(1) (n.) Geophysics. A line joining base stations whose transmissions are synchronized during surveying. See: Base Station, Survey. (2) (n.) Geophysics. A reference line, such as a "shale baseline, " a line representing the typical value of a given measurement for a shale on awell log, or the zero-amplitude line of a seismic trace. See: Bias. (3) (n.) Geophysics. The original survey of a set of surveys covering the same area but acquired over a period of time. In four-dimensional seismic data, it is the first seismic survey, which is then compared to subsequent surveys