• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 948
  • 763
  • 320
  • 306
  • 3
  • 3
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 4357
  • 1520
  • 847
  • 830
  • 830
  • 276
  • 223
  • 171
  • 171
  • 165
  • 149
  • 139
  • 137
  • 135
  • 134
  • 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.

Separation of local and regional information in geomagnetic response functions using hypothetical event analysis

Ritter, Patricia January 1998 (has links)
Magnetic fields of galvanic or inductive origin can be distinguished by their phase behaviour. Pure distortion fields are in-phase with the regional electric field. Hence the magnetic phase of the galvanic response approaches the regional impedance phase, as distortion effects outweigh the inductive response of an anomaly with increasing period. Hypothetical event analysis is suited extremely well to examining the common phase content of a magnetic response function array. This technique can recover the regional strike direction as well as the regional impedance phases. I have tested its application extensively by a series of 3D modelling studies, which incorporate regional 2D and local 3D structures. The approach has also been used to investigate two datasets. For the Iapetus array, which covers an area of 200 km x 300 km in N-England/S-Scotland, the technique revealed a common regional strike azimuth of ca. N125°E in the period range 500s - 2000s. This direction differs from the strike indicated by the induction arrows, which seem influenced mainly by local current concentrations along the east-west striking Northumberland Trough and the north-west striking Southern Upland Fault. Both impedance phases are positive and differ by ca. 10°, which supports the assumption of distortion fields in the dataset and that the regional structure is 2D. In the case of the BC87 profile data from British Columbia (Canada), a regional strike direction of N60°E could be recovered at long periods, which is in agreement with the results derived from earlier magnetotelluric decomposition procedures. The negative phase angle, however, indicates that the local fields are of inductive origin. Hence we do not obtain any information on the regional impedance phase. The modelling studies confirm, that with hypothetical event analysis it is always possible to recover the regional strike direction from distorted data, even if a strong regional vertical field is present in the dataset or if the local fields are generated by induction processes rather than distortion. The determination of the regional impedance phases, on the other hand, is far more sensitive to deviations from the physical distortion model.

Analysis of local earthquake data using artificial neural networks

Dai, Hengchang January 1995 (has links)
The aim of this thesis is to investigate the possibility of using artificial neural networks to develop automatic processing techniques for picking and identifying seismic arrivals. These two key procedures of an earthquake analysis system are extremely labour intensive, and any automation would allow further processing of larger datasets. Two approaches based on the back-propagation neural network (BPNN) are developed to pick and identify seismic arrivals for the dataset which includes 762 three component (3-C) recordings from stations DP and AY of a local earthquake network in Turkey. A BPNN approach was developed to pick arrivals automatically from 3-C recordings and single component (1-C) recordings. In Chapter 4, this approach is applied to the vector modulus (amplitude) of 3-C recordings. A BPNN trained by <I>P</I>-arrivals with high signal-to-noise-ratio (SNR) and background noise from station DP can extend its ability to picking <I>S</I>-arrivals and picking arrivals from other stations and from seismograms with low SNRs. It successfully detects 94.3% of the <I>P</I>-arrivals and 86.4% of the <I>S</I>-arrivals, compared with manual picks. The onset times of 74.5% of the <I>P</I>-arrivals and 63.2% of the <I>S</I>-arrivals are successfully picked with an error of 10 ms (one sample increment). In Chapter 5, this method is adapted to pick seismic arrivals from the absolute value (amplitude) of 1-C recordings. A BPNN trained by <I>P</I>-arrivals and background noise from the vertical component of station DP can extend its ability to the other two horizontal components and other stations. The picking rates are 93.1%, 89.4%, and 83.1% for <I>P</I>-arrivals, and 75.0%, 90.9%, and 87.2% for <I>S</I>-arrivals from the Vertical, E-W, and N-S components respectively. With an error of 10 ms, 66.2%, 59.2% and 63.3% of the <I>P</I>-arrivals, and 52.7%, 61.2% and 57.7% of the S-arrivals are picked from Vertical, E-W and N-S components respectively. In Chapter 6, another BPNN approach is developed to identify <I>P</I>- and <I>S</I>-arrival types from local earthquake data, using the modified degree of polarization (DOP) of 3-C recordings. Only the arrival segments are input in this BPNN approach for processing. Compared with manual analysis, a BPNN trained with nine groups of training <I>P</I>-arrival, <I>S</I>-arrival, and noise burst segments of DOP from station DP can correctly identify 82.3% of the <I>P</I>-arrivals and 62.6% of the <I>S</I>-arrivals from station DP. Another BPNN trained with five groups of training datasets from station AY can correctly identify 76.6% of the <I>P</I>-arrivals and 60.5% of <I>S</I>-arrivals from station AY. In order to understand how the BPNN works, a weight map is designed in this thesis to show the weight patterns of a trained BPNN. This new finding would be applied to any BPNN application, enabling illumination of the "block-box" approach of BPNN analysis. Applying this map to three trained BPNNs shows that it is a useful tool to investigate the interior and performance of BPNNs. For example, the weight map of a BPNN applying to pick arrivals from 3-C recordings shows that its weight pattern is divided into two portions which have different functions in picking.

