Chapter 27: Improving Fractured-rock Characterization Using Time-frequency Analysis of GPR Data Sets
Mehrez Elwaseif, Lee Slater, Mamdouh Soliman, Hay Salah, 2010. "Improving Fractured-rock Characterization Using Time-frequency Analysis of GPR Data Sets", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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Resolution of the ground-penetrating-radar (GPR) method largely depends on the wavelength of the propagated electromagnetic waves, the depth, and the dimensions of the investigated targets. Locating small-scale targets such as fractures can be difficult, especially when the size of the fractures themselves and the spacing between them are smaller than the wavelength of the EM waves. In such cases, fractures will not be resolved individually in the time domain. Examining the frequency (spectral) content of GPR time series might provide additional information that can improve fracture location relative to the results from conventional examination of the time series alone. The S-transform, a powerful time-frequency analysis tool, can be applied to GPR time series extracted from synthetic data over 1D and 2D models of fractured limestone as well as to a data set collected at a fractured limestone site in Egypt. In synthetic scenarios, the value of the S-transform analysis can be investigated under two conditions: (1) discrete fractures spaced at distances greater than one-quarter wavelength and (2) closely spaced fractures separated by distances less than one-quarter wavelength. The synthetic studies include analysis of the dependence of S-transform results on the dielectric properties of a fracture, by simulating both air-and water-filled fractures. Time-frequency analysis can aid in determining the location of discrete, closely spaced fractures when fractures are separated by distances greater than one-quarter wavelength. However, the clarity of fracture expressions from the S-transform depends on the dielectric contrast between the fracture and the host material, so air-filled fractures are not always identified. In the case of fractures smaller than one-quarter wavelength, boundaries where fracture spacing (density) changes abruptly do not result in diagnostic shifts in the frequency content of the S-transform along the time series. However, diagnostic frequency shifts do occur when the dielectric properties of closely spaced fractures change between air-filled and water-filled fractures, with water-filled fractures displaying a higher-frequency content. This frequency-spectrum dependence on dielectric properties of fractures can be used to infer locations of water-filled fractures. In addition, this dependence can help to locate possible localized moisture transport between fracture zones, via capillary effects, at a field site.
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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.