This report presents a fast inversion technique to determine the sound velocity of the topmost sediment layer using broadband reflection loss data in the frequency range 0.5-4 kHz. The measurements of bottom reflections were performed in June 2005 at a test site with a sandy bottom. The acoustic signals were transmitted by a moving source which was allowed to pass a vertical receiver array at close distance. The measured bottom reflections at different angles of incidence towards the bottom were compared to model calculations based on the Rayleigh model of plane wave reflections. A best fit between computed and measured reflection loss data is sought by using an inversion scheme in which different sets of bottom parameters are tried. The inversion results show that the velocity of the sediment at the test site varies between 1480 to 1700 m/s. A computationally intensive full-field model was used to check the accuracy of the Rayleigh model. The run time of an inversion amounts to a few seconds. It implies that the proposed technique can be used in real-time for inversions of data collected on moving platforms. This work is part of an ongoing development of operational techniques for REA (Rapid Environmental Assessment).