A type of seismic refraction in which the shots and detectors are laid out on long straight lines. Successive shots are taken at uniform or almost uniform intervals along each line, and successive detector spreads are shifted about the same distance as the corresponding shot points so as to keep the range of shot-detector distances approx. the same for all shots. Generally, shots are received from opposite directions on each detector spread. The distance range is chosen so that the first, or where desired the second, arrivals will be refracted from a particular formation such as the basement or a high-speed limestone marker. The proper distance is usually determined from time-distance plots based on experimental shooting at the onset of the program.