Chapter 8: Estimating In Situ Horizontal Stress in Soil Using Time-lapse VS Measurements
Ranajit Ghose, 2010. "Estimating In Situ Horizontal Stress in Soil Using Time-lapse VS Measurements", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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The magnitude and temporal changes of in situ horizontal stress at shallow depths in the subsoil are crucial information in geotechnical engineering. Although various methods of monitoring in situ horizontal stress have shown some success, such monitoring remains extremely challenging, especially for sands and stiff clays, and large uncertainties are usual. Laboratory experiments are performed that involve realistic values of stress and porosity, combined with seismic-array data acquisition, to monitor changes in shear-wave velocity (VS) induced solely by changes in horizontal stress. Seismic-array data have been instrumental in distinguishing the small velocity changes associated with horizontal stress changes. Stress-porosity empirical models and micromechanical models have predicted quite accurately the observed trend of variation in VS as a function of horizontal stress. This trend is unique for a given combination of vertical stress, porosity, and soil type. Therefore, by monitoring the temporal change of VS by means of a seismic receiver array fixed at a given depth range and then by using the velocity–horizontal stress trend predicted by the model, one can estimate the temporal change and magnitude of in situ effective horizontal stress. A data-driven inversion approach has been tested on laboratory-experiment data for which the effective horizontal stress is known. The results demonstrate the possibility of estimating in situ effective horizontal stress at a given depth in subsoil, with an uncertainty of less than 15–20%, even when the porosity, vertical stress, and field factor are unknown. This approach shows potential for use on real field data.
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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.