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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.
51

The geology of the Eastern part of the Rift Valley in Baringo District, Kenya

Carney, John Norman January 1972 (has links)
No description available.
52

Characteristics of night time absorption spike events as signatures of magnetosphere ionosphere (M-I) coupling

Aminaei, Amin January 2007 (has links)
Night time absorption spike events (NASE) are common signatures of magnetospheric substorms. Their occurrence in the ionosphere can be easily detected by riometers located at ground based stations. This unique feature is used to achieve a comprehensive study based on 500 NASE occurred during the period 1994-2003 in the IRIS (imaging riometer for ionospheric studies) field of view at Kilpisjarvi, Finland centred at 69.05° N, 20.79° E (L-shell 6.1). NASE generally had similar behaviour which has been mentioned for substorms in the literature. Occurrence of NASE dominates around magnetic local midnight (MLM) with majority of events in the pre-midnight sector. NASE seem to occur more often during high geomagnetic activity according to Kp index variation. Their occurrence during geomagnetic equinoxes is slightly more than that of s~lstices with the peak in the autumn and minimum in the summer time. They also tend to be solar cycle dependent as their appear3?ces during solar minimum dominate in agreement with occurrence of substorms. Our study confIrms most results of previous NASE studies. . . North/westward motion of spike events was dominant with speed in a range of few hundreds to few kms per second. Pi2 pulsations and auroral breakup (found from PIXIE, polar ionospheric X-ray imaging experiment images) are a common feature of NASE. The signifIcant frequency modulation of NASE is in the order of 50 mHz -200 mHz based on wavelet analysis. Apart from these new findings are also discussed in this thesis: Considering the temporal structure and variation of IT.- index, NASE are categorized into 4 classes. Classifications of spike events allow the identification of phenomena such as pseudobreakups from substorms. Another important finding of this study is the location of mapped points of NASE which is in the range of near Earth magnetotail rather than midtail region in favour of current disruption (CD) substorm model as opposed to near Earth neutral line (NENL) model. This yields using geomagnetic field model T-96 and NASE of IRIS and SGO (SodankyHi geophysical observatory) riometers where together covers auroral zone latitudes between 62.42° Nand 77.00 N (L- shell between 3.8-13.6).
53

Deformation history of the Western Helvetic Nappes, Valais, Switzerland

Durney, David William January 1972 (has links)
No description available.
54

The inversion and analysis of crosshole seismic data

East, Robert J. R. January 1988 (has links)
No description available.
55

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

Brown, Hazel A. January 1991 (has links)
No description available.
56

Towards using seismic anisotropy to interpret ductile deformation in mafic lower crust

Tatham, Daniel John January 2008 (has links)
The lower crust forms an important geodynamic control in continental tectonics and the communication and coupling of kinematics between surface and deep-Earth processes. An understanding of the relationship between seismic properties, finite strain and fabric orientation thus provides a useful tool in the remote sensing and interpretation of deformation in the lower crust. This thesis outlines a work-flow model by which the seismic properties of a single and representative lower crustal lithology can be calculated and calibrated against finite strain from petrofabric development across a strain gradient. The work-flow model constitutes a multi-disciplinary approach, incorporating field mapping and sample collection, experimental petrofabric determination, and seismic modelling. A review of compositional estimates of the deep crust, including xenoliths, exposed sections and estimates from wide-angle seismic profiles, indicates the importance of mafic lithologies. The Laxfordian-age high-grade shear zone at Upper Badcall, NW Scotland, exhibits a strain gradient in a deformed doleritic Scourie dyke (Lewisian complex) that intersects the zone at a high angle. From an analysis of field data from detailed mapping, the shear zone is shown to be characterised by generally simple shear, but where the tectonic movement direction varies transversely across the shear zone. Calculation of the strain profile across the deformation zone gives shear strains, y up to 57, but with y < 15 being perhaps more realistic. Cumulative displacements total ~1000m left-laterally, and ~600m vertical displacement, north-side up. Nine samples were collected across the shear zone in the mafic dyke, representing a strain gradient from undeformed protolith to the highest recorded stains. The sample suite is characterised as a hornblende-plagioclase-quartz aggregate that develops macroscopic planar and linear fabrics with strain, from an essentially isotropic protolith. Quantification of the aggregate lattice preferred orientation (LPO) using electron backscatter diffraction (EBSD) showed the dominance of fabric development in the hornblende phase, with (100) poles clustering forming normal to the foliation plane and [001] axes parallel to the tectonic X direction. Plagioclase and quartz retained random fabrics from the wall-rock protolith with increasing finite strain. The hornblende LPO fabric, described by the texture index, J, shows a positive logarithmic relationship with strain, where LPO intensity saturated by y ~10. The strain-calibrated quantitative petrofabric description of each sample is used to calculate their aggregate elasticity tensors (Cij) via a Voigt-Reuss-Bill average, and from which seismic properties are derived using Christoffel's equation. Hence, a framework of petrofabric- and strain-calibrated seismic properties is described for a strain gradient in a representative high-grade mafic lithology. P-wave anisotropies up to ~10% are-recorded in the most deformed samples with Vsmax typically between 6.42-6.63kms/-1. S-wave anisotropies record up to 7.23% AV, in the most deformed samples, with Vpmax ranging between 3.62-3.75kms-1 for all samples. The relationship between petrofabric-derived seismic anisotropy and finite strain across the sample suite show a positive relationship, approximated by a logarithmic function, whereby P- and S-wave anisotropy exhibit a steep positive gradient with strain up to y~10. The sample-wise framework of petrofabric- and strain-calibrated seismic properties is interpolated to estimate the continuum relationship between seismic properties, finite strain and petrofabric orientation. In a move to illustrate the application of results in seismic and structural modelling, case study models of crustal deformation are presented for the eastern Basin and Range province, the North Sea rift, and Tibet. Models are promising in their ability to differentiate between regions of lower crust characterised by a uniform mafic composition but different finite strain state and/or petrofabric geometry, although multiple seismic survey methods may be needed to fully interpret results in terms of strain and fabric orientation. In summary, a multidisciplinary approach combining field mapping and sampling, petrofabric characterisation with EBSD, and seismic modelling provides an efficient and reproducible work-flow for the determination of petrofabric-derived strain-calibrated seismic properties of lower crustal materials.
57

