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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

The Breidavik group on Tjornes, North Iceland: Lithostratigraphy and late Cainozoic glaciations

Eiriksson, J. 1979 (has links)
No description available.
32

Mechanisms of isostatic compensation in areas of lithospheric extension : examples from the Aegean

King, Trevor Alan 1998 (has links)
No description available.
33

A palaeomagnetic, geochronological and palaeoenvironmental investigation of late and post glacial maar lake sediments from NW-Europe

Brown, Hazel A. 1991 (has links)
No description available.
34

The holocene geomagnetic field in Europe

Hogg, T. Eric 1978 (has links)
No description available.
35

A multi-grid inversion of borehole seismic data for characterisation of anisotropy in the near-surface

Mewes, Armin 2008 (has links)
Anisotropy is a valuable proxy for certain ground material properties such as rock type characterisation, fracture direction, fracture intensity identification and ground water flow direction inferred from fracture infonllation. TIns thesis describes a new method for anisotropic seismic inversion and includes extensive synthetic tests, where survey geometry and model heterogeneity levels are varied, wInch help to test under which conditions the method works. The MGMC (Multigrid Monte Carlo) method was chosen because of the hypothesis that it may find the absolute minimunl of the multi-parameter functional without needing penalty tenus or other constraints. The fit to which the functional can be lnininnsed is of very close accuracy to results of the gradient descent methods. Even with such comparable fit, some paranleters were not fully resolved, showing that even at such high levels of accuracy certain trade offs are still possible between parameters. Extensive synthetic tests show that the method works well, as long as certain conditions of angular coverage and model heterogeneity levels are accounted for. Also, the results indicate that residual maps are not at all indicative of correctly resolved anisotropy with large residuals sometimes accompanying better resolved images than relatively small residuals. Analyses of field data show encouraging results becau'se the images correspond well to previous inversion work therefore showing that the MGMC approach is a viable alternative to previous methods. Comparing the analyses, using the new MGMC inversion developed here on the one hand and the previous analysis method, which involves comparing results derived from using various combinations of penalty tenus, it was found very useful not have to compare results derived from using various combinations of penalty terms since tIns saves time and may result in a more accurate representation of the true anisotropy structure. The agreement of the results with pre-inversion apparent velocity analysis and core analysis, the independent APPSM (apparent penalty parameter search mechanism), the extremely low traveltime residual map and the inclusion of steep, near vertical raypaths all suggest that the results are reliable with high likelihood, thereby showing tIlat tIle MGMC method not only operates well for synthetic data but for real field data as well.
36

The effect of stress, pore fluid and pore structure on elastic wave velocities in sandstones

David, Emmanuel Charles 2012 (has links)
A model of elastic wave propagation in fluid-saturated sandstones is developed that takes into account pore fluid properties, and stress. It is based on the assumption that the pore space can be represented by a distribution of crack-like spheroidal pores having a distribution of aspect ratios, and a single family of non-closable spheroidal pores. The Eshelby-Wu formalism is used to find exact expressions for the bulk and shear compliances of the pores. Asymptotic analytical expressions are obtained for the compliances of both dry and fluid-saturated spheroidal pores that are crack-like, needle-like, or nearly spherical. These expressions are incorporated into two commonly used effective medium theories, the Mori-Tanaka and the Differential schemes, to obtain expressions for the effective elastic properties of dry and fluid-saturated rocks containing spheroids of a given aspect ratio, as a function of porosity or crack density. The stress dependence of the elastic velocities is modelled by considering that the elastic moduli vary with stress due to crack closure. This pore structure model is able to explain successfully the pressure dependence of ultrasonic dry velocities on many sets of laboratory data. Predictions of saturated velocities are made using either the Gassmann equation, or using an effective medium theory in conjunction with the aspect ratio distribution found from the dry data. For ultrasonic velocities obtained at high frequencies (MHz), the predictions of effective medium theories are more accurate than the Gassmann predictions. Low-frequency measurements (0.02 Hz) of the bulk modulus were obtained on Fontainebleau sandstone, under pressure, and with different pore fluids. For water and glycerin-saturated samples, both the 4% and 13% porosity rock specimens were more compliant at low frequencies than at high frequencies. Finally, a model is proposed for the frequency dependence of the wave velocities, assuming that at a given frequency, some pores obey the Gassmann equation, and others are isolated, with a critical aspect ratio demarcating the two families that depends on frequency and fluid viscosity.
37

The geological environment of post-Caledonian base-metal mineralization in Ireland

Russell, Michael John 1972 (has links)
The Dlnantian host and wall rocks to the Ballyvergin, Gortdrum, Oola, Carrickittle and Tynagh base-metal deposits were analysed for a variety of trace elements with a view to establishing a local sedimentary syngenetic contribution of metals. Against expectation all the trace element aureoles examined proved the epigenetic nature of the sulphide mineralization. The aureoles are of two kinds corresponding to the sulphide deposit types. The copper deposits in the Lower Limestone Shales and the argillaceous Lower Limestones; Ballyvergin, Gortdrum and Oola, are fringed with enrichments of arsenic and lead, whereas the Waulsortian wall rocks to the Tynagh and Carrickittle lead-zinc deposits contain uneven enrichments of many trace elements. A reconnaissance survey in the Waulsortian mud bank complex to the west north west of Tynagh revealed what may be an extensive syngenetic manganese aureole to the Tynagh chert-hematite deposit. The exhalative origin proposed by Derry, Clark and Gillatt (1965) for the Tynagh iron deposit is supported by chemical analysis. Thus hot springs were in existence at Tynagh in mid-Dinantian times. This was probably the case too at Silvermines (Graham 1970). The iron deposit at Keel as well as the thick developments of chert at Silvermines and Aherlow are taken here as additional evidence for a mid-Dinantian age for the onset of mineralization. Although the local structural controls to the sulphide deposits may be related to the Armorican Orogeny, the distribution of the ore deposits is more easily explained in terms of north-south geofracturing caused by the tensile stresses which eventually led to the formation of the Atlantic (Rockall Trough) margin. The recent discovery of the Navan sulphide deposit was broadly predictable by this theory.
38

Geology and petrology of part of the pre-Cambrian complex in the South Savanna-Rewa area of British Guiana

Singh, S. 1964 (has links)
No description available.
39

Gravity surveys around the mouth of the Severn

Thomas, M. D. 1969 (has links)
No description available.
40

The tectonic and metamorphic history of the Monviso ophiolite body, N.W. Italy (western Alps)

Tinkler, W. C. 1982 (has links)
No description available.

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