The progressive growth of onshore shale production (both gas and liquids) to replace depleting and aging oil fields may benefit from the use of surface seismic shear wave data analysis for full characterization of shale reservoir properties and lead to optimum development of these resources. This includes descriptions of azimuthal anisotropy (HTI - transverse isotropy with a horizontal symmetry axis) for characterization of fractures and internal fracture systems. The objective of this study is to document a predicted distortion in polarization of propagating seismic shear waves upon reflection at a subsurface interface and to propose a correction to this distortion. The polarization distortion occurs even in wholly isotropic media. This correction is based on an understanding of shear amplitude behavior as a function of the reflection incidence angle, particularly differences in the reflection angle relation for different shear components. This study includes a demonstration of the efficacy of the proposed correction by applying it to simulated and real direct shear-wave source data. Such corrections should result in a minimized polarization distortion in the reflection process. The apparent consistency of a null value (zero crossing) of the SV-SV reflectivity (near 20-24 degrees) for common density and velocity contrasts as well as the remarkably regular behavior of the SV-SV and SH-SH reflectivity curves following a linear relation in sin2 and tan2 of the incidence angle and offers the opportunity for a simple and stable correction with minimal sensitivity to detailed knowledge of contrasts in velocity and density. The only independent information required for the correction is the angle of incidence where the SV-SV and SH-SH reflections vanish and the range of these angles is typically quite limited. Some key questions were addressed in gaining an understanding of shear wave polarization distortion upon reflection for varying model data: 1) how do we address reflected polarization distortion for purely isotropic medium for varying incidence angles? 2) How do we apply this correction for an isotropic and anisotropic media for both simulated and actual field data 3) How do we address applications to real data and how distorted amplitudes can be corrected to identify actual subsurface HTI anisotropy. Significantly, the polarization distortion correction is implemented as a simple extension of the established Alford rotation for normal incidence shear reflections of varying polarization. This extension leads to the improved analysis of direct shear-source 3D data with inherently distorted polarization. Thus, analysis may be applied to estimate HTI anisotropy previously not realizable in finite offset data subject to polarization distortion. Example applications to actual field data are included. Note that the polarization correction does remove the AVO effects often exploited in analysis of P-P data where polarization is not an issue that is, the AVO amplitude effect is essentially removed from the SV-SV and SH-SH oriented direct shear-wave profiles, which permits proper analysis of the polarization. Further, additional analysis of the polarization correction on field data with documented anisotropy will be required to fully develop the usefulness of this proposed correction. / text

Shear-wave splitting is a phenomenon observed in almost all in situ rocks. Due to propagation through stress-aligned and fluid-saturated microcracks and fractures the initial shear wave splits into two almost orthogonal waves which propagate with different velocities along similar ray paths. The process is characterized by the polarization direction of the faster split shear wave, which is parallel to the orientation of the cracks, and the time delay between the onsets of the two waves. The analysis of shear-wave splitting has been conducted over records of 233 microearthquakes in the vicinity of five seismic stations in SW Iceland. Visual methods have been applied to the data to retrieve the final results for polarization directions and time delays. The main polarization azimuth for the leading split wave is N30°- 60°E which is in full agreement with the mapped alignments of normal faults and volcanic fissures in the surface. The time delays measured at different sites vary in the range of 10-100 ms for the events of best quality. In general, splitting times do not show a clear pattern at all recording sites with increasing depth. The only firm conclusion that can be drawn from the time delays is that at station BLF in the Brennisteinsfjöll fissure swarm, the time delays are smaller than in the Hengill area and therefore the strength of anisotropy beneath that station appears to be lower.

Azimuthal anisotropy, assumed to be associated with vertical, aligned cracks, fractures, and subsurface stress regimes, causes vertically propagating shear waves to split into a fast component, with particle motion polarized parallel to fracture strike, and a slow component, with particle motion polarized perpendicular to fracture strike. Determining the polarization of each split shear wave and the time lag between them provides valuable insight regarding fracture azimuth and intensity. However, analysis of shear wave polarizations in seismic data is hampered by reflection-induced polarization distortion. Traditional polarization analysis methods are limited to zero offset and are not valid if implemented over the full range of offsets available in typical 3D seismic data sets. Recent proposals for normalizing amplitudes recorded at non-normal incidence to values recorded at normal incidence may provide an extension to correcting offset-dependent shear wave polarization distortion. Removing polarization distortion from shear wave reflections allows a larger range of offsets to be used when determining shear wave polarizations. Additional complexities arise, however, if fracture orientation changes with depth. Reflections from layers with different fracture orientations retain significant energy on off-diagonal components after initial rotations are applied. To properly analyze depth-variant azimuthal anisotropy, time lags associated with each interval of constant anisotropy are removed and additional iterative rotations applied to subsequent offset-normalized reflections. Synthetic data is used to evaluate the success of these methods, which depends largely on the accuracy of AVA approximations used in the correction. The polarization correction effectively removes SV polarity reversals but may be limited in corrections to SH polarizations at very far offsets. After the polarization correction is applied, energy calculations including incidence angles up to 20° more effectively compensates individual SV and SH reflection components, allowing for more faithful polarization information identification of the isotropy plane and the symmetry axis. The polarization correction also localizes diagonal component energy maxima and off-diagonal component energy minima closer to the true orientation of the principal axes when a range of incidence angles up to 20° is used. / text

