A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy / Inverse theory provides an important tool that the geophysicist can use to explore the structure of the Earth. This thesis examines several new approaches to the inverse problem, and suggests ways of improving the conventional least-squares technique. Non least-squares inversion was applied to borehole temperature data from South Africa, and when the norm of the inversion was controlled by the statistics of the misfit, It reduced by over 50% the number of iterations required for the inversion to converge upon a solution. Various damping schemes were also examined, and the use of the misfit in controlling the damping is shown to provide the best solution of those studied (Cooper and Jones, in press). Improvements to the efficiency of the inverse process were also achieved by the fitting of parabolic forms to portions of the misfit surface, using both the misfit value and the gradient of the surface. for gravity data. The presence of nearby minima other than the one that the inversion has just converged to can also be detected in this manner. The set of initial models that converged to a particular solution using leastsquares inversion was studied for magnetic data, and it was noted to have a fractal nature. The fractal dimension of the set was found to be inversely proportional to the damping of the inverse problem. The inverse process was pushed into a chaotic state by the modification of the least-squares inversion equation. The chaotic state was studied, and exploited to / AC2017

The need to monitor geophysical properties of first year ice (FYI) in the Arctic is increasing as this type of sea ice becomes more prevalent. One such method of monitoring the Arctic is the use of electromagnetic remote sensing techniques. These methods determine dielectric properties of the illuminated sea ice by interpreting how the electromagnetic waves interact within the medium. In the literature, there are empirical formulas relating these dielectric properties to the geophysical properties of the sea ice. The contributions of this research are the development and testing of a surface based active microwave remote sensor to monitor sea ice growth in the winter through the reconstruction of the time series complex permittivity profile of FYI. / October 2017

Thesis (M. Sc.)--University of Alberta, 2009. / Title from PDF file main screen (viewed on Feb. 19, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science in Geophysics, Department of Physics, University of Alberta. Includes bibliographical references.

Thesis (M.S. in Physical Oceanography and Meteorology)--Naval Postgraduate School, Dec. 2004. / Thesis Advisor(s): Ching-Sang Chiu. Includes bibliographical references (p. 33). Also available online.

Knowledge of the geoacoustic properties of the ocean bottom is essential for accurate modeling of acoustic propagation in shallow-water environments. Estimates of these properties can be obtained through geoacoustic inversion. Among the various inversion methods, the ones based on matched-field processing (MFP) have been increasingly used due to their relatively easy implementation and their good performance. In matched-field inversion (MFI), the objective is to maximize the match between the measured acoustic pressure field and the modeled field calculated for trial sets of geoacoustic parameters characterizing the environment. This thesis investigates the technique of matched-field tomographic inversion, a recent application of MFI that takes advantage of a multiple array-multiple source configuration to estimate range-dependent geoacoustic parameters. A two-stage inversion method based on the ray approach adopted to calculate the modeled pressure fields is developed to increase the efficiency of the estimation. The first stage consists of matching measured and modeled amplitudes of waterborne rays propagating between each source-array pair to estimate the parameters at the seafloor. The second stage consists of matching measured and replica pressure fields corresponding to rays that penetrate the sediment to estimate deeper parameters. In the first stage, the match is quantified using a least-squares function whereas in the second stage the robust pairwise processor is used. Both stages use a simplex genetic algorithm to guide the search over the parameter space. The inversion method is first applied to the two-dimensional (2-D) problem of vertical-slice tomography where four sets (2 sources x 2 vertical line arrays) of multi-tone pressure fields are used to estimate the depth and range variations of geoacoustic parameters. The method is validated via simulation studies that show its good performance in the ideal case where every model parameter except the ones to be estimated are exactly known, and quantify its limitations in non-ideal cases where noise in the data or errors in the array positions are present. The inversion results show that the parameters to which the pressure field is the most sensitive are well estimated for signal-to-noise ratios greater than or equal to 5 dB or for array position uncertainties less than two wavelengths of the source wavelet. The inversion method is then applied to a 3-D environment problem. From the different array configurations studied, it is found that the accuracy of the parameter estimates increases with decreasing propagation range. Finally, the method is applied to experimental data for a vertical-slice configuration. The relatively poor match obtained between the replica and measured data is attributed to the large uncertainty in the array position and the simplistic parameterization of the environment. / Graduate

The problem of fitting a material property of the earth to a certain model by analysing a returned seismic signal is investigated here. Analysis proceeds with methods taken from the theory of inverse problems. Seismic wave inversion is tack- led by minimisation of the objective function with respect to the model parameters. Absorbing boundary conditions are implemented using an exponentially decaying ansatz. / Physics / Ph. D. (Physics)

Graduation date: 2000

Thesis (M.Sc.)--Memorial University of Newfoundland, 2007. / Typescript. Includes bibliographical references (leaves162-165). Also available online.

The problem of fitting a material property of the earth to a certain model by analysing a returned seismic signal is investigated here. Analysis proceeds with methods taken from the theory of inverse problems. Seismic wave inversion is tack- led by minimisation of the objective function with respect to the model parameters. Absorbing boundary conditions are implemented using an exponentially decaying ansatz. / Physics / Ph. D. (Physics)

Approved for public release, distribution is unlimited / Linear sound sweeps from a towed J15-3 sound source were collected at a moored VLA hydrophone array in the South China Sea during the ASIAEX experiment in May 2001. Measured signals were filtered and pulse compressed. The processed data showed a high signal to noise ratio. Given an a priori chirp sonar survey, a two layer bottom "first guess" model was constructed. A broadband coupled-mode model was used to perform an exhaustive frequency variant sensitivity study of VLA pressures to changes in bottom properties as a basis for the geoacoustic inverse problem. Study results provided information on the observability of the various geoacoustic parameters and a procedure for the inversion. Matched field processing of the VLA data, using the same coupledmode model, was then performed to calculate ambiguity diagrams from which geoacoustic parameter estimates were obtained. Since VLA pressure fields were not sensitive to changes in the sediment attenuation coefficient, a matched field technique that correlated the slope of modeled transmission loss to the negative slope of 10log of the observed energy was performed in order to obtain estimates of the attenuation. These estimates showed a frequency dependent attenuation coefficient in the 50-600Hz frequency band. / Lieutenant, United States Navy