Sea Technology

SEP 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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www.sea-technology.com September 2017 / st 13 Risk vs. Cost. Balancing risk against cost as the survey areas get larger and environments become more challeng- ing is incredibly difficult. Larger areas and more complex geology require more and closer-spaced borehole and CPT locations to effectively characterize soil variability through- out the area of interest. However, sampling and CPT work are expensive, particularly in deepwater, and therefore lead to a difficult balancing exercise, where the increased risks (and associated over-design) of fewer and/or sparser sample locations are offset against acquisition costs. This can prove particularly troublesome if the infrastructure design (e.g., wind turbine layout) changes between the site survey and installation, which often results in significant foundation over-design to accommodate potentially unknown ground conditions. Together, these two issues pose a challenging question for site investigations going forward: Is it possible to gain quantitative information about the nature of the subsurface both earlier, to aid in more efficient site-survey design, and at a higher spatial fidelity, to better manage risk without re- sorting to a more extensive and costly sampling campaign? What Can We Get from Seismic Inversion? A particular strength of seismic inversion is its flexibil- ity. Acoustic impedance inversion, while limited in how it parameterizes the subsurface, is both computationally inex- pensive and simple enough to be applied to single-channel data. It therefore can be applied to all manner of data, even legacy data as long as it was digitally recorded, records the full waveform (i.e., not enveloped), and the basic data qual- ity is sufficient. Similarly, quantifying the way in which the seismic wavefield is attenuated as it moves through the sub- surface (normally expressed in terms of Q-factor) does not require any more than good-quality single-channel data, yet can provide valuable information about the nature of the sediments. Seismic inversion therefore has significant potential for aiding geotechnical engineering site-survey design before any new data have been acquired, through the inversion of existing data during a desk-based study using some combi- nation of impedance inversion and/or Q-estimation. Q-fac- tor, for example, has been shown to be strongly linked to the cohesive versus granular behavior of sediments and there- fore provides a useful insight into whether the sediment will behave like a clay or a sand. In addition to site-survey plan- ning value, availability of similar preliminary soil informa- tion from the early desktop study can be a valuable input to conceptual and preliminary facility layout and foundation design, and thus help guide and speed the overall develop- ment process. Naturally, seismic inversions tend to be cast in terms of geophysical parameters (e.g., Q-factor, impedance, velocity, bulk density) that have a more straightforward influence on the recorded seismic reflection response, rather than geo- logical or geotechnical parameters (e.g., lithology, perme- ability, undrained shear strength) that have a more complex relationship with the reflection response. However, the abil- ity of seismic inversion to aid in managing risk depends al- most entirely on its effectiveness in predicting useful geolog- ical and geotechnical parameters away from, or between, core, borehole and CPT locations. While casting inversions explicitly in terms of geological and/or geotechnical param- All-in-one acoustic tracking, navigation and communications for small AUVs Small AUVs need small instruments and they don't get much smaller than our new AvTrak 6 Nano OEM. It measures just 88mm long and 56mm wide yet we've managed to combine USBL tracking, support for LBL and bi- directional communications in one easy-to-fit, easy-to-integrate design. A li-ion battery gives you 10 days standby life to help recover your vehicle if lost and it's compatible with all the Sonardyne 6G systems fitted to ships and unmanned systems in use across the oceans. Enhance the mission capabilities of your marine robotic platforms. Search AvTrak 6 Nano OEM WHY NOT CUT IT OUT AND TRY IT FOR SIZE? Autonomy

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