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We investigate upper crustal anisotropy of the Endeavour Segment of the Juan de Fuca Ridge using shear wave splitting measurements of ~3000 earthquakes recorded during three years using the Keck seafloor seismic network. We apply a new cluster analysis of shear-wave splitting measurements to our database. The methodology reduces the use of subjective criteria and improves the accuracy of measurements in the presence of noisy data. Fast polarization directions at a given seismic station are constant and stable during the deployment; however, fast-polarization directions between stations vary significantly. We presume that the lack of consistency of shear wave splitting among seismic stations reflects the spatial distribution of anisotropy in the vicinity of the ridge axis. We infer that the variation of fast polarization directions and delay times is caused by spatial variations in shallow hydrogeological structures and the stress field. Local faults and fissures are unlikely to be the primary cause of this anisotropy since most of the fast polarization directions are not consistent with the ridge parallel trend of faults. Stress perturbations induced by magmatic injection into the axial magma chamber or spatial variation in the rates of a hydrothermal heat transfer may contribute to the observed heterogeneity in seismic anisotropy.

[Truncated abstract] Seismic tomography has been extensively used in geophysics for different purposes such as geological mapping and prospecting for oil and gas. In geophysics, ultrasound or electromagnetic waves are normally used to provide the tomographic information. In the geotechnical area, seismic tomography is emerging as a promising technique that can be used to determine the spatial variability of shear wave velocities and hence the small strain stiffness of geomaterials. Although some studies have been undertaken to incorporate seismic measurement into centrifuge modelling, there has been to date no attempt to build a complete seismic tomography facility with high resolution for use in a geotechnical centrifuge. Such a powerful facility can help in better understanding of soil behaviour by providing a complete picture of the spatial variation of the soil property of concern. The main aim of this study was to develop a high-resolution seismic tomography (ST) system for the beam centrifuge at the University of Western Australia (UWA) by which the shear wave velocity and hence maximum shear modulus could be determined anywhere in the centrifuge model. ... This limitation was the requirement to use an a priori model. The exact solutions in the different examples presented in this chapter were known, and they were used as a priori models into the inversion process. However, in practice the exact solution is unknown, and the aim of the tomographic inversion is to obtain a solution that best describes the measured data. Carrying out inversion without using an a priori model can yield an output model that hints at the nature of the model. This output can then be used as the starting point in an iterative process, in which the output from one step is used as an a priori model for reinverting the original data in a subsequent step. In this case, this process slightly improved the output tomogram and decreased the value of root mean squares of travel time residuals (Rrms). An alternative inversion strategy was proposed based on the results obtained in this study. It involves using a searching algorithm. A searching process can be carried out based on the output from the first iteration (without using an a priori model). The search can involve varying the parameters that describe buried anomalies, such as the size of the anomaly, the velocity value in the anomaly, and the location of the anomaly. The aim is to search for the combination of anomaly parameters that minimises the resulting error parameters (mainly Rrmx, but also the average error and the standard deviation of the error). For more subtle cases, such as the velocity model under a footing, where inversion without using an a priori model did not recover the input model, a searching algorithm involving applying perturbations to the exact Boussinesq model can be performed. Not only can the searching procedure involve adding perturbation to the velocity values in the Boussinesq model, but it can also add perturbation to the shape of the velocity distribution below the footing. The searching process can continue until a model that fits the data with a minimum error is found, i.e., minimising Rrms.

