Abstract: A system composed of a seismic profiler and a hydrophone streamer towed behind the profiler has been considered for acoustic velocity measurements of sea-bed sediment layers. It is indicated that the T²-X² approach with Common-Depth-Point processing is the basic way to measure acoustic velocities of arbitrarily dipped layers. Based on Shah's ray-tracing formulation, equations for velocity computations of the layers below sea-bottom are derived and procedures to solve them explained. Through the analysis of errors in measuring velocities, which are caused by deviation of the time-distance curve from an ideal hyperbola, instability of towing speed, echo-time jitters and streamer declination, the problem of determining the major parameters of the measurement system is clarified.