An experimental study has been made of the propagation of a low-amplitude mechanical impulse in a granular medium. The response of granular matter to an applied impulse is primarily governed by the behaviour of the contacts between its constituent particles. The contact behaviour depends on the existing static load on the bed, on the history of previous loading, and on the characteristics of the applied impulse. A procedure has been developed to enable beds with a consistent and reproducible initial state to be deposited and prepared, thus permitting a certain isolation of the effects of loading history. Measurements of the variation of velocity of propagation and attenuation of transmitted impulses with applied static load, impulse duration and intensity, and distance of propagation, have been made on beds of four size fractions of dry sub-angular sand, and comparative experiments performed on beds of common salt and silicon carbide. The velocity of propagation of the impulse is found to vary with the static load raised to some power. The experimental values of this exponent are close to the one-sixth power dependence predicted by Hertz contact theory, but the results show systematic departures from this value related to the size fraction of material. The magnitude of the received impulse is found to decay exponentially with distance, at constant static load, yielding a distance attenuation factor. The magnitude and direction of the variation of this attenuation factor with static load is observed to depend in an involved manner on the particle size fraction and the impulse duration. Literature relating to the deposition and packing of granular media, the effects of applied stress and vibration, wave propagation, and theoretical analyses of the mechanics of granular matter, has been reviewed. The contact theory of Hertz, as developed in particular by Mindlin, forms the basis of an analysis of the observed phenomena.

The aeromagnetic map of Great Britain shows a number of granites associated with magnetic anomalies. One of the largest of these granites, the Cairngorm pluton, has a complex overlying aeromagnetic anomaly, yet measurements show that the surface rocks have a magnetic susceptibility too low to account for this aeromagnetic field. The purpose of this investigation was to determine the likely reasons for this discrepancy employing a range of computer based techniques, and using new land-based data. Potential field data for the Cairngorm area, together with a region of the Southern Uplands, was subjected to frequency domain reduction and filtering. Hand specimens from the granite and host rock were collected for susceptibility measurements. The results have been integrated with profile models to explain the variations observed in the aeromagnetic and ground based magnetic field. The Cairngorm granites appear to be zoned at depth, with a variable thickness cap of low density, low magnetic susceptibility granite. The presence of intermediate composition rock in the upper reaches of the granite mass provides evidence for stoping, whilst the presence of intermediate composition rock at deeper levels is thought to be due to multiple magma pulses and/or differentiation. The Loch Doon granite has a low magnetic susceptibility subsurface core extending off its eastern flank. A number of correlations between the geophysics and geochemistry of the granites investigated were noted. The concept of gravrnag space has been developed and applied as a framework in which to explore the relationships between the granites of Scotland and northern England. Relationships between the background geophysics and the geochemistry of granites in the area of the proposed Iapetus Subduction zone have also been investigated.

Gravity techniques have been used firstly to determine the sub-surface structure of the Guayabo caldera together with the related part of the Guanacaste Arc, and secondly to study the temporal evolution of the Miravalles Geothermal Field. Detailed density and stratigraphy studies of both the drilled and exposed geology have both complemented and constrained the caldera gravity models. A new caldera volume - erupted mass balance model is proposed that computes the buried, intra-caldera mass and volume parameters in terms of the mass deficiency commonly associated with silicic calderas. The 10 km diameter topographic expression of the Guayabo caldera comprises nested collapse structures, the boundaries of which are defined within the caldera by distinct gravity discontinuities. The modelled structure shows that the Guayabo caldera formed by multistage collapse across discrete sub-parallel sets of ring faults, rather than by chaotic collapse. The resultant overall structure resembles a funnel caldera, and it is possible that similar structures may be present but remain undetected in comparable calderas. The 200- 400 m collapse across the faulted topographic margins is far less than the previous estimate of 2400 m, where the entire drilled stratigraphy was thought to comprise the fill to the caldera. Most of the stratigraphy drilled on the eastern margin of the caldera pre-dates the formation of the caldera, contrary to the previous theories. Furthermore, the caldera volume far exceeds the volume of the Guayabo tuffs and post-tuff subsidence, possibly related to the build-up of the recent Miravalles stratoconeso n the caldera margin, is required to explain the imbalance. An important result of the revised caldera model is that the geothermal reservoir host rocks continue beneath the caldera margin, and therefore the volume of the reservoir may be greater than previous estimates. The 35 mGal gravity couple over the inner margin of the Nicaraguan depression is modelled as the near-surface outcrop of the Cretaceous ophiolitic basement, previously thought to be present at several km depth beneath the Guanacaste arc. Gravity data show that the extensive Plio-Pleistocene silicic pyroclastics of Guanacaste do not have detectable deepseated low-density plutonic equivalents. Repeat gravity studies at Miravalles identified 40 tGal fluctuations on a monthly time-scale, due to groundwater movement in the Guayabo caldera rather than geothermal-related sources. Experimental data have shown that LaCoste and Romberg gravimeters record repeatable gravity decreases of up to 700 pGal when subjected to vibration at specific harmonic frequencies. This accounts for the gravity decreases observed during well testing in 1988, that could not be explained by reasonable mass changes. Talring these points into consideration, a repeat gravity monitoring program was designed to both test and complement the future reservoir simulation models.

