Sea Technology

SEP 2017

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14 st / September 2017 impedance, whereas for the latter phases of site investiga- tion a more geotechnical engineering-heavy IST is likely to be more effective. Undoubtedly, however, to get the most out of seismic inversion, it is critical to develop a clear and precise under- standing of what one wants to achieve at each and every stage of the project. This will vary survey to survey, proj- ect to project, but such high-level, project-wide oversight is necessary in ensuring both effective and efficient use of this multifaceted tool. If identified early in the project planning phase and clearly defined deliverables are assigned, seismic inversion has the potential to be an extremely useful tool in the site investigation toolbox. As its advantages become more widely appreciated, and its application becomes in- creasingly feasible, we believe inversion will have a grow- ing role in offshore site characterization. And, inversion is a key component in the evolving development of robust pre- dictive earth models for engineering applications. Further Reading For a list of suggested further reading, contact Mark Vardy at ST eters is extremely difficult and still very rare for hydrocar- bon applications, there are a variety of ways to derive more useful properties from the inverted geophysical parameters. Artificial neural networks, for instance, have shown promise in predicting geotechnical properties, effectively capturing spatial variation in subsurface properties that would pose significant design problems were facilities to be located even a short distance from a ground-truth site. The Future with Seismic Inversion? While seismic inversion is not a panacea, it is a tool that has proven extremely useful for hydrocarbon reservoir char- acterization and could become similarly useful for site in- vestigations. At the most fundamental level, high-resolution seismic reflection surveying is a standard site investigation method and significant advances can be made by applying even the simplest seismic inversion techniques, such as im- pedance inversion and/or Q-estimation, without changing survey methods or the need to acquire new profile data. If changes are made to the survey methods employed, such as through the regular acquisition of boomer or sparker multi- offset data, then more advanced inversion techniques that derive shear-wave as well as compressional-wave properties could be applied. Shear wave properties provide a direct link to the geotechnical properties of the sediment matrix under small strains, which are crucial in a large range of offshore engineering projects. Inversion offers major op- portunities for remotely characterizing soil conditions in the foundation and top-hole zones, thus increasing efficiency and reducing risks and costs of offshore development. The key questions moving forward with seismic inver- sion as a tool for site investigation center around how to maximize its effectiveness. When is inverting extant data during the desk-based study useful and how extensive do the procedures need to be in order to best inform the sub- sequent survey strategy? For example, would just mapping spatial variability in acoustic impedance provide enough information to aid in site-survey planning, or can more fo- cused site investigation operations only reliably be planned using more detailed sediment properties (such as porosity, grain size and effective stress)? Similarly, what is the best composition of the IST and how will this vary both between projects and different stages of the same project (e.g., desk- based study versus engineering design modeling)? In the early phases of site investigation, it may be more cost effec- tive to have a geophysics-heavy IST that is more comfortable at interpreting raw geophysical parameters such as acoustic Dr. Mark E. Vardy is a marine geophysicist working at the National Oceanography Centre, Southampton, U.K. He earned his Ph.D. in marine geophysics from the University of Southampton and specializes in the near-surface application of novel geophysical tech- niques. In particular, he has worked on decimeter- resolution 3D seismic imaging, pre-stack depth imag- ing and various forms of seismic inversion. Andrew W. Hill is a leader in the field of marine geo- physical site investigation. Based in Houston, he is BP's global technical authority for marine geohazards characterization. With an M.S. in marine geology and geophysics from UCL, he has spent his entire career in marine geophysical site investigation, the last 29 years with BP. Kerry J. Campbell is principal geoscientist with Fu- gro in Houston. He has about 43 years of experience helping to pioneer integrated characterization and geohazards assessment of offshore sites worldwide. Campbell is the author/co-author of about 45 publi- cations on marine engineering geology and geophys- ics and marine geohazards. He holds B.S. and M.S. degrees in geology from the University of Massachu- setts, Amherst. MATHEWS ASSOCIATES, INC. 407-323-3390 SALES@MAIFL.COM R E L I A B L E P O W E R S Y S T E M S Whatever floats your !!!

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