The geology of the Blaenau Ffestiniog area, Merionethshire

Bromley, A. V. January 1964 (has links)
No description available.
58

Reconstructing Buried Alluvial Landscapes : The Application of Multiple Geophysical and Geoarchaeological Techniques

Mansfield, Carol January 2007 (has links)
No description available.
59

Sedimentary studies in Lake Windermere

Holmes, P. W. January 1964 (has links)
Late Quaternary sediments, soils,and parent rocks in and around the Windermere North Basin are studied by modern sedimentological techniques. Tho data obtained is examined statistically. Till points, scattered randomly on the Sand/silt/clay triangle, were redistributed into a "still later group", glaciolacustrine clays and main Windermere sediments; and a coarse fluvioglacial group which is better sorted and more rounded than the till. Fine sands, negatively skewed due to finer admixtures, typify certain lake environments. Beach deposits are better sorted and more rounded than the till from which they were partly derived; they are more rounded and coarser on the eastern exposed beaches than on sheltered ones; they are positively skewed due to inefficient fluviatile saltation. Three assumptions: a. Flocculation caused real mechanical bimodality of organic sediments. b. Spasmodic sedimentation was by underflowing "turbidity" currents during storms. c. Deposition occurred when settling (discrete or flocculated) overcame mechanical dispersal. Loss of river water impetus, and organic content and biological activity, controlled Poet Glacial sed1mentation. The relative importance of the two main rivers has alternated. Two basic mineralogical populations formed Late Quaternary material: a)Pink Late Glacial form, non hydrated. Post Glacial form, illite and aluminosilicates, hydrated and flocculated due to organic content and acidic environment (occurs up to 0.020 mm). b) Silt. Chloritic rock fragments (magnesium rich ripidolite, iron rich thuringite); calcite was removed from all but buried varves by Post Glacial acid corrosion. A marked change in lake sedimentation occurred with the spread of plant growth at the end of Late Glacial times 11,000 years ago. Controlled erosion, flocculation, and a high organic content characterised Post Glacial reddish gyttja sedimentation. Human settlement in the last 200 years caused further changes: accelerated erosion due to forest removal, reducing lakewater conditions due to sewage caused stronger flocculation in the blackish surface ooze.
60

Fracture mechanics of volcanic eruptions

Matthews, C. January 2009 (has links)
Seismology is a key tool in the forecasting of volcanic eruptions. The onset of an eruption is often preceded and accompanied by an increase in local seismic activity, driven by fracturing within the edifice. For closed systems, with a repose interval of the order of a century or more, this fracturing must occur in order to create a pathway for the magma to reach the surface. Time-to-failure forecasting models have been shown to be consistent with seismic acceleration patterns prior to eruptions at volcanoes in subduction zone settings. The aim of this research is to investigate the patterns in seismic activity produced by a failure model based on fundamental fracture mechanics, applied to a volcanic setting. In addition to the time series of earthquake activity, statistical measures such as seismic b-value are also analysed and compared with corresponding data from the field and laboratory studies. A greater understanding of the physical factors controlling fracture development and volcano-tectonic activity is required to enhance our forecasting capability. The one dimensional, fracture mechanics grid model developed in this work is consistent with the theory of growth and coalescence of multi-scale fractures as a controlling factor on magma ascent. The multi-scale fracture model predicts an initial exponential increase in the rate of seismicity, progressing to a hyperbolic increase that leads to eruption. The proposed model is run with variations in material and load properties, and produces exponential accelerations in activity with further development to a hyperbolic increase in some instances. In particular, the model reproduces patterns of acceleration in seismicity observed prior to eruptions at Mt. Pinatubo (1991) and Soufriere Hills (1995). The emergence of hyperbolic activity is associated with a mechanism of crack growth dominated by interaction and coalescence of neighbouring cracks, again consistent with the multi-scale fracture model. The model can also produce increasing sequences of activity that do not culminate in an eruption; an occurrence often observed in the field. Scaling properties of propagating fractures are also considered. The seismic bvalue reaches a minimum at the time of failure, similar to observations from the field and measurements of acoustic emissions in the laboratory. Similarly, the fractal dimension describing the fracture magnitude distribution follows trends consistent with other observations for failing materials. The spatial distribution of activity in the model emerges as a fractal distribution, even with an initially random location of fractures along the grid. Significant shifts in the temporal or spatial scaling parameters have been proposed as an indication of change in controlling factors on a volcanic system, and therefore represent a relatively unexplored approach in the art of eruption forecasting.

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