• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 664
  • 320
  • 282
  • 251
  • 3
  • 3
  • 2
  • 1
  • 1
  • Tagged with
  • 3543
  • 791
  • 305
  • 290
  • 290
  • 236
  • 141
  • 132
  • 125
  • 117
  • 115
  • 114
  • 114
  • 113
  • 105
  • 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.
21

Scaled geotechnical centrifuge modelling of gelifluction

Smith, James Seymour January 2004 (has links)
This thesis describes the laboratory modelling of gelifluction processes using the geotechnical centrifuge technique. Thirteen 1/10 scale planar slope models were frozen from the surface downwards on the laboratory floor and thawed, also from the surface downwards, under gravitational acceleration of 10 gravities (approximately 98.1 ms'2). A natural sandy silt soil from Quaternary periglacial slope deposits collected in SW England formed the base test material and slope models at gradients 4, 8, 12 and 16 were constructed using this soil. 10% and 20% by weight increments of glaciolacustrine silt and Kaolinite clay were added to the natural soil and a series of slope models were constructed at gradients of 4, 8, and 12 using these soils. Each slope model was subjected to four cycles of freezing and thawing except for the four slope models that underwent rapid slope failure. During thaw, soil temperatures and pore water pressures were recorded continuously, together with soil thaw settlement and surface displacement. Following each experiment, models were sectioned to observe displacement columns that showed the profiles of soil movement and allowed volumetric displacements to be calculated. It was shown that thaw settlement and slope gradient strongly affected the rate of surface movement and the subsurface profile of movement. Increasing slope gradient generated greater amounts of subsurface and surface movement as a function of increased gravitational shear stress. Thawing ice lenses inclined parallel to the slope gradient provided localised zones of microshearing in response to localised low frictional resistance. Rates of movement increased between the 4 and 8 models, but a greater increase occurred between the 8 and 12 models. A slope failure was initiated within the 16 slope model. Rates of gelifluction were dominantly influenced by increasing silt content impacting upon the distribution of segregated ice and the reduction of frictional shear strength. Increasing silt content generated high positive porewater pressures commonly in excess of hydrostatic and consequently greater amounts of pre-failure strain. A clear behavioural threshold was identified between the 10% and 20% silt soils, with far greater gelifluction in the latter than the former. Increasing clay content had a less pronounced impact upon rates of gelifluction when compared to increasing silt due to cohesion. Rates of movement increased between the 10% and 20% clay in response to lower shear strength. A sawtooth style of pore pressure response caused by water escape events within the 20% clay prevented maximum potential pressures being achieved and possibly impacted upon the overall rate of gelifluction. A successful simulation of both landsliding and slow mass wasting processes was undertaken and future applications for the technique have been outlined.
22

The thermal and hydrogeological regimes of the accretionary complexes along the Pacific margins of Colombia and Panama

O'Neill, P. S. January 1996 (has links)
No description available.
23

The geology of the region between the Alness River and the Dornoch Firth

Armstrong, Matthew January 1964 (has links)
The stratigraphy, petrography and structure of the Old Red Sandstone between the Alness River and the Dornoch Firth have been examined in detail.
24

The geology of Qaersuarssuk Julianehaab district, south Greenland

Watt, William Stuart January 1963 (has links)
An area of Precambrian basement with later dyke intrusions is described. The area is predominately of granite which has been intruded by later basic dykes. Following the intrusion of these dykes there has been reactivation of the granite. This reactivated granite locally shows intrusive features. Gneiss and a little meta-sediment also occur in the area. Some textural features in the granite are described and interpreted as the result of potash metasomatism leading to the formation of large potash feldspar porphyroblasts in a granodioritic matrix. The basic dykes (discordant amphibolites) are described in some detail and comments are made on their use for the division of the basement. The later dyke rocks consist of a variety of types including dolerites, trachy-dolerites, nepheline - micro-syenites, alkali - micro-syenite? camptonites and a single example of a spherulitic soda-rhyolite. Noteworthy among tie dyes are a group which contain numerous, large feldspar crystals (up to 50 cm in length) and feldspar aggregates (up to 1.5 m across). Some of these big feldspar dykes are composite with margins of alkali - micro-syenite. As most of the dykes with alkaline affinities are sub-parallel a division is made on their relations to movements along wrench faults. With the exception of the earliest types this gives an intrusion sequence opposite to that expected from alkaline magmas formed by successive stages of fractional crystallization. Later than all other dykes and all faulting is a dolerite dyke that is characterized by its vertical banding.
25

Investigation of correlations of precipitation electricity to different receivers

