Chapter 18: High-resolution SH-wave Seismic Reflection for Characterization of Onshore Ground Conditions in the Trondheim Harbor, Central Norway
Polom Ulrich, Hansen Louise, Sauvin Guillaume, L’Heureux Jean-Sébastien, Lecomte Isabelle, Charlotte M. Krawczyk, Vanneste Maarten, Longva Oddvar, 2010. "High-resolution SH-wave Seismic Reflection for Characterization of Onshore Ground Conditions in the Trondheim Harbor, Central Norway", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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The area around Trondheim Bay in central Norway is affected by landslides, both onshore and within the fjord, with several events documented to have occurred in the last century. As urban development, including land reclamation, is taking place in the harbor, assessing in situ soil conditions is paramount for infrastructure and operational safety. To obtain better insight into the harbor setting in terms of subsurface structures and potential coastal geohazards, a high-resolution multichannel SH-wave seismic-reflection land survey was carried out during summer 2008, which complements a dense network of high-resolution, single-channel marine seismic profiles over the deltaic sediments in the fjord. The SH-wave seismic reflection was chosen because the resulting interval shear-wave velocity provides a nearly direct proxy for in situ soil stiffness, a key geotechnical parameter. In total, 4.2 km of 2.5D SH-wave profiles was acquired along roads and parking places. Highly resolved images of the sediments were obtained, overlying the bedrock at a depth of about 150 m. The high quality of the data is ascribed to the quieter ambient noise conditions of the nighttime data collection and an efficient suppression of Love waves arising from the presence of a high-velocity layer at the surface. Five main stratigraphic units were identified based on reflection patterns and amplitudes. Distinct SH-wave reflection events enabled detailed S-wave velocity determination down to the bedrock. Subsequently, interval velocities were remapped into soil stiffness. Low S-wave velocities of about 100 m/s occurring in the upper 50 m of the fjord-deltaic sediment succession suggest low sediment stiffness (50 to 100 MPa) directly below the stiffer man-made fill that is 10 to 15 m thick. The results indicate that SH-wave seismic reflection is well suited for urban ground investigation.
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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.