A simple seismic refraction method used for delineating subsurface geologic features (e.g. salt domes, buried valleys, and back-filled mineshafts) by contrasting their seismic velocities with those of the surrounding materials. Geophones are set out around a segment of arc in a fan-like array centred on one or more shot locations. A base line of geophones is used in relation to one of the shot locations to provide a time–distance curve where no subsurface feature is present, thus calibrating the travel times for a given range. Using the fan arrays, travel times of refracted rays are measured to each detector; any ray encountering a zone of anomalously high or low velocity will arrive ahead of or behind the expected travel time for that shot-to-geophone range.
Subjects: Earth Sciences and Geography.