Environmental and instrumental effects on high precision gravimetry : a case study in Britain

Hopewell, Hugh January 1999 (has links)
This thesis investigates the removal of environmental and instrumental effects from high precision gravity records and applies the findings to measurements made in Britain. Ocean loading calculations have been developed by manipulating ocean tide model grids to be of variable size. This has improved the fit to irregular coastlines and the approximation of grid cell masses to point mass loads. Ocean loading is calculated for 12 components at the 69 sites of the British Precise Gravity Network (BPGN). The new ocean loading correction amplitude for Newlyn has increased from 16 to 23 <i>μ</i>gal; the set standard deviation of a 24 hour FG5 absolute gravity data set reduces from 4.3 to 1.8 <i>μ</i>gal. Adjustment of the BPGN with the new correction reduces the mean site standard error from 3.32 ± 0.60 <i>μ</i>gal to 2.96 ± 0.46 <i>μ</i>gal. FG5-103 Bidston measurements show a seasonal change of 8 <i>μ</i>gal. Water table data after cor­rection for air pressure show a correlation with gravity, having a lag of 3 months. Using 6 months of continuous LaCoste & Romberg (LCR) Earth-tide gravimeter data, the air pressure admittance at Bidston varies seasonally from 0.35 to 0.50 <i>μ</i>gal mbar<sup>-</sup><sup>1</sup>. Frequency dependent admittances correcting for instrumental humidity effects and environmental air pressure and rainfall effects removed structure in the data. Admittances derived from LCR Earth-tide data for air pressure and rainfall do not improve the absolute data. Using synthetic data, the frequencies of damped sinusoids found from FG5 residuals are dif­ferent to those originally present. Noise found at 9.5<i> </i>Hz causes gravity changes of < 1 <i>μ</i>gal when fitted to the equation of motion with real data. However, gravity changes by 6.3 <i>μ</i>gal upon removal of its cause. Fitting frequencies from 4 -13 Hz to this data creates changes in the order of 10 <i>μ</i>gal. Existing noise correction algorithms may miss errors of this size.

An estimation of seismic anisotropy in the crust from three different time and distance scales

Graham, Gerhard January 1994 (has links)
In this thesis shear-wave splitting is analyzed in shear-wave arrivals from three different time and distance scales to identify the occurrence and extent of anisotropy in the crust. The refracted shear waves recorded by the closely spaced seismic network of the third Turkish Dilatancy Project from regional earthquakes in western Turkey have similar polarizations to those previously observed above local earthquakes, but with less scatter and substantially greater time delays (up to 1.0 s) between the split shear waves. This is interpreted as indicative of the presence of anisotropy in the lower crust with similar anisotropic symmetry to the crack-induced anisotropy of the upper half of the crust. Mine tremors in the deep gold mine of South Africa have seismic magnitudes ranging from M<SUB>L</SUB> -4 to M<SUB>L</SUB> more than 5. An examination of three-component acceleration and velocity seismograms recorded on the surface above an active gold mine indicates that the shear-wave polarizations have a nearly uniform alignment. This combined with the measured time delays are consistent with shear waves propagating through the effective anisotropy of parallel, vertical microcracks throughout the rockmass. It is concluded that the dry fractures caused by the high stresses during normal mining processes have negligible effect on the shear-wave polarizations at the wavelengths at which shear waves are recorded at the surface. The anisotropy observed at the surface appears to be due to microcracks aligned by the regional stress regime rather than disturbances in the local stress regime due to mining operations. High frequency subsurface recordings of mine tremors that occur in confined volumes surrounding the work face of the mining excavations show shear-wave splitting over distances of a few hundred metres. Plate Carée cylindrical projections of the observed shear-wave polarizations and time delays were compared with theoretical patterns of polarizations and time delays from different synthetic models. From this comparison it became clear that a model containing fluid-filled, non-vertical cracks striking approximately north-south could be used to produce the observed polarizations and time delays to some extent.

