Chapter 11: Sensitivity Studies of Fundamental- and Higher-mode Rayleigh-wave Phase Velocities in Some Specific Near-surface Scenarios
Carlos Calderón-Macías, Barbara Luke, 2010. "Sensitivity Studies of Fundamental- and Higher-mode Rayleigh-wave Phase Velocities in Some Specific Near-surface Scenarios", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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Insufficient low-frequency data can pose a problem when inverting fundamental-mode Rayleigh-wave phase velocities using two-channel (SASW) or multiple-channel (MASW) active-source methods. Depth of penetration and accuracy of the inverted models are particularly sensitive to the low frequencies. Ambiguities might be reduced with supplemental geologic or geophysical information about the near surface, such as passive-source seismic data and well information. Dispersion data from higher modes might complement the fundamental mode in resolving shear-wave velocities in some cases because higher modes are more sensitive to shear-wave velocities at greater depths. However, interpretation of higher-mode phase velocities poses a challenge because the higher modes are not always easy to separate from the fundamental mode and possibly from other scattered energy. The energy partition into fundamental and higher modes results in different frequency-dependent signal-to-noise ratios for the different modes. Furthermore, convergence of the solution’s data error to the hypothetical global minimum appears to be more complex when fundamental and higher modes are included in the data error to be minimized. Some of the difficulties encountered when inverting higher-mode surface waves are investigated for some simplified shallow (< 10-m) earth models of engineering significance: a sediment profile with a constant stiffness gradient, a low-velocity sediment overlying shallow bedrock, and a high-velocity layer overlying and underlying softer sediment layers.
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Near-surface seismology and ground-penetrating radar (GPR) have enjoyed success and increasing popularity among a wide range of geophysicists, engineers, and hydrologists since their emergence in the latter half of the twentieth century. With the common ground shared by near-surface seismology and GPR, their significant upside potential, and rapid developments in the methods, a book bringing together the most current trends in research and applications of both is fitting and timely. Conceptually, near-surface seismology and GPR are remarkably similar, and they share a range of attributes and compatibilities that provides opportunities to integrate processing and interpretation workflows, which makes them a perfect pair to share pages in a book.
With growth in numbers and professional emphasis have come sections, focus groups, and even professional societies specifically promoting near-surface geophysics. The emergence of near-surface geophysics groups, beginning in the late 1990s and extending into the early twenty-first century, has fueled a diversity of opportunities for professional collaborations. A range of workshops and shared publications has been the fruit of collaborative efforts. The near-surface community continues to extend and develop methods and approaches necessary to satisfy increasing demands in some of the socioeconomically pertinent disciplines such as civil and environmental engineering and hydrology. This book represents the first formal cooperative effort undertaken by the near-surface communities of the Society of Exploration Geophysicists, the American Geophysical Union, and the Environmental and Engineering Geophysical Society.