We see that the horizons that we draw on seismic sections provide us with a two- dimensional picture only. A three-dimensional picture is necessary to determine whether closure exists, the area within the closing contour, the location of the highest point on the structure, and so on. To obtain three-dimensional information, we usually shoot lines in different directions. Most reflection surveys are carried out along a more-or-less rectangular grid of lines, often with common shotpoints at the intersections of lines to facilitate correlating reflections on the intersecting profiles.