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This project is concerned with the development of software to invert seismic reflection data for acoustic impedance, with application to the YY-reservoir area in Gialo Field, Sirte Basin. The problem was that of inverting post-stack seismic reflection data from two seismic lines into impedance profiles. The main input to the inversion process is an initial guess, or initial earth model, of the impedance profile defined in terms of parameters. These parameters describe the impedance and the geometry of the number of layers that constitute the earth model. Additionally, an initial guess is needed for the seismic wavelet, defined in the frequency domain using nine parameters. The inversion is an optimisation problem subject to constraints. The optimisation problem is that of minimising the error energy function defined by the sum of squares of the residuals between the observed seismic trace and its prediction by the forward model for the given earth model parameters. To determine the solution we use the method of generalised linear inverses. The generalised inverse is possible only when the Hessian matrix, which describe the curvature of error energy surface, is positive definite. When the Hessian is not definite, it is necessary to modify it to obtain the nearest positive definite matrix. To modify the Hessian we used a method based on the Cholesky factorisation. Because the modified Hessian is positive definite, we need to find the generalised inverse only once. But we may need to restrict the step-length to obtain the minimum. Such a method is a step-length based method. A step-length based method was implemented using linear equality and inequality constraints into a computer program to invert the observed seismic data for impedance. The linear equality and inequality constraints were used so that solutions that are geologically feasible and numerically stable are obtained. The strategy for the real data inversion was to first estimate the seismic wavelet at the well, then optimise the wavelet parameters. Then use the optimum wavelet to invert for impedance and layer boundaries in the seismic traces. In the three real data examples studied, this inversion scheme proved that the delineation of the Chadra sands in Gialo Field is possible. Better results could be obtained by using initial earth models that properly parameterise the subsurface, and linear constraints that are based on well data. Defining the wavelet parameters in the time domain may prove to be more stable and could lead to better inversion results.

No description available.

No description available.

No description available.

This study provides an analysis of earthquake hazard in the Middle East for insurance and reinsurance purposes. The analysis incorporates important lessons learned from the 1985 Mexican earthquake. It has the following components: a) An in-depth examination of the Mexican earthquake. This has highlighted the strong influence of superficial geology in controlling exposure to earthquake hazard, and of building type and height in controlling vulnerability to damage; b) An analysis of the escalating earthquake risk in the Middle East. It is concluded that this is attributable to rapid rates of population growth, urbanisation and economic expansion, and to the development of marginal areas that are more exposed to earthquakes; c) A regional analysis of the distribution of earthquake hazard, based on 20th century data and a catalogue of historical earthquake activity that has been compiled during the research programme. It is shown that the areas of greatest hazard tend to coincide with the most densely inhabited parts of the region. The analysis has also provided evidence of temporal fluctuations in seismic activity between contiguous tectonic zones; d) The presentation of a new scheme for earthquake hazard zonation, which is designed to meet the specific requirements of insurers and reinsurers. An evaluation of this scheme, using Israel as a case-study, has proven its worth as a basis for detailed insurance-oriented examinations of earthquake hazard and risk; e) A discussion of earthquake risk control. It is concluded that the data and techniques presented in this study can be used to derive hazard and risk assessments that are more accurate than those currently available to the insurance industry. By using such assessments to control its own vulnerability to earthquake loss, the industry can help to stem the escalation of risk that has recently been witnessed in the Middle East.

The Alborz Mountain range separates the northern part of Iran from the southern part. It also isolates a narrow coastal strip to the south of the Caspian Sea from the Central Iran plateau. Communication between the south and north until the 1950's was via two roads and one rail link. In 1963 work was completed on a major access road via the Haraz Valley (the most physically hostile area in the region). From the begining the road was plagued by accidents resulting from unstable slopes on either side of the valley. Heavy casualties persuaded the government to undertake major engineering works to eliminate ''black spots" and make the road safe. However, despite substantial and prolonged expenditure the problems were not solved and casualties increased steadily due to the increase in traffic using the road. Another road was built to bypass the Haraz road and opened to traffic in 1983. But closure of the Haraz road was still impossible because of the growth of settlements along the route and the need for access to other installations such as the Lar Dam. The aim of this research was to explore the possibility of applying Landsat MSS imagery to locating black spots along the road and the instability problems. Landsat data had not previously been applied to highway engineering problems in the study area. Aerial photographs are better in general than satellite images for detailed mapping, but Landsat images are superior for reconnaissance and adequate for mapping at the 1 :250,000 scale. The broad overview and lack of distortion in the Landsat imagery make the images ideal for structural interpretation. The results of Landsat digital image analysis showed that certain rock types and structural features can be delineated and mapped. The most unstable areas comprising steep slopes, free of vegetation cover can be identified using image processing techniques. Structural lineaments revealed from the image analysis led to improved results (delineation of unstable features). Damavand Quaternary volcanics were found to be the dominant rock type along a 40 km stretch of the road. These rock types are inherently unstable and partly responsible for the difficulties along the road. For more detailed geological and morphological interpretation a sample of small subscenes was selected and analysed. A special deve loped image analysis package was designed at Aston for use on a non specialized computing system. using this package a new and unique method for image classification was developed, allowing accurate delineation of the critical features of the study area.