The seismic cone penetration test (SCPTu) is the most efficient means for geotechnical site characterization and the evaluation of seismic ground hazards. In this thesis, software systems including ShearPro, ClusterPro, and InSituData, are developed to automate post processing of these SCPTu data. ShearPro is developed to automate the post-processing of the shear wave signals. ClusterPro uses the proposed three-dimensional cluster analysis approach for soil stratification. InSituData facilitates the post processing of penetration data for seismic ground hazards analysis. A new three-dimensional soil classification chart is also proposed in this thesis to help discern soil layers that may be subject to seismic ground hazards, such as loose liquefied sands and silty sands. These methods are then applied to SCPTu data collected at previously-identifed paleoliquefaction sites located in the New Madrid Seismic Zone (NMSZ). For liquefaction evaluation, the cyclic stress ratio (CSR) is computed using site response analysis by DeepSoil and a measured profile of shear waves derived from the 30-m SCPTU soundings and deep suspension loggings in AR and TN. The natural resistance of the soil to liquefaction, termed the cyclic resistance ratios (CRRs), is evaluated based on both deterministic procedures and probabilistic procedures. Based on liquefaction evaluation results at selected paleoliquefaction sites, regional CRR criteria for liquefaction are developed for the NMSZ. As even the latest major earthquakes in NMSZ occurred nearly 200 years ago, aging effects might be an important factor to consider in utilizing the liquefaction criteria to assess the seismic parameters associated with the previous earthquakes. The aging effects in the NMSZ were investigated through large scale blast-induced liquefaction tests conducted in the NMSZ. Then a procedure to estimate seismic parameters associated with previous earthquakes is proposed. It utilizes both the liquefaction criteria based on SCPTu tests and the empirical attenuation relations developed for the corresponding regions. The approach is validated through data evaluation related to the 1989 Loma Prieta earthquakes in California and then applied to previous historic earthquakes in the NMSZ.

Nonlinear site response analyses are generally preferred over equivalent linear analyses for soft soil sites subjected to high-intensity input ground motions. However, both nonlinear and equivalent linear analyses often result in large induced shear strains (3-10%) at soft sites, and these large strains may generate unusual characteristics in the predicted surface ground motions. One source of the overestimated shear strains may be attributed to unrealistically low shear strengths implied by commonly used modulus reduction curves. Therefore, modulus reduction and damping curves can be modified at shear strains greater than 0.1% to provide a more realistic soil model for site response. However, even after these modifications, nonlinear and equivalent linear site response analyses still may generate unusual surface acceleration time histories and Fourier amplitude spectra at soft soil sites when subjected to high-intensity input ground motions. As part of this work, equivalent linear and nonlinear 1D site response analyses for the well-known Treasure Island site demonstrate the challenges associated with accurately modeling large shear strains, and subsequent surface response, at soft soil sites. Accounting for the uncertainties associated with the shear wave velocity profile is an important part of a properly executed site response analyses. Surface wave data from Grenoble, France and Mirandola, Italy have been used to determine shear wave velocity (Vs) profiles from inversion of surface wave data. Furthermore, Vs profiles from inversion have been used to determine boundary, median and statistically-based randomly generated profiles. The theoretical dispersion curves from the inversion analyses as well as the boundary, median and randomly generated Vs profiles are compared with experimentally measured surface wave data. It is found that the median theoretical dispersion curve provides a satisfactory fit to the experimental data, but the boundary type theoretical dispersion curves do not. Randomly generated profiles result in some theoretical dispersion curves that fit the experimental data, and many that do not. Site response analyses revealed that the greater variability in the response spectra and amplification factors were determined from the randomly generated Vs profiles than the inversion or boundary Vs profiles.

During the past two decades, University of Kentucky researchers have been acquiring seismic refraction/reflection data, as well as seismic downhole data, for characterizing the seismic velocity models of the soil/sediment overburden in the central United States. The dataset includes densely spaced measurements for urban microzonation studies and coarsely spaced measurements for regional assessments. The 519 measurements and their derivative products often were not in an organized electronic form, however, limiting their accessibility for use by other researchers. In order to make these data more accessible, this project constructed a database using the ArcGIS 9.1 software. The data have been formatted and integrated into a system serving a wider array of users. The seismic shear-wave velocity models collected at various locations are archived with corresponding x-, y-, and z-coordinate information. Flexibility has been included to allow input of additional data in the future (e.g., seismograms, strong ground-motion parameters and time histories, weak-motion waveform data, etc.). Using the completed database, maps of the region showing derivative dynamic site period (DSP) and weighted shear-wave velocity of the upper 30 m of soil (V30) were created using the ArcGIS 9.1 Geostatistical Analyst extension for examination of the distribution of pertinent dynamic properties for seismic hazard assessments. Both geostatistical and deterministic techniques were employed. Interpolation of V30 data yielded inaccurate predictions because of the high lateral variation in soil layer lithology in the Jackson Purchase Region. As a result of the relatively uniform distribution of depths to bedrock, the predictions of DSP values suggested a high degree of accuracy.