No description available.

TEM sounding curves are distorted by geological noise in complex environments and often cannot be interpreted using practical 1D interpretation schemes which assume homogeneous, horizontal strata. Systematic experiments with a Geonics EM42 and MarkI and MarkII SIROTEM systems in complex environments show that geological noise is dominant over system noise and errors in loop receiver geometry which seldom exceed 2% of the measured signal. Inversion results in complex areas depend on the loop configuration and type of receiver, unlike those from 1D situations. Small loops are less susceptible to geological noise and have a better vertical resolution than large loops which, however, do have greater depths of penetration. When carrying out soundings along profiles it is important to match the station spacing and loop size to the scale of lateral discontinuities. Data from four geologically complex areas in Cyprus, Spain, Northern Ireland and England have been collected and interpreted to investigate the applicability of using TEM depth soundings in such environments. The results have shown than in some areas, soundings can provide useful quantitative information on depths and resistivities. Even in extremely noisy areas the method can be used to locate anomalies for follow up work with other geophysical methods. Investigations of various interpretation schemes including pseudosections, time slices and 1D inversions have demonstrated limitations in complex environments. A new interpretation scheme based on a direct calculation of approximate depths from an apparent resistivity curve has been developed which removes some of the ambiguities due to the resistivity dependent velocity of the EM waves. A general purpose TEM reduction and interpretation program has been written to carry out apparent resistivity and direct depth calculations, forward modelling and 1D inversions. Tests on synthetic and field data show a good agreement with sections produced from inversions and borehole data.

No description available.

Experiments have been performed at Troms?, Norway and Arecibo, Puerto Rico to investigate the effects produced by high power radio waves (heating) on the high and low latitude ionospheres, respectively. The modifications induced in the F-region have been observed with both incoherent scatter radars and tow power HF diagnostic radio waves. These observations provide information on both heater-induced large scale changes, and small scale density structures. The spatial and temporal development of the perturbation in electron temperature caused by heating, measured at Troms? for the first time, compares well with theory. Observations of thermal striations, which may be the result of the thermal self-focussing of the heater wave, are also presented. The large-scale heater-induced changes in the electron density measured by EISCAT have been compared with those derived from measurements of the phase of reflected HF diagnostic signals. Both methods yield consistent results with regard to whether the electron density is enhanced or depleted but there are, however, differences in the magnitudes of the measured changes. The sign of the perturbation can reveal aeronomical information on the chemical and diffusion characteristics of the unheated ionosphere. The first observations of tow latitude heating by means of low power HF diagnostic waves are presented. The anomalous absorption measured at tow latitudes is less than that at high latitudes, in agreement with theory. The scale length and amplitude of the associated small-scale density structures are comparable at both high and low latitudes. A detailed comparison has been undertaken of the effect of heating at high and low latitudes and the results compared with current theoretical models. The two independent diagnostic techniques (incoherent scatter and low power HF waves) provide a consistent picture of the ionospheric perturbation induced by both heating facilities.

