Chapter 25: Detecting Perched Water Bodies Using Surface-seismic Time-lapse Traveltime Tomography
David Gaines, Gregory S. Baker, Susan S. Hubbard, David Watson, Scott Brooks, Phil Jardine, 2010. "Detecting Perched Water Bodies Using Surface-seismic Time-lapse Traveltime Tomography", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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Applications of seismic time-lapse techniques generally are constrained to large-scale investigations associated with petroleum exploration and exploitation. There is growing interest in using geophysical methods to monitor nearsurface phenomena, such as fluid flow in fractured or karstic bedrock, hydraulic infiltration, and anthropogenic manipulations during environmental remediation. Previous near-surface geophysical time-lapse studies have focused on electrical or electromagnetic (EM) techniques (including ground-penetrating radar) or borehole methods. To evaluate the utility of surface seismic time-lapse traveltime tomography, a site was monitored through time along a single 2D profile. The objective was to attribute increases in seismic P-wave velocity with the development of perched water bodies in the upper 4 m of the subsurface. The study was conducted in the Y-12 area of Oak Ridge National Laboratory in Tennessee, U.S.A., in conjunction with a broader multidisciplinary investigation on the fate and transport of contaminants. Because of previous anthropogenic alterations of the site associated with remediation efforts (e.g., replacing as much as 7 m of contaminated soil with poorly sorted limestone gravel fill during construction of a seepage basin cap), the near-surface hydrogeology was extremely heterogeneous and was hypothesized to have a large influence on differential infiltration, contaminant distribution, and contaminant remobilization. The seismic data were processed using a wavepath eikonal traveltime (WET) tomography approach, and a modified trend-analysis technique was applied to remove the larger spatial component associated with geologic variability. The final “residual” velocity-anomaly images were compared with wellbore hydrologic data and error analyses and were used to interpret the presence and geometry of perched water in the shallow subsurface. The study suggests that velocity estimates obtained from surface-seismic traveltime tomography methods are effective for indicating the spatial and temporal distribution of perched water bodies at the Oak Ridge site in the upper 4 m of the subsurface.
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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.