An astrobiological study of high latitude Martian analogue environments

Cousins, C. R. January 2010 (has links)
The search for life on Mars is in part reliant on the understanding of Martian environments, both past and present, in terms of what life may inhabit these environments, how this life may be preserved in the rock record, and how this rock record may be detected during future missions to Mars. In particular, the upcoming European Space Agency mission ‘ExoMars’ has the primary aim to identify evidence of past or present life on Mars, and the work presented here is carried out within this context. Volcanism is a geological process common to both Earth and Mars, and this work sought to conduct a multidisciplinary astrobiological study of terrestrial volcanic and associated hydrothermal environments that exist geographically at high latitudes. Specifically, subglacial basaltic volcanic environments were explored in terms of phylogenetic diversity, preservation of biosignatures, and habitability under Martian conditions. Additionally, these and other volcanic environments were utilised in the development and testing of the Panoramic Camera – an instrument that will form an integral component of the ExoMars rover instrument suite. Results presented within this thesis demonstrate that subglacially erupted lavas provide a habitat for a diverse bacterial community, and that when such a community is subject to present-day Martian analogue conditions, survivability is significantly enhanced when a simulated subglacial volcanic system (i.e. heat and ice) is present. However, the generation of bioalteration textures – a biosignature common to glassy basaltic lavas – appears to be less common in subglacially-erupted lavas than their oceanic counterparts. Lastly, this work demonstrates the ability of the ExoMars PanCam in the detection of astrobiological targets, and shows the importance of utilising Martian analogue terrains both for biological studies, and also for testing rover instrumentation in preparation for upcoming missions.

Negotiating diasporic mobilities and becomings : interactions and practices of Europeans of Moroccan descent on holiday in Morocco

Wagner, L. B. January 2011 (has links)
In recent years, the annual summer journey of Moroccan families from Europe towards ‘home’ has become a state-acknowledged event as their arrivals number in the millions. These holidays serve an important role in molding ideas and practices of ‘Moroccanness’ for post-migrant generation diasporic visitors, as it is during this time that those of Moroccan descent raised in Europe have the most exposure to Morocco in situ – to language, community and space as they are practiced within the territorial boundaries of the nation – instead of their habitually practiced ways of being Moroccan (or not-being Moroccan) elsewhere. Using an approach based in linguistic anthropology alongside a geographical consideration of embodiment and mobilities, this thesis focuses on communicative and consumption practices of such individuals during their visits to Morocco. I traveled with participants, observing and recording interactions amongst diasporic visitors and community members resident in Morocco, and engaging with them in their practices of touristic leisure consumption. Thinking about ideas of ‘Moroccanness’ as a node in assemblage, unfixed yet specific, I demonstrate how their communicative and consumption practices shape an evolving sense of what it means to be ‘Moroccan’ for diasporic visitors. Their ideas of ‘Moroccanness’, which take shape both as rooted in diasporic connection and as touristic appreciation through consumption, resonate with the sense of ‘being Moroccan’ during their holidays and when they return ‘home’ to Europe. Yet, their diasporic orientation towards Morocco as a place of leisure consumption has ramifications on the relationship between future diasporic generations and the territory as ‘homeland’. My main theoretical contributions are: reimagining ‘diasporic’ in materialist terms, as an action instead of a state of being; and reimagining ‘hybridity’ as a set of interactions responding to multiple attractors in multiplicity, rather than an unstable condition of being neither one, nor the other.

