Few studies of metal pollution in lakes have been carried out in urban environments. This research aimed to determine both temporal and spatial changes in metal concentration in sediments of lakes in London, and identify the current extent of contamination in lake ecosystems as a whole. The novel use of sediment archives to reconstruct potential toxicity was also explored. Sediment cores were taken from seven lakes across London and analysed for Ni, Cu, Zn, As, Pb and Hg concentrations. The cores were dated using a combination of radiometric and SCP dating, and lake age. Temporal trends were found to vary within and between sites, due to metal behaviour, catchment disturbance and the proximity to and type of metal source. PCA showed that there was some evidence for a regional pattern of contamination. Across all sites metal concentrations were very high, exceeding guideline values both in the past and at present. At various times the concentration of Pb had reached levels that were over 2000% higher than the guideline value. Increasing levels of enrichment and flux towards the surface of the cores also showed that contamination was not declining. Metal concentrations were also determined in deposition, water, and biota at one of the lakes. The concentration of Pb was found to exceed guideline values in both water and fish. The extent of Pb contamination in London lakes is therefore a major cause for concern. The potential toxicity of the combination of metals analysed in the sediment cores was reconstructed, through the calculation of mean toxicity quotients. Every core exceeded the potential toxicity threshold at all depths. Comparison to laboratory toxicity test data carried out on sediments from the OPAL lakes showed that the sediments were likely to be toxic, which would have implications for lake ecosystems should they be disturbed.

This thesis comprises two parts. The main part is involved with laboratory studies of the palaeosecular variation of the geomagnetic field as recorded in lake sediments. The natural remanent magnetization of the sediments cored from the two Italian maar lakes, Lago di Monticchio and Lago di Martignano, has been studied. Further studies were carried out on the sediments of Lago di Martignano to determine the cause of large variations in the magnetic intensity of the sediments with an age of ˜ 6000 years BP and it was concluded that this was caused by the arrival of Neolithic man and the advent of agriculture in the catchment area. The directional record for this lake was also compared to the established record for north west Europe. Several declination and inclination features could be correlated between the two records. The record from Lago di Martignano can be accepted as a regional palaeomagnetic reference curve for central Italy. Cores up to 50m long were taken from Lago di Monticchio. Whilst not yet firmly dated, it is agreed that this record spans the last 250,000 years. A relative palaeointensity record has been calculated and spectral analysis has been carried out. It is concluded provisionally that the palaeointensity recorded in the sediments was effected by the astronomical frequencies associated with precession of the earth, the eccentricity and the obliquity of the Earth's orbit. The second part of this thesis is concerned with modelling the palaeosecular variations as recorded in sediments around the world through the Holocene, i.e. the last 10,000 years. The properties of sequential secular variation records from sediments are compared with palaeosecular variation scatter determined from sets of lava flows. It is concluded that a comparison between PSV recorded in lava flows and lake sediments is valid.

No description available.

Observations and theories of elastic anisotropy in the Earth's crust and upper mantle are reviewod and discussed. Crampin and Taylor's (1971) procedure for computation of seismic surface-wave characteristics in a plane-layered anisotropic half-space is described and a modification to incorporate a surface water layer is developed. Simplified models of anisotropic ocean-basin structure are examined, with particular reference to surface-wave particle-motion. Two types of anisotropic alignment are considered, one resulting from syntectonic recrystallisation of olivine in a horizontal-shear-zone, the other from olivine glide-plane slip with horizontal or vertical slip-planes. Alignment of the first type can cause slightly anomalous particle motion, of tilted-Rayleigh-type, in all surface-wave modes. The variation of anomaly amplitude with period in the fundamental mode can indicate the approximate depth to the anisotropic layer. Alignment of the second type can cause highly anomalous particle-motion, of inclined-Rayleigh-type, in the third generalised mode, corresponding to the isotropic second-Rayleigh mode. The anomaly amplitude is rather insensitive to details of structure. For either type of alignment, the sense of tilt, or inclination, varies with direction of propagation, in a manner characteristic of the structural symmetry. Some practical problems in observing surface-wave particle-reaction in real ocean-basins are discussed. Several seismograms are presented showing tilted-Rayleigh-type particle-motion for modes corresponding to the isotropic fundamental-Rayleigh and Love modes. Observations are consistent with theoretical models if aligned olivine b-axes are tilted down in the direction of plate-motion. Carter's (1970) syntectonic recrystallisation model then suggests that the lithosphere is dragging the asthenosphere. Preliminary results for the variation of anomaly amplitude with period suggest a high degree of allignment, stronger in the lithosphere then in the athenosphere.

