guanabara bay bottom sediments and seabed characteristics were analysed using high-resolution (7 khz) sub-bottom profiles associated with particle size analyses of 92 bottom sediment samples. eight types of echo-characters were identified revealing the strong relation with the particle size distribution. sandy bottom areas presented strong echo reflections, without sub-bottom penetration (echo types i and iii), while in muddy areas sub-bottom reflections showed the acoustic basement delineating buried sugar-loaf hills and infilled-valley features (echo type iv). the presence of shallow gas within the sediments is indicated by acoustic blanket and a series of bottom-multiple reflections (echo types va and vb). erosion by bottom currents and artificial mechanical dredging are suggested by truncations of sub-bottom reflections and a wrinkled seabed surface (echo types vi and vii). crystalline basement outcrops on the seabed are recognized by multiple or single hyperbolae with varying elevations above the bay bottom (echo type ii).