The geomagnetic field is found to change quite frequently on a world-wide scale. In the three months' period near sunspot maximum, such changes are found on 90 "per cent of all days. Most of these changes are not registered either as sudden commencements or as sudden impulses, and are tentatively called in this thesis 'worldwide changes'. The frequent occurrence of world-wide changes seems to be consistent with the idea that world-wide features of the geomagnetic field are always influenced by a permanently flowing corpuscular stream from the sun. The physical state of the corpuscular stream may be as variable as that of the solar atmosphere, and sudden changes in it will give rise to sudden, world-wide changes in the geomagnetic field. The morphology of world-wide changes is studied, and the form of the change, the distribution of magnitude and the mode of spreading over the earth are clarified. It is found that world-wide changes can be classified into two groups according to the sign of the main part of the change which appears all over the world. Those with an increase in the total force are called positive changes and those with a decrease are called negative changes. Except for the sign of the change, negative changes are morphologically identicalto positive changes. Since the morphology of sudden commencements and sudden impulses is the same as that of world-wide changes, they must be produced by a common mechanism, and an explanation of negative changes is a new, fundamental requirement imposed upon any theory of these changes. The observed change in the geomagnetic field may originate at the magnetospheric boundary where the solar corpuscular stream interacts with the geomagnetic field. The change may be modulated by the screening effect of the ionosphere before it is observed at ground level. Although this effect has been shown to be negligible for changes with a time scale of the order of world-wide changes, incorrect assumptions have been made in existing theories. More accurate calculations show that this effect is actually significant for a certain mode of the incident field. [ ... ] / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Geomagnetic micropulsations are described and possible origins discussed. Previous work in Canada is reviewed with particular attention to the normal daytime Pc oscillations with periods from 10 - 30 seconds. A description is given of the work done correlating the amplitude and direction of the exciting vector at two stations. Field work at Ralston, Alberta during July -August 1959 is described in detail. A digital computer is used to obtain the auto-correlation coefficient and the covariance of the horizontal components of the vector. From these two paramenters the dominant frequency, amplitude, and polorization of the vector are obtained. The conclusion is reached that normal daytime Pc's seem to occur in a small band centred around a dominant frequency. Two such dominant frequencies may occur simultaneously and the vectors can have different polarization. The amplitude and direction of a given Pc seems to follow a closed diurnal pattern reaching a maximum slightly before local noon. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Over the years, the study of the aurora has drawn on many different aspects of physics and geophysics in an attempt to explain this common phenomenon. Two prominent magnetic effects which are associated with auroral displays are geomagnetic bays and Pi 2 micropulsations. Both these geomagnetic perturbations attain a maximum amplitude in the auroral zone, but may also be seen in mid and low latitude regions. A bay may be described as the magnetic field of a westward flowing current in the auroral zone coupled with an eastward flowing return current in the mid-latitude and polar cap regions. The means by which Pi 2 micropulsations propagate from the auroral zone centre of activity to mid and low latitude field points is not as yet known. To study the propagation characteristics of Pi 2 micropulsations, a set of five fluxgate magnetometer units was set up stretching across the continent from Victoria, B.C. to Montreal, P.Q. with all stations lying on approximately geomagnetic latitude 56°N. Continuous recordings of H, D and a filtered H component (bandpass 25 - 200 sec.) were made from June 1, 1965 till Aug. 15, 1965. Further information on pertinent disturbances was obtained from stations distributed over the continents of North America and Antarctica. A theoretical study of some of the possible modes of propagation was carried out and specific characteristics of each mode were established in order to permit a check with the experimental data obtained. The pulsations of interest were digitized and power spectra as well as Fourier spectra were computed for the H and D components. The position in longitude of the source of the disturbance was determined making use of pulsation amplitudes, bay amplitudes, and the direction of the disturbance vectors in the horizontal plane. An intensive investigation of the polarization of the pulsations in the horizontal plane was carried out, making use of the complete chain of several cycles but drawing each cycle separately for comparison purposes. This analysis disagrees with previous claims that Pi 2 pulsations are polarized anticlockwise after local midnight and clockwise before local midnight. On the basis of the analysis performed with the data, it was found that the mode of propagation most likely responsible for the transfer of Pi 2 activity from high to mid latitudes could be described by a plane electromagnetic wave propagating approximately normal to the earth's magnetic field lines through the lower E-region of the ionosphere. It is found that the maximum period in a Pi 2 event increases with increasing Kp index in the range Kp ~0 + to 2o. This complements the earlier observation that the dominant period in a Pi 2 event decreases as the Kp index increases; this fact is also verified in the analysis of the experimental data. It is found that the number of peaks in the frequency spectrum of a Pi 2 event increases approximately linearly with increasing Kp . / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate