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I present new models of the elastic structure of the Pacific upper mantle that address the formation and evolution of oceanic plates. Using a surface-wave dispersion dataset, I perform anisotropic tomography to construct two-dimensional phase-velocity maps and three-dimensional velocity models of the Pacific basin. My three-dimensional elastic models describe both the radial and azimuthal anisotropy of seismic waves. In order to constrain these models, I develop regularization techniques that incorporate a priori information about the nature of the oceanic upper mantle, including both the age dependence of seismic velocities and the expected scaling relationships between azimuthal anisotropy parameters derived from realistic peridotite elastic tensors. I observe a strong cooling signal in the upper-mantle seismic velocities that is consistent with halfspace cooling of the lithospheric plate; deviations from this simple cooling signature are related to the influence of mantle plumes or other thermal alteration of the lithosphere. As plate age increases, the depth to the thermally controlled lithosphere-asthenosphere boundary increases as well. This thermal boundary, as seen in the negative gradient in seismic velocities, is consistent with the depth at which there is a transition in anisotropy fast-axis orientation. This change in anisotropy orientation is due to the transition from frozen-in lithospheric anisotropy to asthenospheric anisotropy that is related to geologically recent shear beneath the base of the plate. The anisotropy orientations and strength that we observe throughout the plate are only consistent with A-type olivine fabric. There are regions where anisotropy orientations do not align with paleospreading directions in the lithosphere or absolute-plate-motion in the asthenosphere, suggesting that small-scale convection, mantle flow, and plumes could all lead to changes in the orientation of seismic anisotropy. There is a dependence on the strength of anisotropy on spreading rate at shallow depths; this implies that corner flow at faster-spreading ridges is more effective at aligning olivine crystals in the direction of shear. I also present a new set of local surface-wave amplification maps spanning the contiguous United States. I perform a synthetic-tomography experiment in order to assess our ability to resolve variations in surface-wave amplification due to variations in local elastic structure. Local amplification derived from synthetic seismograms is very highly correlated with direct predictions of amplification, suggesting that we are able to resolve this signal well and that local amplification observations reflect elastic structure local to the station on which they are measured. Local amplification can be used as a complementary constraint to phase velocity in order to map upper-mantle elastic structure.

Ocean-bottom seismometers (OBS) allow wider angle recording and therefore, they have the potential to significantly enhance imaging of deep subsurface structures. Currently, conventional OBS data analysis still uses first arrival traveltime tomography and prestack Kirchhoff depth migration method. However, using first arrival traveltimes to build a velocity model has its limitations. In the Taiwan region, subduction and collision cause very complex subsurface structures and generate extensive basalt-like anomalies. Since the velocity beneath basalt-like anomalies is lower than that of high velocity anomalies, no first-arrival refractions for the target areas occur. Thus, conventional traveltime tomography is not accurate and amplitude constrained traveltime tomography can be dangerous. Here, a new first-arrival stochastic tomography method for automatic background velocity estimation is proposed. Our method uses the local beam semblance of each common-shot or common-receiver gathers instead of first-arrival picking. Both the ray parameter and traveltime information are utilized. The use of Very Fast Simulated Annealing (VFSA) method also allows for easier implementation of the uncertainty analysis. Synthetic and real data benchmark tests demonstrate that this new method is robust, efficient, and accurate. In addition, migrated images of low-fold data or data with limited observation geometry like OBS are often corrupted by migration aliasing. Incorporation of prestack instantaneous-slowness information into the imaging condition can significantly reduce migration artifacts and noise and improve the image quality in areas of poor illumination. Here I combine slowness information with Gaussian beam depth migration and implement a new slowness driven Gaussian beam prestack depth migration. The prestack instantaneous slowness information, denoted by ray parameter gathers p(x,t), is extracted from the original OBS or shot gathers using local slant stacking and subsequent localsemblance analysis. In migration, we propagate both the seismic energy and the principal instantaneous slowness information backward. At a specific image location, the beam summation is localized in the resolution-dependent Fresnel zone where the instantaneousslowness-information-related weights are used to control the beams. The effectiveness of the new method is illustrated using two synthetic data examples: a simple model and a more realistic complicated sub-basalt model. / text

