Sea Technology

JUL 2017

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16 st / July 2017 fine in which places the trenching could be done without reaching the project's burial depth. Conclusion Nowadays, shallow, high-res- olution 3D seismic surveys are on their way to becoming a ma- jor practice in subsea engineering projects. This means that projects put more resources into geophysi- cal survey in order to have a full, detailed geologic model of the interest area. In addition, these surveys reduce the financial resources to be spent on geotechnical surveys, which are much more expensive. In cable route surveys where engineers must have precise information about the shallow sub-bottom geology, the sin- gle-frequency approach is not ideal. With multifrequency, high-resolution 3D surveys, the combination of the informa- tion given by each system's different frequencies can lead to better interpretation of the seafloor for better planning. Acknowledgments The authors want to thank engineers Mauricio Lamenza and Mario Perrone for allowing the publication of the data for scientific purposes. ST César Félix is a geoscientist who received his M.S. from the University of São Paulo in geological oceanography. He has extensive experience in projects for oil and gas, harbor assessment and telecommunications. Félix was the technical manager on this project while working for EGS Brasil. He is now an indepen- dent consultant in geosciences and hydrography. Dr. Arthur Ayres Neto is a marine geologist with a Ph.D. in marine geophysics from the University of Kiel. After 17 years of working in the survey industry, since 2006, he has been a professor of engineering and environmental marine geophysics at Universidade Federal Fluminense. Neto worked as a consultant on this project. Dr. Luiz Antonio Pereira de Souza is a geologist with a Ph.D. in marine geology from São Paulo University. He works at the Institute for Technological Research in São Paulo, Brazil, focusing on shallow-water geophysics for projects related to infrastructure, telecommunications, pipelines, dams and hydroways. He was a consultant on this project. nize geological layers by the decimeter level. Employing this method, one can investigate thicker layers and the occur- rence of rock subcrops. In this project, two high-frequency chirp systems (2 to 8 kHz and 10 to 20 kHz) were employed to provide nominal resolution of less than 10 cm. This made it possible to resolve very thin superficial layers, with low impedance contrast, as well as boundaries between distinct sediments on the seabed, which were not clear on the back- scatter and side scan mosaics. The multifrequency seismic data also showed that errors in the interpretation of exposed structures could be avoided. For example, the 900-kHz side scan mosaic and chirp data revealed an obstruction defined by a very rugged and high- amplitude object that could be easily mistaken for a rock outcrop. However, when analyzing the boomer or sparker sections, it was possible to see that the rocky baseline was much deeper below the seafloor, indicating that it was not a rock outcrop. The opposite situation was also observed. While boomer and sparker data showed a homogenous superficial layer up to 2-m thick, the chirp data were able to identify that part of the area was covered by a thin muddy layer, with low impedance contrast relative to the layer below, but with consistent differences in its composition (mud and bioclas- tic mud, respectively), with implications for the trenching process. The higher resolution data also helped evaluate the exact position of the rocky basement when it was too close to the seafloor (with an uncertainty of 50 cm). This minimized changes on the planned route and helped to de- Example of the seismic attribute RMS calculated within the seafloor and the project's depth of burial limit identifying subcropping rock. R E L I A B L E P O W E R S Y S T E M S 407-323-3390 SALES@MAIFL.COM MATHEWS ASSOCIATES, INC. Whatever floats your !!!!!

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