The area mapped lies astride the Tonale Line, a major, east-west, sub-vertical lineament marked by 10 to 70 m of brecoiated pseudotachylite. This zone of repeated movement divides the area into two major units, the Southern and Central Alps. Regional Alpine northward thrusting is restricted to the Central Alps which lie north of the Tonale Line. The Southern Alps consist of Greenschist grade metasediments which have been folded by the Heroynian and Alpine events. These rooks are intruded by the Adamello Massif, a composite calo-alkaline body of Oligocene age, which has a narrow metamorphic aureole. The tonalite and biotite-quartz-diorite within the area mapped are foliated adjacent to their northern margins, and strain values have been calculated from deformed xenoliths. The Central Alps consist of three distinct units: 1. The Stavel Gneiss, a 0.7 to 1 km wide zone of massive quartz-K-feldspar-phengite gneiss, lies adjacent to the Tonale Line. The gneiss formed from banded politic gneiss under high PH20 at moderate temperatures. Only Late Alpine folding is seen. Mineral compositions for phengites and co-existing feldspars are given and factors determining phengite composition and d spacing discussed. 2. The thrust sheets north of the Stavel Gneiss are composed of Caledonian Upper Amphibolite grade politic gneiss, marble and amphibolite. The rocks form a large 'backfold' which apparently 'roots' to the south. Heroynian, Early and Late Alpine folding and metamorphism have affected the gneisses although these events are localised in some areas. The mylonite zones which separate individual thrust sheets have been studied using available strain markers, microtestures, magnetic anisotropy and minor fold geometry. The mylonites formed by ductile flow under high shear stress along narrow wet zones, initiated as 'contact strain' areas. 3. North of the Peio Line, a Late Alpine mylonite zone, are a series of semipolitic schist showing evidence of Heroynian and Early Alpine deformation and metamorphism. These metasediments form the upper part of a larger thrust sheet.

Bias in global seismic magnitude determinations caused by inadequacies in distance/depth calibration functions is reduced, by developing new formulae for surface-wave magnitude <I>M<SUB>s</SUB></I>, and new distance/depth calibration terms for body-wave magnitude <I>m<SUB>b</SUB></I> . Bias in <I>M<SUB>s</SUB></I> and <I>mb </I>is investigated using the complete ISC and NEIC datasets between 1978 and 1993. Analysis of the ISC dataset shows that the density function for magnitude against frequency of <I>M<SUB>s</SUB></I> values is smooth but significantly asymmetric, while that for <I>m<SUB>b</SUB></I> is symmetric and is close to normally distributed. Examination of <I>M<SUB>s </SUB></I>: <I>m<SUB>b</SUB> </I> for this dataset reveals some anomalous earthquakes which plot as explosions according to the <I>M<SUB>s </SUB></I>: <I>m<SUB>b</SUB> </I> discriminant. Also, the frequency-distance plot for reported surface wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the <I>P</I>-wave shadow zone for 120° < Δ < 100°. To reduce bias in surface-wave magnitude two new formulae are proposed, with constants obtained using all ISC data: <I>M<SUP>e</SUP><SUB>s </SUB></I>= log (<I>A/T</I>) <I><SUB>max </SUB></I>+ 1.155 log (Δ) + 4.269 <I>M<SUP>t</SUP><SUB>s </SUB></I>= log (<I>A/T</I>) <I><SUB>max </SUB></I>+ <SUP>1</SUP>/<SUB>3 </SUB>log (Δ) + <SUP>1</SUP>/<SUB>2 </SUB>log (sin Δ) + 0.0046Δ + 5.370. For <I>M<SUP>e</SUP><SUB>s</SUB></I> the conventional logarithmic dependence of the distance correction is retained, while for <I>M<SUP>t</SUP><SUB>s</SUB></I> the theoretically-known relationship for the dispersion and geometrical spreading contributions is exploited. Comparison of these formulae with other work confirms the inadequacy of the distance-dependence term in the Gutenberg (1945) and Prague formulae. The <I>M<SUP>e</SUP><SUB>s</SUB></I> formula, as well as that of Herak and Herak (1993), give less bias at all epicentral distances to within the scatter of the observed dataset. <I>M<SUP>t</SUP><SUB>s</SUB></I> provides an improved overall distance correction, especially beyond Δ=145°.