Understanding the ecological response of marl lakes to enrichment : a combined limnological and palaeolimnological approach

Wiik, E. M. E. January 2012 (has links)
Eutrophication is a worldwide phenomenon affecting the ecology of water bodies, yet little is known of the timing, magnitude and characteristics of the resulting change in highly calcareous (marl) lakes. This project aimed to fill this knowledge gap through the combination of multi- proxy palaeolimnological analyses (last few hundred years) and comprehensive limnological monitoring (2009, 2010) of three English marl lakes, Cunswick Tarn, Hawes Water, and Mal- ham Tarn. The specific objectives were to characterise the centennial-scale eutrophication response of macrophyte, microalgal and invertebrate communities, and to assess ecological status and reference conditions for the sites. Eutrophication-driven declines in macrophyte colonisation depth, diversity, and evenness have occurred in all sites. While such changes were expected in the most eutrophic site, Cunswick Tarn, relatively marked changes also in Mal- ham Tarn and Hawes Water evidence high ecological sensitivity. Detailed palaeolimnological analyses of Cunswick Tarn revealed not only gradual, but also rapid and dramatic change and importantly synchroneity in response across biological groups. Shifts in community composi- tion closely followed changes in marl deposition with (i) substantial increases over a decade in the early 1900s characterised by increases in charophytes, molluscs and plant-associated cladocerans, indicating a persistence of benthic pathways and high water transparency (ii) decreases around the 1920s, accompanied and followed by progressive increases in floating- leaved macrophytes, cyanobacteria and pelagic cladocerans. Stable isotope analysis of fossil resting eggs of Daphnia in Cunswick Tarn sediment appeared to indicate key changes in the balance between benthic and pelagic production through changes in both carbon uptake and patterns of Daphnia abundance and seasonality. Although marl lakes may initially buffer eutrophication through increased benthic production, key ecological changes identified early in the eutrophication process suggest marl lakes are as susceptible to eutrophication as other lake types. Near-complete collapse of their characteristic ecology is possible with moderate levels of enrichment.

Studies of the petrologic and geochemical diversity of the lunar regolith

Snape, J. F. January 2012 (has links)
This study comprises petrologic investigations of the lunar meteorite Northeast Africa (NEA) 001 and 16 fragments from the Apollo 12 soil 12003. These samples are examples of the variety of materials found in lunar feldspathic highland and mare regoliths, and provide insights into the petrogenesis of different lithologies and regolith mixing processes. They have been analysed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry. NEA 001 is a feldspathic regolith breccia containing clasts of multiple lithologies including; feldspathic impact melt, ferroan noritic anorthosite, magnesian feldspathic clasts and low-Ti and very-low-Ti (VLT) basalts. Feldspathic impact lithologies in NEA 001 are more mafic than estimations for the composition of the upper feldspathic lunar crust, indicating that they have incorporated more mafic lower crustal material. The VLT basalt clasts have low incompatible trace elements (ITE) concentrations, and may represent products of primitive basaltic volcanism. The feldspathic nature of the sample, low-ITE concentrations, and the presence of magnesian feldspathic clasts suggest that the meteorite has been sourced from the Outer-Feldspathic Highlands Terrane, probably on the lunar farside. Apollo 12 landed in the eastern region of Oceanus Procellarum. Previous studies of Apollo 12 samples identified three major basaltic suites (olivine, ilmenite and pigeonite basalts) local to the region. Examples of all three of these have been identified within the 12003 soil collected near the Lunar Module. Two samples are identified as possibly representing a previously unrecognised basaltic suite. One sample is identified as a possible addition to the feldspathic suite; a potential fourth basaltic suite currently consisting of only one other sample. Three non-igneous samples are identified, including two breccias and one impactite. The petrologic analysis of these samples forms part of an ongoing basaltic diversity study which will also include radioisotope dating of these and other Apollo 12 soil samples.

