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Speaker: James Gaiser, Gaiser Geophysical Consulting
Utilizing shear-wave (S-wave) data acquired with compressional waves (P-waves) is becoming more common as joint imaging and inversion techniques improve. A strong motivating factor is that S-wave information is important for quantifying most petrophysical properties. Recorded shear wavefields provide valuable travel times and amplitudes to determine VP/VS and S-wave impedance for lithology discrimination, brittleness and fluid-content validation. Neural-network joint inversion of seismic attributes from P-waves and converted P- to S-waves (PS-waves) has even been useful to estimate sand porosity. The unique property of S-wave splitting anisotropy is very important, providing fast S1- and slow S2-wave information to help characterize the nature of porosity in terms of crack/fracture orientation and magnitude, fracture gradient and qualitative permeability information.
Shear-wave signal strength varies significantly for the various wavefields. Traditionally, P-waves are used to obtain S-wave information from amplitude variation with offset (AVO) or polar angle theta (AVA). However this signal can be weak because it is second order in sin2θ, which is very small at vertical incidence and near offsets. Although pure-mode S-waves obtained from horizontal sources have stronger signal because they are first order (cosθ) in radiation amplitude and reflectivity, they are expensive and environmentally unfriendly. Converted PS-waves are particularly attractive for S-wave signal strength because they are first order (cosθ) in P-wave radiation and only first order (sinθ) in S-wave reflectivity. On the other hand, converted S- to P-waves (SP-waves) from vertical sources are first order (sinθ) in S-wave radiation amplitude and reflectivity. The combined effect is second order (sin2θ), and additionally SP-waves are nearly overwhelmed by P-wave reflections on the vertical component.
Interest in SP-waves radiated from vertical sources and buried explosives exploits conversion to P-waves as primary reflections for reducing acquisition costs and for application to legacy data. However, recent investigations overstate the extent of SP-wave illumination and show isotropic processing results with narrow bandwidth frequency and wavenumber data. I demonstrate that illumination with SP-waves is limited in general to polar angles up to around θ =30° or 35° for VP/VS =2 or 3, respectively. At greater angles, S-waves are typically in the P-wave evanescent range and cannot excite SP-wave reflections. Contrary to recent claims, these sources for P-wave do not radiate SH-waves polarized in horizontal planes in all azimuths. I examine radiation properties for isotropic media with expressions for amplitude derived in vector slowness coordinates. Also, I extend these expressions to transversely isotropic media with a vertical symmetry axis to show agreement with synthetic seismic data that only quasi SV-waves are radiated and become more narrowly focused towards 45°. Furthermore, in azimuthally anisotropic media (orthorhombic), synthetic data show that fast S1- and slow S2-waves polarized parallel and perpendicular to fractures may appear as SV- and SH-waves. For a partially saturated fracture model studied here, S1-wave radiation has broader azimuthal illumination than slow S2-waves, which are more narrowly focused in azimuth. These produce SP-wave splitting signatures on vertical component reflection data that are nearly identical to PS-wave signatures on radial horizontal component data. However, separating these fast and slow SP-waves without horizontal components is an additional processing challenge.
Speaker Biography: James Gaiser, Gaiser Geophysical Consulting
James E. Gaiser received a BS (1972) in geology from Indiana University, an MS (1977) in geophysics from the University of Utah and PhD (1989) in geophysics from the University of Texas at Dallas. In 1977 he joined ARCO in the geophysical analysis and processing group before moving to research and development in 1981, where he worked on vertical seismic profiling (VSP), elastic wave imaging, and seismic anisotropy. He worked with Western Geophysical from 1992 to 2000 conducting research in coherent-noise suppression, depth migration and multicomponent processing, and continued with Western Geco until 2007. He joined ION Geophysical – GXT Imaging Solutions in 2007 and worked until 2009 on interferometry, and 3D converted wave imaging. After several years as senior scientist with Geokinetics Inc., where his research activities were 3D imaging and velocity model building in anisotropic media, noise attenuation, and converted waves, he worked with CGG as R&D Manager of multicomponent research from 2013 to 2015. Since then, he is a consultant with his company, Gaiser Geophysical Consulting.
He has authored numerous patents and articles, shared the award for SEG best presentation in 1981, received SEG honorable mention for his presentation in 1993, shared SEG honorable mention for his TLE article in 2003, and received the SEG Life Membership award in 2007. He served on the SEG Development and Production committee from 1993 to 1997, was District 2 Representative for the SEG council from 1994 to 1997. He served as vice president of the Denver Geophysical Society (DGS) in 2004 and president in 2005. He currently serves on the SEG Research committee (2007 to present), is a lecturer for the SEG Continuing Education program and presented the 2016 Distinguished Instructor Short Course on “3C Seismic and VSP”. He is a member of SEG, DGS, CSEG, EAGE, RAS, SPE and AAPG.
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