The West Orkney Basin is situated in a frontier hydrocarbon region of the United Kingdom Continental Shelf. This study presents a reappraisal of the tectono-stratigraphic development and petroleum potential of the basin, and is based on a recent compilation and partial reprocessing of all the available 2D reflection seismic for the area. Evidence for the presence of Devonian lacustrine source-rocks in the basin is demonstrated by the recognition of a syn-rift sequence overlying basement, which comprises two packages of contrasting seismic facies characteristics, which are correlateable to onshore Devonian source-rock and reservoir facies. The syn-rift sequence is truncated at unconformity; that is related to Late Carboniferous inversion of the Great Glen-Walls Boundary Fault system. A second major phase of rifting within the basin, with formation of new faults and reactivation of pre-existing Devonian faults, is interpreted to have initiated in the Late Permian and dwindled into the Early Jurassic. Subsequent extensive exhumation events occurred in the Mid-Jurassic to Early Cretaceous and Cenozoic, with removal of about 2.5 km of Upper Triassic to Lower Jurassic sediments and perhaps 0.5 to 1 km of Upper Cretaceous rocks. Timing of hydrocarbon generation from Devonian source-rocks was modelled using Genesis 1D basin-modelling software from Zetaware, and the results from this indicate that it most probable that the majority of hydrocarbon generation in the basin preceded the end of the second phase of rifting in the basin (Late Permian to Early Jurassic). Therefore, the major risks with play-concepts based on a Devonian source-rock are considered to be seal integrity during multiple and prolonged uplift events.

Low velocity hydrous minerals in the subducting plate deliver water to the mantle and are thought to cause intermediate depth Wadati-Benioff zone (WBZ) seismicity through dehydration embrittlement. High frequency seismic energy (> 2 Hz) from intermediate depth earthquakes that occur within this low velocity oceanic crust is retained and delayed by the crustal waveguide while lower frequency (< 0.5 Hz) energy travels at the faster velocities of the surrounding mantle. These guided waves therefore spend longer interacting with the low velocity oceanic crust than any other seismic phase, and have the potential to reveal a large amount about the velocity structure of the WBZ. Dispersive arrivals recorded in the forearc of Northern Japan are directly compared to synthetic waveforms produced from full 2D and 3D waveform simulations. Comparing the relative amplitude and arrival time of a given frequency using the velocity spectra and spectrogram respectively, allows the full dispersive P-waveform to be constrained. Analysis of dispersive arrivals from upper plane WBZ events at 150 – 220 km depth place the first observational constraints on the metamorphic reactions occurring before full eclogitisation of the subducting oceanic crust. I show that blueschist and lawsonite bearing rocks may persist well beyond the depths inferred from established thermo-petrological subduction zone models, and that full eclogitization may occur at much greater depths than is inferred by receiver function studies. The persistence of meta-stable hydrous minerals explains the occurrence of WBZ seismicity at 200 - 250 km depth, and may be due to the partially hydrated oceanic crust. Dispersion from events that occur well below the upper plane of WBZ seismicity can be explained by the occurrence of low velocity hydrated outer rise normal faults at intermediate depths. At depth, these faults are inferred to be 2 - 3 km thick and 12 - 15 % slower that the surrounding mantle, suggesting they are 50 - 71 % serpentinised. We suggest that the extended P-wave coda observed at stations close to the trench in Northern Japan are explained by low velocity dipping faults of a range of scale lengths forming a scattering medium. This scattering medium is simulated using a von Kármán function, and the synthetic waveforms produced are compared to the observed P-wave coda, that decays in amplitude with distance from the trench. The magnitude of this spatial coda decay is sensitive to the average bulk velocity of the scattering medium and provides a constraint on the hydration of the lithospheric mantle subducted beneath Japan. This first in-situ constraint on the degree of slab mantle hydration at intermediate depth suggests that 170.4 - 318.7 Tg/Myr/m of water is subducted beneath Northern Japan by the slab mantle. In summary we have shown that up to 94 % of the water subducted beneath Northern Japan is transported by the lithospheric mantle, and that upper and lower planes of WBZ zone seismicity are directly related to hydrous mineral assemblages, and so may occur through dehydration embrittlement. This work shows that guided waves have the potential to resolve new details of the WBZ velocity structure and the techniques developed here can be applied to other subduction zone settings.