Abstract: It is usually unsatisfactory to use the T²-X² approach for measuring acoustic velocities of thin and hard sea-bed layers. Therefore the applications of the ray-parameter method to thin layers and the refraction method to hard layers need to be considered in an acoustic velocity measurement system composed of a sound source and a towed hydrophone streamer. Some practical problems of applying the two methods, such as echo-data processing procedures and error estimations in measuring acoustic velocities, are discussed, and the effectiveness of theoretical analyses has been verified through computer simulations.