This dissertation addresses the problem of inverting two-dimensional seismic data to determine the compressional wave velocity as a function of the spatial position in the medium. An automatic procedure for velocity estimation is set up using a bi-dimensional cross-correlation function to compare the modelled seismograms with the data. The standard wave-equation traveltime inversion algorithm treats each recorded seismic trace independently, and ignores the offset-dependence of reflected and diffracted arrivals in an ensemble of traces in a shot gather. My new inversion method, wave-equation traveltime-<I>offset</I> inversion, focuses on reflected or diffracted waveforms and introduces the bi-dimensional (offset-time) cross-correlation function to tackle this offset dependence. The velocity model is updated using the current velocity model, the observed pressure field, and the fields computed by reverse time propagation of two pseudo-residual functions acting as sources in a particular location. The wave-equation traveltime-offset inversion reconstructs the low frequency content of the velocity model when transmitted arrivals are used. Combined with the full waveform inversion, it succeeds in inverting a synthetic fault model in the crosshole configuration. The proposed method also succeeds in inverting surface reflection datasets while the standard traveltime inversion fails. By taking into account the moveout between traces, the convergence is more stable than with the conventional method. When inverting datasets with surface reflection geometries, the dominant event and the velocity above it are recovered using the traveltime and offset information. The remaining interfaces are defined as well. However, as there is a gap in resolution between the long and short wavelengths, the blocky variations in the velocity model are not observed.

This thesis considers anisotropy of seismic wave propagation at two oil fields in the North Caucasus region of Russia. In both oil fields, the reservoir zone displays a strong lateral variation in productivity which is thought to be caused by variations in fracture intensity. Such fractures may cause azimuthal anisotropy which can be detected in Vertical Seismic Profiles (VSPs) and Walkaway VSPs. The main aim of the thesis is to characterise this azimuthal anisotropy at three of the oil wells in these fields and to compare this anisotropy with productivity. At each of the three wells, I determine azimuthal anisotropy from VSPs by the application of techniques for estimating shear-wave splitting. I find that the polarisation direction of the fast shear-wave at all three wells is aligned approximately NNE-SSW. At two of the wells, forward modelling shows that the shear-wave splitting parameters in the top 1km can be closely matched by a model containing aligned, vertical fractures, striking NNE-SSW, in approximately the top 1 km. I am unable to resolve the anisotropy of the reservoir zone at these two wells. At the third well, strong azimuthal anisotropy of the reservoir zone is indicated by a large decrease of time delay between shear-waves propagating along vertical raypaths. This decrease is interpreted as an orthogonal rotation of the fast shear-wave polarisation direction at a depth just above the reservoir zone. Using forward modelling, I successfully match these observations with three different fractured reservoir models: the first model contains vertical fractures striking orthogonal to the presumed maximum horizontal stress direction; the second model has dipping fractures striking parallel to the maximum horizontal stress direction; and the third model has a distribution of fractures with a high internal pore-fluid pressure. Consideration of only vertical raypaths through the reservoir cannot discriminate between these models. However, modelling of non-vertical propagation from far-offset VSPs suggests that the dipping fracture model is the better model, although the lack of observations above the reservoir at this well means that other interpretations cannot be excluded.

