Forward and Inverse Models of Electromagnetic Scattering from Layered Media with Rough Interfaces.

This work addresses the problem of electromagnetic scattering from layered dielectric structures with rough boundaries and the associated inverse problem of retrieving the subsurface parameters of the structure using the scattered field. To this end, a forward scattering model based on the Small Perturbation Method (SPM) is developed to calculate the first-order spectral-domain bistatic scattering coefficients of a two-layer rough surface structure. SPM requires the boundaries to be slightly rough compared to the wavelength, but to understand the range of applicability of this method in scattering from two-layer rough surfaces, its region of validity is investigated by comparing its output with that of a first principle solver that does not impose roughness restrictions. The Method of Moments (MoM) is used for this purpose. Finally, for retrieval of the model parameters of the layered structure using scattered field, an inversion scheme based on the Simulated Annealing method is investigated and a strategy is proposed to address convergence to local minimum.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64752/1/alirezat_1.pd

Retrieval of Parameters for Layered non-Smooth Interface Media: Theory and Experiment.

Many naturally occurring or manmade objects can be modeled as three layer media with non-smooth interfaces. Most of the existing forward and inverse scattering models that can be applied to such media are either too inefficient or have limited regions of validity. In this dissertation an efficient forward scattering model based on the Extended Boundary Condition Method is developed for a three layer medium. The boundary between the first and the second layers is periodic while the boundary between the second and third layers is rough. The model is then extended by including an arbitrarily shaped cylinder placed into the third layer. Both TM and TE polarizations and PEC and Dielectric cylinder cases are considered. The Method of Moments (MOM) is used to obtain an impedance matrix, which is then transformed into a T-matrix. The T-matrix is transformed into a scattering matrix and cascaded with scattering matrices for the periodic and rough interfaces to obtain a generalized scattering matrix for the total system. A solution to the inverse problem for a three-layer medium is developed using simulated radar data. The retrieval of the layered- medium parameters is accomplished by sequential nonlinear optimizaxiii tion starting from the top layer and progressively characterizing the layers below. The optimization process is achieved by an efficient iterative technique built around the solution of the forward scattering problem. To be efficiently utilized in the inverse problem, the forward scattering model is simulated over a wide range of unknowns to obtain a complete set of subspace-based equivalent closed-form models that relate radar backscattering coefficients to the sought-for parameters, including the dielectric constants of each layer and the thickness of the middle layer. The inversion algorithm is implemented as a modified conjugate-gradient-based nonlinear optimization. It is shown that this technique results in accurate retrieval of surface and subsurface parameters, even in the presence of noise. To validate forward and inverse scattering models, a compact tower-based radar system is built. The data collected with the instrument is used to demonstrate sensitivity of radar measurements to changes in soil moisture and the potential for estimating surface and subsurface parameters.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89647/1/yuriy_1.pd

CRS-stack-based seismic reflection imaging for land data in time and depth domains

Land data acquisition often suffers from rough top-surface topography and complicated near-surface conditions. The resulting poor data quality makes conventional data processing very difficult. Under such circumstances, where simple model assumptions may fail, it is of particular importance to extract as much information as possible directly from the measured data. Fortunately, the ongoing increase in available computing power makes advanced data-driven imaging approaches feasible; thus, these have increasingly gained in relevance during the past few years. The common-reflection-surface (CRS) stack, a generalized high-density velocity analysis and stacking process, is one of these promising methods. It is applied in a non-interactive manner and provides, besides an improved zero-offset simulation, an entire set of physically interpretable stacking parameters that include and complement the conventional stacking velocity. For every zero-offset sample, these so-called kinematic wavefield attributes are obtained as a by-product of the data-driven stacking process. As will be shown, they can be applied both to improve the stack itself and to support subsequent processing steps...thesi

Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: implications for its evolution and recent reactivation

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on 'growing' the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years