A study was undertaken to improve the signal quality and the resolution of the velocity profile for deep downhole seismic testing. Deep downhole testing is defined in this research as measurements below 225 m (750 ft). The study demonstrated that current testing procedures can be improved to result in higher signal quality by customizing the excitation frequency of the vibrator to local site conditions of the vibrator-earth system. The earth condition beneath the base plate can be an important factor in the signal quality subject to variations with time when tests are repetitive. This work proposes a convenient method to measure the site localized natural frequency and damping ratio, and recommends using different excitation frequencies for P- and S-wave generation. Properly increasing the excitation duration of the source signal also contributes to the quality of the receiver signal. The source signature of sinusoidal vibratory source is identified. Conventional travel time analysis using vibratory source generally focuses on chirp sweeps. After testing with impulsive sources and chirp sweeps and comparing the results with the durational sinusoidal source, the sinusoidal source was then chosen. This work develops an approach to identifying the source signature of the sinusoidal source and concludes that the normalized source signature is relevant only to four parameters: the fixed-sine excitation frequency, the duration of excitation, the damping ratio of the vibrator-earth system, and the damped natural frequency of the vibrator-earth system. Two of the parameters are designated input to the vibrator and the other two parameters are measured in the field test using the proposed method in this work. A new wavelet-response technique based on deconvolution and consideration of velocity dispersion is explored in travel-time analyses. The wavelet-response technique is also used for development of a new approach to correcting disorientation of receiver tool. The improved downhole procedures and analyses are then used in the analysis of deep downhole test data obtained at Hanford, WA. Downhole testing was performed to a depth of about 420 m (1400 ft) at Hanford site. Improvements in resolving the wave velocity profiles to depths below 300 m (1000) ft are clearly shown. / text

<strong>Upper mantle discontinuities from PP- and SS-precursors</strong> The 410km discontinuity is examined using two global datasets of precursors to the PP and SS phases. The precursor amplitudes are used to constrain the impedance contrast, and examine lateral variations in the reflection coefficient of the discontinuity. P- and S-wave reflection amplitudes vary over different scale lengths, which could be due to the presence of melt, water or other chemical heterogeneities in the transition zone. Models for 410-topography are also derived, and the relationship between topography and transition zone seismic velocity anomalies examined. A moderate negative correlation exists between long wavelength 410-topography and transition zone velocity anomalies. However, when shorter wavelengths are included the relationship becomes more complex, and in some regions positive correlations are seen. This suggests that long wavelength variation of discontinuity topography and seismic velocity, is due to thermal effects. However, at shorter wavelengths the influence of chemical heterogeneities becomes important. Different spectra for thermal and chemical heterogeneity suggests that chemical anomalies can survive in convecting mantle. <strong>Lower mantle reflectors and S-wave scattering</strong> The lowermost mantle is investigated using a phase stripping technique and two migration methods: a backprojection, and a scheme with weights based on the Generalised Radon Transform. Resolution of the results is tested by migrating synthetic datasets. In some regions the results can be simulated using fairly simple distributions of point scatterers, but elsewhere the results require more complicated structures. The results identify several important properties of the D" region including reflectors within the D" region and a complex pattern of positive and negative scattering potentials near the core-mantle boundary. The results also show the presence of an intermittent D" discontinuity which is not a continuous, nor a global, feature. This suggests that the D" discontinuity is caused by either localised structures, such as thermal or chemical heterogeneity, or a global boundary with a variable impedance contrast.

Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2008. / Committee Chair: Zhigang Peng; Committee Member: Andrew V. Newman; Committee Member: Leland T. Long

This thesis consists of two major parts. In the first part, I monitor the temporal change of S-wave velocity in shallow soil layers using seismic data collected in an experiment at Panola Mt. Atlanta, GA, 2006. I use the cross correlation function to find the arrival time differences for different water levels, and then calculate the change of Rayleigh wave phase velocity according to different frequencies in the range 5 to 50 Hz. After that, I find a reference 1-D layered P and S-wave velocity model from the measured Rayleigh wave dispersion curve, and put 6 sets of Gaussian perturbations into the reference velocity structure to invert for the actual temporal change of velocity structure in the experiment. I find a clear increase of S-wave velocity in the water injection area, and the S-wave velocity gradually recovers to the initial value after we stop pumping water. In the second part, I analyze temporal changes in fault zone site response along the Karadere-Düzce branch of the North Anatolian Fault, starting 8 days before and ending 72 days after the 1999 Mw7.1 Düzce, Turkey, earthquake. The analysis involves comparisons of strong motion seismic records at station VO inside the Karadere fault and station FP about 300 m away from the fault. I compare all available seismic waveforms at these stations, including those generated by foreshocks, the mainshock, aftershocks and seismic noise, and cut them into 10 s windows with a 5 s overlap. Fourier amplitude spectra are computed for seismic data in each window, and the average amplitude spectra for the two horizontal components are used to obtain the spectral ratio for each on/off fault pair of seismic records. The spectral ratios are smoothed over every 10 points in the frequency domain (0.5 Hz). The results show a shift of the spectral peak to lower frequencies during the main shock. The peak frequency reduces from 4.3 Hz several days before the main shock to 2.9 Hz (67.4% of the pre mainshock value) right after the mainshock. It quickly recovers to 3.8 Hz (64% recovery of the dropped value) after a day, and then gradually recovers to 4.0 Hz (79% recovery of the dropped value) after 72 days. I also compare the results from all the seismic data including direct S-wave, S coda waves and seismic noise and from coda waves only and find that the results from coda waves which are generally less scattered than those from all the data, and show lower amplitude of spectra ratio with higher peak frequencies. The observations suggest a nonlinear behavior of the fault zone material under strong ground motion of nearby major earthquakes. Finally I attempt to link the two parts by identifying their implications for the nonlinear site effects.

Knowledge of fracture orientation and density is significant for reservoir and aquifer characterization. In this study, field experiments are designed to estimate fracture parameters in situ from seismic and GPR (radar) data. The seismic experiment estimates parameters of orientation, density, and filling material. The GPR experiment estimates channel flow geometry and aperture. In the seismic study, lines of 2D data are acquired in a vertically fractured limestone at three different azimuths to look for differences in seismic velocities. A sledgehammer, vertical source and a multicomponent, Vibroseis source are used with multicomponent receivers. Acquisition parameters of frequency, receiver spacing and source-to-receiver offset are varied. The entire suite of seismic body waves and Rayleigh waves is analyzed to characterize the subsurface. Alford rotations are used to determine fracture orientation and demonstrate good results when geophone orientation is taken into account. Results indicate that seismic anisotropy is caused by regional faulting. Average fracture density of less than 5% and water table depth estimates are consistent with field observations. Groundwater flow direction has been observed by others to cross the fault trend and is subparallel to a secondary fracture set. In this study, seismic anisotropy appears unrelated to this secondary fracture set. Vp/Vs and Poisson's ratio values indicate a dolomite lithology. Sledgehammer and Vibroseis data provide consistent results. In the GPR experiment, reflection profiles are acquired through common-offset profiling perpendicular to the dominant flow direction. High frequency waves are used to delineate fluid flow paths through a subhorizontal fracture and observe tracer channeling. Channeling of flow is expected to control solute transport. Changes in radar signal are quantitatively associated with changes in fracture filling material from an innovative method using correlation coefficients. Mapping these changes throughout the survey area reveals the geometry of the flow path of each injected liquid. The tracer is found to be concentrated in the center of the survey area where fracture apertures are large. This demonstrates that spatial variations in concentration are controlled by fluid channel geometry.