In the first part of this thesis we introduce a new numerical method that combines a numerical method with an analytical method. We model the seismic wave propagation in fractured rock using the pseudospectral method. The fractures are treated as planes of weakness using the concept of the linear slip deformation or displacement discontinuity model. The implementation of fractures with a vanishing width in the finite difference grid is done using an equivalent medium theory. The objective is to investigate the effects of lengthscale (size) and spatial distributions of fractures on the characteristics of propagating waves. We demonstrate that the waveforms can be significantly affected by the presence of fractures with different lengthscales reltive to the wavelength, and we also show that different spatial distributions of fractures can give characteristic features on the wavefields, implying that information about fracture distributions in natural rock may be obtained directly from seismic data. In the second part of the thesis, we deal exclusively with scattering attenuation. Synthetic modelling studies with and without intrinsic attenuation show that the contribution of scattering attenuation is significant. Scattering involves no energy loss, but produces a more extended, lower amplitude wavetrain by the resulting interference. It is dependent on the nature of small-scale fluctuations in the earth parameters and is found to be frequency dependent. For the numerical simulation, we use the method introduced in the thesis that can accurately model the effects of scattering. The various fracture patterns examined are patterns of development of a population of fractures involving nucleation, growth, branching, interaction and coalescence created by a multiscale cellular automaton model. The objective is to examine the behaviour of scattering attenuation at different fracture patterns characterised by different statistical properties, fracture population geometry and criticality. We examine scattering attenuation in a range of frequencies for each one of the fracture patterns and demonstrate the frequency dependence. The comparison of the pattern of scattering attenuation with frequency between different fracture patterns shows that there is a change that can be attributed to the changes in the statistical properties of the fracture population. We conclude by examining the existence of direct links between fracture properties and scattering attenuation patterns, which can be used for the characterisation of fractured reservoirs.

Interest in converted waves (<i>C</i>-waves) has been growing significantly in recent yeas due to the advent of four-component (4C) ocean-bottom-cable (OBC) seismic recordings. This has changed the way geophysicists obtain fluid and lithology information about hydrocarbon reservoirs through joint <i>P-</i> and converted-wave analysis. Since 4C OBC surveys use conventional air-gun sources, which generate <i>P</i>-waves only, the shear-waves recorded by the 4C sensors on the ocean bottom are mode-converted shear-waves. The main focus of this thesis is to find ways to process and understand these mode-converted shear-waves in the presence of seismic anisotropy which is common in marine sediments. To this end, I examine and model the data characteristics of 4C seismic data, review the basic theory of converted-wave processing, develop new kinematic theories for converted-waves propagating in anisotropic, inhomogeneous media, and apply these new methods to field 4C data. I focus on two types of anisotropy: transverse isotropy with either a vertical (TIV) or horizontal (TIH) axis of symmetry. As an emerging technology, the characteristics of 4C seismic data have not been fully understood, and there are many acquisition related problems yet to be solved. The characteristics of 4C seafloor data have been studied by field data analysis and by synthetic modelling. I have found: 1) the water-column reverberations in the vertical geophone are much weaker than those in the hydrophone, because of the different sensor responses to the source- and receiver-side multiples; 2) the presence of a low shear-wave velocity gradient in the seabed prohibits <i>P</i>-to-<i>S</i> conversion, and this implies that most shear-waves recorded in 4C data are converted at deep reflector; 3) due to current sensor design, there is a shear-wave energy leakage from the inline horizontal geophone to the vertical geophone, resulting in geophone coupling problem. I have also studied the problem of sensor orientation and presented geophone orientation algorithms for both gimballed and non-gimballed geophone systems.