As a first deep geophysical survey, thirteen magnetotelluric (MT) and magnetovariation (MV) soundings in the period range of 0.0085 - 6000 s have been carried out in the Methana Peninsula and Trizina area (Greece) which form a part of the active Hellenic Volcanic Arc (HVA). The intensity of artificial disturbances in Methana prevented the acquisition of good quality data, and led to the exclusion of data in the 10-100 s range from further interpretation. The measured data are first processed using robust methods, then decomposed using the methods of Groom and Bailey and of Bahr. Thin sheet modelling follows with the aim of detecting effects due to the surrounding seas. All the data show anisotropy and were found to be distorted by galvanic distortion, facts which become more apparent at periods below 1 s. At the same period, effects from the sea also become important. The above problems indicated: (i) the insufficiency of 1-D modelling (ii) that the data are at least 2-D and (iii) that a careful consideration of all the above effects must be given in order to provide a meaningful electrical model. The analysis of distortion effects and the measured induction arrows enable the determination of a regional azimuth to which subsequently the data are rotated. Seven sites are used, and a 2-D model is proposed. The model indicates a zone of high conductivity (7-32 ohm m) beneath the centre of the peninsula at a depth of 1-5 km, which is interpreted as being connected with the volcanic history of Methana, as a remaining and cooling part of former magmatic activity. Although with the long period MT data it was not possible to locate a deep magma source connected with the indicated anomaly, induction arrows at T > 2400 s taken from the regional area point towards the South and South West, perpendicular to the strike of the well known deep structure of the Hellenic Volcanic Arc. Further attempts to integrate the results from Methana with other areas along the HVA lead to correlation of the regional azimuths with horizontal stress directions in the South Aegean sea.

This study involves the production of arrays of parallelepiped magnetite particles in the size range 0.1 - 1 μm. This was achieved by utilising electron beam lithography techniques which are often employed in the design of semi-conductor devices. These particles are required in order to understand the mechanism of reversal of magnetisation for pseudo-single-domain (PSD) particles important to paleomagnetic studies. The samples used by previous researchers are magnetite powders. In such samples, despite attempts to disperse the particles onto non-magnetic matrix, there is no way of eliminating particle clumping, and magnetostatic interaction. Low temperature magnetic measurements obtained using samples of cubic arrays of magnetite particles shows that the effect of particle clumping is to increase the amount of saturation isothermal remanence (SIRM) lost at the Verwey transition. The dependence of SIRM lost at the Verwey transition on particle size in the PSD size range, is consistent with the vortex domain structure predicted for unconstrained 3-D micromagnetic studies. The results shows that 'true' magnetic memory is a stress related phenomenon. Magnetic properties of cubic arrays of magnetite particles measured at room temperature are not consistent with the mechanism of magnetic reversal of coherent rotation of atomic magnetic moments, but are generally in good agreement with that of vortex nucleation and propagation in general. The method of domain classification using the coercivity ratio often used in rock magnetism, is shown to be not useful in classifying PSD in the submicron size range. Magnetic properties of rectangular parallelepiped magnetite particles are more complex than expected from the simple demagnetisation shape anisotropy contribution.

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This thesis examines shear-wave propagation in the Paris Basin and demonstrates that the sedimentary rocks must have, at least, an orthorhombic anisotropic symmetry. It is suggested that the most likely cause of such an orthorhombic symmetry is the combination of fine-layering and crack anisotropy. Observations from two near-offset VSPs indicate the large variation of shear-wave delay expected for propagation directions close to a point singularity, and constitute probably the first recognition of the effects of a singularity in observed data. However, lateral variations of crack density in the near-surface could also account for this observation and thus, without further geological information about the near-surface, it cannot definitely be concluded that the anomalous low delay at one VSP is due to singularity. A transition zone (close to a singularity) where there are rapid changes of shear-wave polarization and delay can account for the anomalous motions observed on the wide-offset VSPs.

