A method for obtaining a continuous well log of formation acoustic compressional velocity is described. The instrument is composed of an acoustic pulse generator separated from a receiver by a five-foot acoustic insulator. Means are provided for generating an acoustic pulse and automatically selecting and recording the first-break travel time as detected by the receiver. The instrument has a range of 5,000 to 25,000 feet per second and an accuracy better than 5 percent. The log is taken in a conventional well logging manner at speeds of 100 to 150 feet per minute. Typical logs are illustrated along with their probable correlation to seismic reflection horizons and lithology in the borehole.