

@mastersthesis {7191,
	title = {Two-dimensional finite-difference time-domain full-waveform inversion of crosshole georadar data},
	year = {2007},
	note = {C829697},
	school = {Eidgenoessische Technische Hochschule Zuerich (Switzerland)},
	type = {phdPh.{D}.},
	address = {Ann Arbor},
	abstract = {Borehole georadar is an increasingly popular method for probing the shallow subsurface. A typical setup for a crosshole georadar experiment consists of an emitting dipole-type antenna located in a borehole and a corresponding receiver antenna located in a neighboring borehole. The nominal center frequencies of commonly used antennas range from \~{}20 to \~{}250MHz, which correspond to dominant wavelengths of \~{}5 to \~{}0.4m, in the subsurface. Prior to developing a full-waveform inversion algorithm it is therefore essential to improve our understanding of the complex electromagnetic wave propagation. To study the radiative properties of borehole georadar antenna systems, I developed a modeling tool based on finite-difference time-domain (FDTD) solutions of Maxwell{\textquoteright}s equations in cylindrical coordinates. To minimize reflections from the model boundaries, efficient uniaxial perfectly matched layer (UPLM) absorbing boundary conditions are implemented along the top, bottom and right model edges and symmetrical conditions along the cylindrical axis. Using an accurate local refinement technique allows to account for the detailed aspects of borehole radar systems, slim boreholes and materials with very high permittivities (e.g., water). Numerical experiments were conducted to benchmark the modeling tool and to analyze different antenna geometries. Results of these studies reveal that radiation characteristics of finite length Wu-King-type antennas correspond well with typically employed dipole-type approximations as long as they are placed in same environments. In contrast, substantial differences in the radiation characteristics are found when placing antennas in water-filled boreholes. I further studied effects of antennas placed in air- or water-filled boreholes on two real data sets acquired in dry crystalline rock (Grimsel Rock Laboratory in Switzerland) and in water-saturated sediments (Boise Hydrogeophysical Research Site in the USA). Both experiments using realistic transmitter antennas provide better agreements between observed and modeled data than simulations based on infinitesimal dipole transmitters and receivers. In my thesis, I present a full-waveform inversion scheme for crosshole georadar data based on FDTD solutions of Maxwell{\textquoteright}s equations in a 2-D Cartesian coordinate system. Using a Cartesian coordinate system rather than a cylindrical system offers more freedom with respect to placing transmitters and receivers in the model space and allows the implementation of highly accurate and fast generalized perfectly match layers as absorbing boundary conditions. My waveform inversion approach directly considers permittivity and conductivity distributions and has the potential to resolve not only the coarse but also the fine scale features of the probed subsurface. The scheme requires a stepped or cascaded inversion approach in which the first step consists of a permittivity inversion that mainly accounts for the phase information of the data, followed by a conductivity inversion, which focuses on the remaining amplitude information. Numerical experiments demonstrate that my 2-D inversion scheme is capable of adequately resolving, the locations, the shapes and the magnitudes of bodies with spatial extent considerably smaller than a dominant wavelength embedded in homogeneous background media. Similar good results are reported for band-limited Gaussian noise contaminated data. More realistic synthetic experiments consisting of layers and artificial objects (pipes and tunnels) embedded in stochastic background media suggested that dielectric permittivity distributions are only slightly improved compared to corresponding ray-based results. Generally, the waveform inversion of the electrical conductivity clearly resolves more details of the artificial objects. In order to invert field data, I developed a 3-D to 2-D transformation scheme that accurately corrects for (i) 3-D geometrical spreading, (ii) a π/4 phase shift, and (iii) a frequency scaling effect of 1/[Special characters omitted.] , where ω is the angular frequency. The unknown source wavelets are estimated with a scheme that accounts for permittivity and conductivity medium parameter distributions together with deconvolutions of the observed data. The same field data sets are used as for the initial study of the radiation characteristics of different antenna types. The resolution of all full-waveform tomograms is shown to be significantly higher than that of the relevant ray tomograms. Boundaries between distinct geological features and small heterogeneities are sharply imaged in the full-waveform tomograms. These studies demonstrate that waveform inversion is feasible and yields markedly more information about the probed subsurface. It thus may provide key constraints on engineering parameters, such as water content, salinity, porosity, clay fraction or ore grade. (Abstract shortened by UMI.)},
	keywords = {0373:Geophysics, Earth sciences, Full-waveform inversion, Geophysics, Georadar},
	url = {http://search.proquest.com/docview/304782035?accountid=14270},
	author = {Ernst, Jacques Robert}
}
