Chapter 20: Refraction Nonuniqueness Studies at Levee Sites Using the Refraction-tomography and JARS Methods
Published:January 01, 2010
Julian Ivanov, Richard D. Miller, Jianghai Xia, Joseph B. Dunbar, Shelby Peterie, 2010. "Refraction Nonuniqueness Studies at Levee Sites Using the Refraction-tomography and JARS Methods", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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The utility of two varied approaches to first-arrival time analysis of seismic data acquired at several unique levee sites is demonstrated by solving the inverse refraction-traveltime problem (IRTP). These data were evaluated using conventional refraction tomography and joint analysis of refractions with surface waves (JARS). The JARS approach uses a reference model, derived from surface-wave-calculated shear-wave velocity estimates, as a constraint in reducing refraction nonuniqueness. At those levee sites, conventional refraction-tomography and JARS methods provided different solutions, equally matching the observed data. This observation suggests both approaches are equally possible from a numerical perspective. The JARS images reveal horizontal layering patterns, laterally uniform velocity trends, mild velocity variations, and channel-like features consistent with geologic expectations. In addition, the JARS approach demonstrated the capability for imaging low-velocity layers/zones, something not seen using conventional refraction or refraction-tomography techniques. As a result of these qualitative observations, without ground truth to support an earth model (e.g., from wells), the JARS approach can be viewed as an additional method for finding solutions to the IRTP. However, from all evidence in those studies, the JARS approach represents a possible solution and an example of the potential adverse affect of nonuniqueness. These empirical results support the understanding that for a given refraction data set, significantly different and equally possible velocity-model solutions can exist, resolving which is truly best using invasive ground truth.
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Advances in Near-surface Seismology and Ground-penetrating Radar
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.