Owolabi, I. Esan January 1964 (has links)
Measurements were made of the precipitation currents to two identical shielded receivers, constructed according to the design first suggested by Scrase (1958), the earth's electric potential gradient by the Field Mill method, wind speed (using a cup-generator type anemometer), and wind direction (using a rotary potentiometer). The two rain receivers, situated on the flat roof of the Physics building of the Durham University, could be separated horizontally to 50 metres. Using an Elliott 803 digital computer, and by suitable programming, correlation coefficients between the two currents were calculated for various, tine lags between one current and the other, both when the receivers were placed side by side and when separated by 30 metres. The conclusion is reached that no significant difference could be obtained between the correlation coefficients calculated when the receivers were side by side and when they were at such a short distance apart, especially under continuous rainfall conditions. Comparisons were, therefore, made between the precipitation current measured at the Laboratory and that measured 900 metres away. Significant correlation was obtained for instantaneous measurements of the currents when there was no wind, but when the wind was blowing roughly in the direction of the receivers, a definite time lag was found between similar variations of the currents. By applying the same method of correlation-time lag analysis to the current, I, and the potential gradient, F, the author found on one occasion, for sleet and snow, that a significant negative correlation existed between the two parameters for a time lag-..of about 1+0 seconds of the current on the potential gradient, consistent with the idea that the time of fall of precipitation particles should be considered when fitting the functional relationship I = a(F + C): the discrepancy between the values of constants a and C for Summer and Winter results found by Ramsay (1959) could thus be resolved. Some of the records, especially those of rain showers, did show that the space charge on the falling precipitation (as pointed out by Magono and Orikasa (1960)) is an important factor in the "Mirror- Image" phenomenon in low potential gradients, The author's results indicate, also, that the melting process is a plausible mechanism of precipitation charging.
26

A seismic refraction study of the earth's crust beneath S.W. Britain

Holder, Andrew Peter January 1969 (has links)
This thesis describes a seismic refraction project undertaken in south west Britain in November 1966, designed to investigate crustal structure associated with a granite batholith. The results and interpretation of the data collected are presented in the form of crustal structure sections along the three lines of shots. These results are then compared with those obtained in other areas of the British Isles and the geological implications are discussed, suggestions for the design of future crustal structure experiments based upon the experience gained in south west England are also included. Several methods of analysis have been employed. Least squares straight lines have been fitted to first arrival travel time data and time term analyses have been applied to the major phases thus identified. Amplitude and velocity filtering measurements have been made of both first and secondary arrivals. The importance of a large amplitude secondary arrival, identified as a supercritical reflection from the Moho, is emphasised and an explanation of its amplitude characteristics is provided in terms of a lower crust exhibiting a gradual increase of velocity with depth. The average crustal velocity along the line of the south west England peninsula and between Land's End and Brittany is about 6.1 km/sec with an upper crustal velocity of about 5.8 km/sec. The crust is about 27 to 28 km thick with an almost horizontal Moho and a sub-Moho P(_n) velocity of about 8.07 km/sec. The granite batholith of south west England extends to a depth of about 11 km beneath which the lower crust exhibits a gradual increase of velocity with depth. For the region between Land's End and Ireland there is some evidence for a higher P(_n) velocity and average crustal velocity with a dipping Moho such that the thickness of the crust beneath southern Ireland may be about 30 km.
27

Interpretation of oceanic magnetic anomalies using a linear inverse technique

Hutton, Michael Alexander January 1970 (has links)
A direct magnetic interpretational technique has been developed and applied to oceanic magnetic anomalies. The method of interpretation computes a distribution of magnetization, within a specified two-dimensional model - given the direction of magnetization, from the observed magnetic anomalies. The technique is based on the numerical solution of a linear integral equation which is approximated by a finite set of linear algebraic equations. These equations relate (n) observed magnetic anomaly field points to (m) unknown magnetization values. Solution of this system of equations is carried out by computer, using matrix operations. The programming procedure allows model elements of irregular cross-section to be incorporated within the magnetic layer and provides a solution to both the completely determined and overdetermined problem (i.e. n≥m). Details of this procedure are presented together with an evaluation of methods of application. Interpretations of magnetic profiles in the North Atlantic Ocean, the Gulf of Aden and the Pacific Ocean are presented in terms of computed distributions of magnetization confined to Layer 2. Results are discussed in terms of the Vine-Matthews hypothesis of sea-floor spreading and certain apparent differences in the bulk magnetization of the oceanic crust. Model studies confirm the feasibility of a thin magnetic layer (0.5 km), situated just below the sea-floor. The approximate shape of this magnetic layer is deduced from known magnetization values obtained from dredged rock samples. Interpretation of magnetic data from the Pacific Ocean indicates that both vertical and inclined source bodies, within Layer 2, represent plausible models, although extensive subhorizontal bodies (dipping at 10 and less) are unlikely.
28