Quantifying stress and strain in diamond

Howell, D. January 2009 (has links)
Birefringence in diamond is an anomalous optical property for a nominally isotropic material. The occurrence of birefringence in diamond is a result of the photoelastic effect; the change of refractive index caused by stress (Nye, 1957). With the development of the MetriPol system (Glazer et al., 1996), a new technique is now available that allows rapid and accurate measurement of birefringence. Detailed knowledge of the photoelastic effect in diamond is vital to be able to use this new quantitative birefringence analysis with confidence. Initial investigation of the current literature on the photoelastic constants of diamond showed some confusion between the results of different groups. So to confirm the values believed to be the most accurate, a new technique was developed during the course of this study that allows a quantifiable stress to be applied to a crystal sample while it is being viewed under a microscope. This has allowed for an investigation of the photoelastic constants of diamond with the MetriPol system. The data produced is within ± 15% of the results of Grimsditch et al. (1979) and has confirmed the signs of the photoelastic constants, as well as the fact that the refractive index of diamond decreases with increasing hydrostatic pressure. Lang (1967) postulated five causes of stress and strain within natural diamond. These being lattice parameter variations, dislocations, fractures, inclusions and plastic deformation. Preliminary investigation of each of these five causes in this study highlighted the importance of understanding the geometry of the stress field when using quantitative birefringence analysis. This lead to further investigation of the birefringence halos that form around mineral inclusions that remain under remnant pressure. Application of the MetriPol analysis to measuring the remnant pressure required a theoretical model to be produced to understand the relationship between the measured value of retardation and the peak level of anisotropy. The birefringence analysis and theoretical model were initially applied to garnet inclusions in diamonds from Udachnaya, Siberia. While this gave a suitable result for the remnant pressure and calculated source pressure and temperature conditions, it could not be verified by another other analytical technique. Verification of the model was obtained by measurements on a coesite inclusion in a diamond plate from Finsch, South Africa, using various Raman techniques (including 2D mapping and point analysis). However, when the source pressure and temperature conditions were calculated from the remnant pressure, the results were well below the diamond and coesite stability fields. This calls into question the suitability of the coesite-in-diamond geobarometer, first proposed by Sobolev et al. (2000). A series of high pressure high temperature (HPHT) deformation experiments were performed on a set of diamonds, to investigate the association of brown colour with plastic deformation. With the use of a deformation DIA type press, a uniaxial stress was applied to diamonds that were already under HPHT conditions. In two tests where the uniaxial stress was not introduced there was no colour change in the sample. In four tests where the uniaxial stress was introduced, the colour of the samples changed from either colourless or yellow, to brown.

The Marwar Group (Rajasthan, India)

Papp, M. L. January 2013 (has links)
The Marwar Group is a Neoproterozoic - Early Cambrian sequence of sedimentary rocks exposed in NW-India that are similar to successions found in Oman(Huqf-Supergroup) and the Salt-Range in Pakistan. Attempts to correlate these locations have been hampered by the poor age control and limited exposure of the Marwar Group. The goal of this project is to resolve this by meeting three core objectives: (1) establish a better chronostratigraphic framework for the Marwar Group based on evidence from detrital zircon U-Pb dating, (2) conduct detailed eld mapping to test and rene existing stratigraphic models and (3) produce a detailed 13C isotope prole for the carbonates that can be used for regional correlation. Mapping of the Neoproterozoic-Cambrian outcrops of the Nagaur Basin in Marwar, Rajasthan are subdivided into the Jodhpur Sandstone, the Bilara Carbonate and the Nagaur Sandstone. Results from the sandstone rocks of the Jodhpur and Nagaur Formations yielded zircon ages that belong to three main age groups at; 0.7-1.1 Ga, 1.6-1.9 Ga and 2.4-2.7 Ga, consistent with a common provenance. The youngest zircon ages indicate the maximum age of the Marwar Group is 0.75 Ga. I discovered that the carbonates of the Bilara Formation, were deposited in a restricted environment with a range of 13C values that are typical for a restricted basin. Therefore the 13C values are not suitable to be correlated to isotope curves from other, open marine, basins and they do not provide improve on the existing chronostratigraphic framework of the Marwar Group.

Page generated in 0.0547 seconds