Gravity surveys measure the effect on the earth's gravitational field of variations in the densities of subsurface rocks. Basement rocks in general have a higher density than the overlying sediments and where this is the case anomalously high gravity values are recorded when the basement rocks approach the surface. Conversely, low gravity values are recorded over depressions in the basement surface. Gravity surveys can, therefore, be used to outline sedimentary basin development and show structural trends in the basin. Variations in densities also occur in the sedimentary rocks and when older and denser rocks are brought nearer the surface in the cores of anticlines and other structures anomalously high gravity values are recorded. The density of salt is generally lower than that of the surrounding rocks and anomalously low gravity values are frequently associated with salt structures.