The magnetotelluric (MT) method is a geophysical technique which uses observations of the naturally occurring electromagnetic (EM) field variations to determine the electrical impedance as a function of frequency. Owing to the variable signal and often high level of noise contamination, it is usual to collect large numbers of observations and perform parameter regression in order to gain reliable estimates of this Earth response function. It is then transformed into more interpretable, albeit non-unique, resistivity versus depth models. In the course of this study, the processing, modelling and interpretation of MT observations from Northern England are considered. This includes previously unpublished data as well as a re-examination of spatially more extensive measurements which were acquired during earlier investigations. Existing MT field equipment has been adapted to allow local and remote reference observations to be collected and the results show that the amplitude of the impedance is contaminated by a lower level of noise induced bias. In addition, the resolution of the resulting model of the near structure can be enhanced by supplementary D.C. resistivity measurements. Considerable attention is focused on the development of new methods for processing and modelling MT observations. In particular, the background theory and implementation of a robust and constrained method for the estimation of the impedance is described in some detail. This method is demonstrated on real and synthetic unreferenced observations and shown to improve the consistency and the reliability of the resulting response function by reducing the level of noise contamination. One dimensional models are obtained for 42 MT response functions from Northern England. These are computed using a new 'exploratory' parametric technique which not only aims to find the best fitting model, but also to investigate its range of uniqueness. Additional, more interpretative modelling methods are also developed to improve the consistency of the models from adjacent sites. An experimental procedure which uses the constraining information provided by 2-D gravity model sections is also considered. At a subset of the sites over the Alston Block and the southern margin of the Northumberland Trough a strictly 2-D MT model is derived.

No description available.

From August 1994 until May 1997, a network of 8 three-component digital seismographs operated at the Açu reservoir. A detailed analysis of this data forms the first part of this thesis. The earthquake locations revealed remarkably well-defined NE trending fault structures beneath the reservoir due to the simple seismic-wave velocity structure of the area. This digital data is shown here to be very suitable for focal mechanism determination and also to investigate seismic anisotropy in the area. This is possible due to the low attenuation of the seismic waves and good signal-to-noise ratio in the seismograms provided by the crystalline rocks of the area. The focal mechanism determination and the shear-wave study of this seismicity confirm that the area is in E-W compression and also that the shear-wave splitting is controlled by the seismic anisotropy associated with the Precambrian foliation. Moreover, this study shows that the seismicity migrates between different faults and within individual faults over different time periods. To investigate this behaviour and relate it to changing water levels in the reservoir and the resulting groundwater flow beneath, a fully three-dimensional groundwater flow code (PARADIGM) is used. PARADIGM is employed to simulate the pressure field in both the rock matrix and an idealised fault geometry. This flow modelling forms the second major component of this thesis. The significance of my approach is that I explicitly consider the physical meaning of hydraulic conductivity and storativity and their impact on fluid flow. In this model, both realistic information on the hydrogeological regime of the area and measurements of real fault zone hydraulic properties are included. The combination of the spatio-temporal analysis of the seismic activity and the numerical simulations carried out in PARADIGM provide new insights into the mechanism that causes the migratory behaviour.

In this study, the extensive terrestrial record of glacigenic streamlined landforms in Fennoscandia is used to analyse and reconstruct the internal organisation of the Late Weichselian Scandinavian Ice Sheet and its evolution during the deglaciation. The analyses involved the identification of coherent sets of lineations that can be linked to flow systems within the ice sheet. The presence of superimposed sets of glacial lineations makes it possible to work out relative ages for coherent lineation patterns. In a number of cases, absolute dating is possible because lineation patterns can be connected with dated ice marginal positions. The ice sheet organisation in Scandinavia was dominated by the presence of a number of ice streams. In many areas these zones of fast flow are to a large extent controlled by the presence of valleys and troughs. In the case of Central and South Finland and Russian Karelia, however, a persistent system of ice streams/ice lobes developed that was not topographically controlled. The scale of these ice streams is at least an order of magnitude larger than the scale of the underlying topographic variations, both in the vertical and the horizontal dimensions. The regular distribution of these ice streams, their persistence during the Late Weichselian deglaciation and their independence of local topography and geology implies that they are the result of internal dynamic processes and reflect ice flow organisation within the ice sheet. The landform distribution reflect these lateral variations in ice velocity. Ice stream areas are dominated by streamlined bedforms, while in the inter stream areas, hummocky fluvio-glacial deposition is more important. Due to the limited ice activity in the inter stream areas, landforms pre-dating the deglaciation have been preserved here. During deglaciation diachronous, superimposed, patterns of diverging glacial lineations were formed in the ice stream areas.

No description available.

During a six month period in 1984 synchronous Electromagnetic data were recorded from a 5 site MT array located near the town of Izmit, western Turkey. The array was located over an active earthquake swarm which forms part of the active North Anatolian Fault (NAF). The recordings covered a band width of 10 to 10000 seconds. Both Single Site (SS) and Remote Reference (RR) processing techniques were employed to produce impedance tensors for MT, Vertical and Horizontal fields. Results for consecutive 5 day groups provide maximum resolution for the time dependency study, while 30 day groups provide high resolution sounding curves that are used in the construction of a geoelectric model for the area. Interpretation of the MT sounding curves indicates that induction is controlled by a thin sheet mechanism which gives rise to static shifts in all the MT curves. Regional geoelectric strikes correlate well with the azimuths of regional stress (ESE-WNW). A 1-dimensional (1-D) geoelectric model was derived based on the MT phase curves of sites 222 and 333. The first layer, 10 to 14 km thick with a resistivity of 60 to 100 ohm.m., correlates well with the brittle fracture seismogenic zone of the upper crust in this region. This zone overlies a more resistive layer of 800 to 1000 ohm.m. Time dependency results highlight the need to adopt robust methods for data collection and processing. Within the limits of data resolution and because of low level seismic activity, no precursory changes in the electromagnetic transfer functions were observed. Horizontal field transfer functions provided the most consistent and accurately resolved results.

