03. The anisotropy parameters for the cap rock, which is a shale in most of the wells, was estimated as follows: epsilon = 0.11 and delta = 0.06. These parameters were applied in an AVO analysis, performed for the vertical well 31/2-L-41. An approximation using 3-term Shuey equation was applied for this purpose. The anisotropic case was compared with the isotropic case. This showed that there is an evident difference between the isotropic and the anisotropic model at large offsets. The exact solution from the Zoeppritz's equations was also included for comparison. This proved to be very close to the approximate solution.Amplitude values from seismic gathers were included in the AVO analysis. This showed that the amplitudes from the gathers increased with incidence angle, as for the isotropic and the anisotropic model. However, the increase in amplitude was much less than for the models.From this work, the estimation of the anisotropy parameters were shown to have a large uncertainty, even after filtering. To make the estimation of delta more stable, more deviated wells to cover the whole inclination angle range, especially from 27-40 degrees, are required. Epsilon is dependent on the vertical and the horizontal P-wave velocity only, thus there should be less uncertainty in estimating this parameter.From the AVO analysis, including the amplitudes from the gathers, no conclusive statements could be established due to the fact that the amplitude values had to be scaled to fit the amplitude values with the models. The amplitudes of the gathers were scaled to the amplitude of the isotropic case at far offset, thus the result could have been altered if scaled to the anisotropic case.