1. A program to map geologic structure by using head waves. Head waves involve energy that enters a high-velocity medium (refractor) near the critical angle and travels in the high-velocity medium nearly parallel to the refractor surface. The objective is to determine the arrival times of the head waves to map the depth to the refractors in which they travel. For a summary of refraction interpretation methods, see Sheriff and Geldart (1995, 95–98, 425–446). 2. Refraction surveying also includes the detection and mapping of high-velocity masses such as salt domes (see fan shooting) and the delineation of masses by shooting into seismometers located in deep wells (see Figures A-15 and T-8).