Cores of sediment were collected from six lakes in southern and eastern Finland. Measurements of the natural remanent magnetization (PIRM) and the magnetic susceptibility were made on both the unopened cores and on subsamples in order to define secular variation curves for Finland. Tests showed the remanence to be stable. Age estimates were obtained from pollen analyses since radiocarbon dating of the sediments proved to be unsatisfactory. The correlation between cores from any one lake was facilitated by use of the intensity and susceptibility logs. Swings in both the declination and inclination records obtained from the cores were correlatable within and, to some extent, between lakes. Using a variety of magnetic methods it was concluded that fine grained magnetite carried the PIR14 of the sediments. Haematite was present in some of these sediments but did not appear to carry any of the 11RbL. It was found that the HR14 was decreased by drying and by cooling through -10°C. In both cases randomization of fine grains which carried the HR14, brought, about by physical changes in the sediment, was responsible for the loss of intensity. Rotation of magnetic grains after deposition of the sediment appeared to be the main mechanism by which the lake sediments acquired an HRM. Further experiments showed that the grains were not fixed in place simply by dewatering of the sediments but that the growth of gels in the sediment stabilized the particles. Some grains in the sediments were still free to rotate and to acquire a remanence. Samples of bedrock, soil, drift and stream sediment were collected from the catchmeat areas and magnetic measurements made on them to determine the source of the magnetic minerals in the lake sediments. The measurements indicated that the magnetic minerals were derived from the glacial drift. Haematite was present as a secondary mineral in the drift but was broken down during erosion and transport of detritus to the lakes.

<I>P</I> waveforms from small intraplate earthquakes observed at teleseismic distances often exhibit discrete arrivals which are easily interpreted in terms of a direct <I>P</I> phase and associated surface reflections. These arrivals contain information about the <I>P </I>and <I>S</I> radiation pattern at the source. This thesis attempts to quantify how much information about the source is contained in seismograms from small disturbances. In the first study, regional surface waves and <I>P</I> waves recorded at teleseismic distances by short period (SP), long period (LP) and broadband (BB) seismographs are analysed to determine the source parameters of the Völkershausen seismic disturbance of 13 March 1989. This disturbance is the source of some controversy, with many proposed source mechanisms, including a shallow multiple mine collapse. Application of the relative amplitude method and forward modelling of the <I>P</I> waveforms recorded at teleseismic distances suggest that the data are consistent with a sub-vertical normal dip slip fault striking northwest-southeast at a depth of 6.5 km. Forward modelling of the surface waves recorded by the Blacknest broadband network shows that the observed surface waves are consistent with this source model. Examination of SP seismograms from the French network and waveforms from the Gräfenberg array, in the distance range 100 to 500 km, show no large <I>R<SUB>g</SUB></I> phase, which would be expected for a shallow mine collapse. In a second study, over 50 small earthquakes in the 1976 Gazli sequence were analysed based on SP array station recordings. Pearce <I>et al</I> (1980) studied 21 of these earthquakes using the relative amplitude method (Pearce 1977, 1980), assuming an <I>a priori</I> double couple source, and found that the observed waveforms were consistent with 45° reverse dip slip faults striking northwest-southeast.

This thesis is concerned with the evolution of seismicity as described by the value of <i>b</i> in the Gutenberg-Richter relation, and the fractal dimension, <i>D</i>, of the distribution of earthquake epicenters. It is found that, in the case of two areas of California, there is a negative correlation between the <i>b</i>-value and <i>D</i>. These observations lead to the development of a one-dimensional fracture-mechanical model of seismicity which includes both negative and positive feedback processes. The fracture toughness of the fault elements is initially generated by a random walk function, but as the applied remote stress is increased, the fracture system becomes organised into a fractal set. The values of <i>b</i> and the fractal dimension of the crack population are measured at each step. It is found that the model predicts the observed negative correlation between the <i>b</i>-value and the <i>D</i> of earthquake epicenters. The short-range interactions described in the model give rise to a system with a long-range order, similar to current models of critical processes. It is found that in the long term there is an extremely irregular evolution of <i>b</i>-value. A preliminary extension of the model to two dimensions using a cellular automaton is presented. The model is further tested using a high quality dataset collected in a structurally simple area in Brazil. Two clusters of seismicity are examined in detail. It is found that in one cluster there is a strong negative correlation between <i>b</i> and <i>D</i>, whereas in the other cluster, which is characterised by a relatively small number of large events, there is a weak positive correlation. It is suggested that this positive correlation arises from processes connected with fluid migration in the crust. The fractal nature of heterogeneity in the crust in western Norway is examined by measuring coda-<i>Q</i>. It is found that the attenuation in this area is very low, with <i>Q</i> values of 1400 at 10Hz, and that the fractal dimension of lithospheric heterogeneity is low. The origin of the stresses responsible for intra-plate seismicity is studied by estimating the stress tensor in Sweden by inversion of data from earthquake fault-plane solutions. The stresses in the southern part of Sweden are found to be uniform and suggest a NW-SE compression, as predicted from present-day plate motions. In the north of the country, however, the stress varies over a short distance, and appears to have been significantly perturbed by recent deglaciation.

No description available.