No description available.

This work is an analysis of Rayleigh waves with frequencies of around 1 to 3 Hz, observed from four explosions fired in Scotland during the large scale refraction project LISPB. Time domain measurements are made from reduced tra.vel time sections showing recordings from the linear LISPB array in northern Scotland and for isolated paths around the LOWNET network in the Midland Valley and to the EKA array in the Southern Uplands. These show a division of the country into seven separate provinces with boundaries marked by sharp velocity changes and sudden attenuation, co-incident with features of the mapped surface geology. In the frequency domain, group and phase arrival times and amplitude are obtained for each Rayleigh wave recorded. After least squares analysis, phase and group slowness (s (f), s (f)) are c u obtained for each of the seven provinces and the specific dissipation factor (Q~1(f)) for five. s (f) ranges from 0.329 to 0.610 s/km and generally increases c with frequency, consistent with s (f) which is always larger. u ranges from 0.015 to 0.050. These data are inverted using both linear and Hedgehog methods to obtain shear wave velocity (~) and the shear wave dissipation factor (Qp1) in layered models for the upper 2 km of the crust in each province. generally increases with depth, due to compaction and near surface weathering. It also increases with geological age, which ranges from the Carboniferous of the Midland Valley to the Moine of northern Scotland. A low velocity zone beneath the Ochil Hills indicates that Devonian lavas overlie Old Red Sandstone whichoutcrops further north. Q~ generally increases with depth also. Introducing a Q~1(f) which varies as f-O.5 makes no marked change to the models, with a tendency for larger values at shallow depth and smaller values deeper down

No description available.

No description available.

No description available.

This thesis is concerned with the statistical studies of microwave signals received from beyond the radio horizon. A measurement campaign has been conducted in collaboration with Rutherford Appleton Laboratories and CNET (§) in France in order to collect propagation data across the English channel. The transmitter sites were located at Lannion and Cap d'Antifer in the North French coast and the receiver sites at HighcIiffe and Portsmouth in the South of England. Several original findings have been obtained by the research. For the two types of path considered of 155 and 250 km respectively, the cumulative distributions of transmission loss level L display two separate regions; one represents the troposcatter region and the other one represents the anomalous region. The separation between the two regions occurs between 10 and 20 dB below the free space path loss irrespective of the type of path considered. The two regions appear to be well modelled by two distinct normal distributions with their particular mean and standard deviation. The cumulative distributions of the transmission loss derived from measured data were also compared with prediction models available from Ii tera ture. The duration D of exceedance or enhancement above signal level threshold was investigated in terms of the joint statistics of transmission loss and duration of exceedances Prob (D' ~ DIL' s L). Two different distributions were analysed and modelled; the number distribution Pn and the time distribution PI. P,dDIL) represents the fractional number of cases where durations have been greater than D once the path loss is less than L, whereas PdDIL) 5ives the relative time with path loss less than L due to all durations greater than or equal to D. It is found that both Pn and PI are well modelled by log-normal distributions for all types of propagation conditions and these distributions are very skewed. For PI the (§) Centre Nationale D'Etudes des Tclecomunications, Paris and Lannion Laboratories mode computed can range from fractions to several minutes whereas the median can range from many tens of minutes towards the troposcatter levels down to a few minutes close to the free space loss threshold of L = 0 dBf. The logarithm of the median duration of Pt was found to be linearly related to L (dBt). As regards the standard deviation, an (lnD) and at(lnD) are very close in agreement with the theory and range from 1 to 4 when D is in minutes. They were also found to be linearly related to L (dBD. The research has shown that there is a strong seasonal dependence of the path loss and the monthly attenuation may vary over a wide range throughout the year, with variation being greatest on the longer path. The research has also shown that during a 24 hours period there is, in general, a rise in the probability of exceeding a given level at around 1000 GMT and this probability reaches its peak at around 1900 GMT to then begin to decrease at about 2000 GMT. This pattern is much more pronounced in the summer months than in the winter months. The daily variation is also more pronounced for the shorter paths than for the longer paths. The time between 11 no to 1900 GMT appears to be the strongest period of potential interference. The research has included a preliminary analysis of the correlation between signal level strength and the local surface meteorological parameters: temperature, water vapour pressure and atmospheric pressure. As expected good correlation with temperature was observed particularly with weekly averaged maximum daily values. However; atmospheric pressure is the most dominant parameter on signal enhancements. Finally, short term fading of clear air troposcatter signals was investigated and the power spectral density analyses show significant components up to about 5 Hz.

This thesis describes research into multipath frequency-selective fading over a wide bandwidth (100's of MHz). In particular it focuses on the use of wideband fade measurements f0r the study of the underlying multipath propagation. The early chapters are concerned with a survey of the 0verall topic of multipath and of the physical processes involved, including the geometrical optics model of propagation. The thesis then discusses wideband channel measurement techniques such as the popular frequencysweep method. It also describes in some detail the "wideband experimental system" developed by Portsmouth Polytechnic and Rutherford Appleton Laboratory. This system uses a PRES cross-correlation approach rather than a frequency-sweep, and it is the equipment that has been used in this research. The central interest of the research is the estimation of multipath "ray" delays and amplitudes using wideband fade measurements. Because of its inherent limitations of resolution and sidelobes, the Fourier transform is rejected in favour of modern "parametric" spectral methods. These are discussed, and one in particular, the modified Prony algorithm, is selected. It is applied to a database of 250 MHzbandwidth measurements made in the laboratory using the wideband system and a "multipath simulator" network to simulate real operational situations. The results obtained demonstrate the algorithm's ability to resolve rays closer than the Fourier limit and to assign amplitudes which enable accurate reconstruction of the original measurement. Statistical analysis of the results also reveals a de~~ee of systematic bias in the delay and amplitude estimators. As well as the laboratory measurements, the algorithm is applied to a selection of "in-the-field" multipath measurements obtained in the UK and in France. These illustrate the applicability of the method and they also highlight certain limitations. Appendices to the main thesis include a study on the use of a simple 2-ray propagation model to facilitate the interpretation of the delays and amplitudes obtained. There is also a detailed mathematical analysis of the PRBS technique as used in the wideband system which indicates the system-dependent nature of the transfer-function estimates generated.

No description available.

No description available.

No description available.