Identification of acoustic patterns in gas-rich sediments of coastal environments of Rio de Janeiro

摘要

The presence of free gas within shallow Quaternary marine sediments has been known for some considerable time. While some geophysicists state that gassy sediments represent a problem for imaging subsurface layers by adversely affecting seismic signal penetration, many researchers of varied backgrounds view them with interest for being an indicator of organic matter accumulation. The project's main objective is to identify gassy sediments and map its spatial distribution in three different coastal environments of the city of Rio de Janeiro (Guanabara and Sepetiba bays and Rodrigo de Freitas Lagoon), through the use of high-frequency seismic surveys. The results show that the occurrence of gas is common in the three environments. However, the occurrences are different in form and areal extension. In the Guanabara Bay the occurrences are mainly as acoustic turbidity zones extending for several kilometers. In the Rodrigo de Freitas Lagoon and Sepetiba Bay, the gas occurrences are more restricted in area usually associated with paleo-topographic depressions (paleo-channels?). In the second turbidity curtains are also visible in some areas. The different gas occurrences are indicative to understand the geological evolution of coastal environments after the last maximum glacial. Moreover, gassy sediments represent a risk for subsea engineering making their precise identification important factor for the project.