The most common VSP in vertical (or neat-vertical) holes, a zero-offset VSP, uses a single source located near the wellhead. In an offset VSP, the source is located some distance from the wellhead, often 900-2,100m, to give data away from the borehole. An alternative way of obtaining such data is a walkaway VSP, in which the source locations are moved to successively larger distances from the wellhead. Azimuthal VSP surveys locate sources in different directions from the wellhead to investigate changes with azimuth. Combinations of the foregoing are used in deviated holes in directional VSP surveys; in marine directional VSP surveys, a source boat often travels so that it is vertically above the geophone with another source located near the wellhead.