A method of analyzing acoustic data comprising, determining fluid sound speed through connate fluid. The method involves sampling the fluid, sending an acoustic signal into the fluid between a first and a second reflective interface. Data is recorded that represents acoustic signals over time as they are reflecting from the interfaces. A smoothed first derivative with respect to time of the cumulative sum of squares (CSS) of the filtered amplitude data is determined. This first derivative is cross correlated to the time-shifted versions of itself.