A technique which provides a record of the single travel time a sound wave takes to pass through the sidewall of a borehole from a sonde source to a receiver, when these are a constant distance apart and on the same down-hole tool. As the tool is drawn up the borehole to the surface a record of the variations in travel time is plotted as a function of down-hole depth. Measured in units of 10^-6 feet per second, it provides the inverse of the formation velocity.