Near-surface seismic: More than a problem of scale
Dr Rick Miller
Over the last 30 years, various seismic techniques have been successfully used to unravel some of the most intriguing and challenging near-surface problems faced by geologists, hydrologists, and engineers. In general, geophysicists define the nearsurface as the upper few hundred meters of the Earth’s surface. With the unique behavior and wavefield characteristics commonly observed on seismograms at timedepths as great as 1 km, I define the upper 1 km as near-surface for this discussion. Within this upper 1 km of the Earth’s crust, many assumptions central to conventional exploration approaches are invalid. Adapting conventional seismic techniques to near-surface applications requires more than simple scaling to accurately and confidently image or characterize the shallow subsurface. It seems intuitive that considerations, concepts, and workflows for the seismic wavefield should scale consistent with depth; however, making that assumption has led some to incorrectly conclude that inherent limitations of the seismic-reflection method make it unreliable at shallow depths and associated higher frequencies. High-frequency near-surface applications of the method do have some inherent limitations, but they are related to resolution. Resolution limitations stem from both the Earth’s preferential attenuation of higher frequencies and its relatively rapid changes in physical properties, both vertically and horizontally, in the near-surface. In this talk, I present key aspects of both acquisition and processing, focusing on differences between the approaches and the most important considerations that distinguish high-resolution from conventional imaging. The bane of both conventional and near-surface seismic-reflection surveying is surface waves. Over the last decade, surface waves have transformed from noise to signal for many near-surface applications. This evolution has allowed the entire seismic wavefield to become signal for near-surface practitioners. My presentation includes examples of enhancement techniques that have been and are being tailored to each wave type, providing better and more redundant characterization for an ever-increasing range of near-surface settings. Seismic wavefield characteristics uniquely associated with the near surface include an extremely large velocity gradient, a high percentage of dispersive energy within the optimum recording window, high attenuation coefficients, lateral heterogeneity of physical properties, and minimal modal and wave separation. This talk will highlight some of the most troublesome problems and the associated solutions developed to accurately interrogate the near surface with seismic methods.
Richard Miller (Rick) is a senior scientist at the Kansas Geological Survey (KGS), a research and service division of the University of Kansas (KU). He also holds a courtesy appointment as associate professor of geology at KU. Rick received a BA in physics from Benedictine College, an MS in physics (emphasis geophysics) from KU, and a PhD in geophysics from the University of Leoben, Austria. He has been at the KGS since 1983. His scientific interests focus on the application of shallow, high-resolution seismic methods to a wide assortment of problems ranging from energy to engineering to the environment. Rick is currently SEG’s second vice president and was the head of the research team that received SEG’s Distinguished Achievement Award in 2002 and was the recipient of the SEG/NSGS Hal Mooney award in 1995. His editorial service includes five years on the TLE Editorial Board (2005 through 2009, chairman during 2009) and as co-editor of the 2010 book Advances in Near-Surface Seismology and Ground-penetrating Radar. He has published more than 85 refereed journal articles and a half-dozen book chapters.
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