Tide gauge observations suggest current rates of uplift of the land with respect to mean sea level of a few millimetres per year ranging from +1.6 at Lerwick to -3.2 at Southend. The measurement of absolute heights separates the land movements from changes in mean sea level. 10 mm of uplift causes a change in gravity of about 2 μgal (1 μgal = 10<SUP>-8</SUP> m s<SUP>-2</SUP>) so a precision of 5 μgal detects a differential rate of 5 mm per year in about 10 years. The British Precise Gravity Network 1993 (BPGN93) was established by the author to provide a datum from which future reobservation could identify vertical crustal movements. It consists of 58 sites spaced about 100 km apart over mainland Britain. They were observed with three specially calibrated LaCoste & Romberg gravity meters and controlled by observations measured with FG5 absolute gravity meters at Edinburgh, Birkenhead, Teddington and Taunton. Very high accuracy relative gravimetry depends on the calibration of the instrument and maintaining reproducible instrumental drift characteristics, correctly modelling environmental disturbances such as Earth tides and developing robust and appropriate statistical models. A search for periodic errors in the micrometer screw of the unique double-dial instrument D145 appeared to find a period of about 400 coarse dial turns, four times that expected from the gear ratios. The meter was balanced using the coarse screw at various settings of the fine screw. The sequence of readings every 10 minutes for up to 5 hours detected non-linear drift caused by changes in the direction of screw turning. Because a linear drift model was analysed using least-squares, this was initially interpreted as a periodic screw error. The 'spring hysteresis model' explains the anomalous result. Solid-Earth tides were calculated using the full Cartwright-Tayler-Edden expansion but other algorithms agree to better than 0.5 μgal. Inconsistent treatment of the static tide can create discrepancies of about 30 μgal for British latitudes. The ocean load tide is only important near the margins of continents, and in particular in SW England, but was implemented for all relative and absolute gravity data. This thesis describes improved analysis techniques for absolute gravimetry, with a detailed study of three corrections, for vertical gradient, datum height and system response, which were incorrectly treated in the manufacturer's software. The vertical gradient of gravity was measured at the eight absolute gravity sites locations in the UK where the absolute gravity has been observed to correct the new data. The FG5 absolute gravity meter measures the position and time of a freely falling corner-cube reflector using fringes produced by laser interferometry, and estimates gravity by fitting them to an equation of motion. Ignoring the vertical gradient of gravity in the equation of motion estimates gravity at the 'effective measurement height', but it is shown that theoretical predictions relating this position to the top of the drop are incorrect because of fringe counting errors. Thus the equation of motion used must include the vertical gradient of gravity, determined by relative gravimetry, so as to estimate gravity estimate at a definite height for comparison with other instruments.

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No description available.

The magnetotelluric method (MT) has been used to study the conductivity structure of the earth. The method uses the time variations of the natural magnetic and the induced electric fields measured simultaneously at the earth's surface. The demand for new MT instrumentation has increased in the past few years. New applications require more complex sensor configurations. Characteristics like size, weight and real time data processing have become increasingly important to improve logistics and mobility, although it is becoming more and more difficult to measure electromagnetic fields without strong contamination from <I>man-made</I> sources. The ability to process MT results in the field is essential for a quick evaluation of the site conditions, enabling the time required to obtain a suitable amount of good quality data to be reduced. SPAM Mk-III (Short-Period Automatic Magnetotelluric) developed at The Edinburgh University introduces new concepts in terms of geophysical instrumentation. The equipment is based on a family of Transputer devices widely used in parallel processing, incorporating the recent technologies and techniques. Its modular design allows flexible network configurations. A complete description of the equipment, design concepts, features and the precautions needed in designing low-noise instrumentation are presented in this thesis. The distinctive SPAM Mk-III capabilities of multi-channel data acquisition were used in two field experiments. In the first experiment the effect of the electric dipole length on the impedance tensor was investigated. Results confirmed that the apparent resistivity and phase are quite consistent and that longer electric dipoles tended to be more susceptible to local interference. In the second experiment the rotation property of the impedance tensor was studied. The experiment reinforces the assumption that for a layered earth (1-D), the impedance tensor is independent of measuring co-ordinates. New Cu-CuSO<SUB>4</SUB> electrodes were developed for the field work. These are to operate in the frequency range of the SPAM Mk-III (0.001 Hz to 2kHz). Several electrodes were tested and the results indicate that noise and stability does not depend on the internal resistance. Also the frequency response of these electrodes show they do not present any filter effects within the range of magnetotelluric application.

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After an extensive desk study and a number of trial surveys, electrical and electromagnetic techniques were chosen for the geophysical investigation of 2 ex-oil distribution terminals in West Granton, Edinburgh, which the City of Edinburgh Council aim to acquire for remediation and redevelopment. An electromagnetic survey of the site was conducted, using a Geonics EM31 conductivity meter, as part of a complete assessment of the surface pollution. A 2-D resistivity survey was later employed to image the geology beneath the site with the aim of locating potential contaminant pathways. The EM31 is directionally dependent and yields different results when its boom is either parallel or perpendicular to subsurface linear anomalies. Use of the second derivative of unidirectional measurements clarified the response of a labyrinth of underground pipes easing interpretation by geophysicists and non-geophysicists alike. The electromagnetic survey also successfully located at number of shallow oil plumes emanating from the ends of buried, broken pipes. The computer controlled 2-D resistivity imaging method provided rapid coverage of the Granton site returning a large number of 2-D apparent resistivity pseudosections. The subsurface pipe network had a significant effect on the resistivity measurements and masked the response of the background geology. In order to reconstruct the background resistivity distribution, the effect of a subsurface linear conductor was quantified using a known analytical solution. Pipes could then be located and their effects removed from apparent resistivity pseudosections. Contour plots (plan views) of the background resistivity distributions across the complete site were then constructed by extracting true resistivity data points from all modelled profiles at equivalent depths. These contour plots proved invaluable for the assessment of the integrity of the superficial clay coverage, for the location of faults and for resolving complex geology. The conclusions obtained from non-invasive geophysical surveying over the Granton site far exceed any interpretations that could be drawn from an invasive survey alone. As a result of the work, the Granton site remediation plans have been substantially redesigned and the benefits of geophysics have been promoted.