Geophysical studies applied to coastal engineering have shown a growing importance in unforeseen problems evolving the execution of dredging or coastal ports. This work objective is to show the importance of geophysical methods application as an important tool in the enforcement of coastal works. Bathymetric, sonographic and high resolution seismic 3D models were able to simulate, in this case, that the conventional deepening dredging to a quota around 20m (DHN) in a desired channel waterway located at Sepetiba Bay, RJ, would be rendered impossible by the presence of a crystalline rock in the channel center. The lack of a prior geophysical research allowed severe disorders, such as the review of the project, both in the positioning and in the costing scope. Thus, in order to mitigate unforeseen financial costly and environmental impacts during the implementation of coastal works, this work encourages the use of geophysics as an indispensable tool for marine research.