Measurements of the derivative of the p wave travel time curve by means of an array network

Corbishley, Derek J. January 1969 (has links)
Data from the four UKAEA seismic arrays have been combined to measure the slope of the P-wave travel time curve (dT/d Δ , or slowness) of events occurring at distances Δ = 30 to 104. Slowness is the quantity that enters into any calculation of the compressional velocities to give the main source of direct detailed information regarding the mantle. Multiple regression analysis was used and corrections, which are azimuthally dependent, estimated to correct for the near surface geology under the arrays. By using all available events and removing the bias introduced by the array geology, the slowness-distance curve should represent the best average for the world. Anomalous features in the slowness curve occur at distances of around 35-36, 48-49 , 60 , 68-70 and 84-85 . These correspond to high velocity gradients within the lower mantle near the depths of 900, 1200, 1550, 1900 and 2500 kms., and support the hypothesis that the mantle is inhomogeneous at depth. A comparison is made between these features and regions considered to be inhomogeneous found at similar distances by other studies. The site corrections obtained for each seismometer are attributed to inhomogeneities in the sub-array geology. The correct ions derived for the arrays situated in Canada (YKA) and India (GBA) are small in magnitude and show the crustal layering to be essentially horizontal. The corrections at the array in Scotland (EKA) are shown to be related to the relative altitudes of the instruments. A velocity of 2.94 km/sec. was derived for the velocity in the top 170 m of crust. The corrections for the Australian array (WRA) show that to a first approximation, the layering in the crust is dipping at 3.6 in the direction N195 E. The corrections have a large azimuthal component and show the presence of an anomaly near the cross-over point of the array.
29

Point discharge in atmospheric electricity

Maund, J. E. January 1958 (has links)
Investigations of the potential gradient beneath the ion stream liberated in point discharge in the atmosphere are described. The electrostatic effects of the ions are analysed theoretically and the results of simultaneous measurements of the potential gradient on the upwind and downwind side of an artificial point, wind speed, wind direction and temperature gradient reported. It is concluded that for distances from the base of the point less than √3 h times the height of the point and excluding conditions of snow and very heavy rain the electro-static field of the ion stream can be computed on the assumption that the ions take up an infinite line distribution. These conclusions are then applied to measurements made on a line of trees at Durham University Observatory and around a sycamore tree at the Durham City Golf course. The results for the line of trees indicate a change in point discharge characteristic with season; for the sycamore tree the current down the tree was found to be less than 1μa for a potential gradient of + 7,000 V/m. The changes of characteristic with season might be correlated with the appearance of the leaves on the trees. The implications of these results are discussed with reference to the assessment of point discharge current density basedon the equivalence of an artificial point and tree of a similar height. It is concluded that this equivalence is not in general justified and that a more comprehensive study of a similar nature is required before a reliable estimate of mean point discharge current density can be obtained from measurements with artificial points.
30

The interpretation of magnetic anomalies between Iceland and Scotland

Ingles, Alan David January 1971 (has links)
The collection of data, and the results of a detailed magnetic survey on the crest of the Iceland-Faeroes Rise are described, A matrix method is developed to transform gravity anomalies to magnetic anomalies, and vice versa, to determine the ratio of magnetism to density in an equivalent layer, to solve for the angle of magnetisation of a body causing a magnetic anomaly and to separate magnetic anomalies caused by different types of source body. The data from the detailed survey area are interpreted as supporting the conclusions of previous authors that the crustal structure of the Iceland-Faeroes Rise is highly anomalous for an oceanic setting, and 16 similar to that of Iceland, with at least two magnetic layers which contain central intrusive complexes; granitic rocks may also be present. The matrix methods developed in the first part of this work are used to interpret gravity and magnetic data from a previous Durham survey on the Iceland-Faeroes Rise. Results indicate that the magnetic anomalies are controlled by seismic structure on NE - SW profiles, but include a component which is independent of seismic structure which is of greater significance on NW - SE profiles. The latter component is identified as magnetisation changes as a function of time. Magnetic and gravity anomalies from the Scottish Continental Shelf region are used to demonstrate further the scope of the matrix methods for combined analysis of gravity and magnetic anomalies.

Page generated in 0.0436 seconds