If you want to find oil and gas accumulations, or produce them efficiently once found, then you need to understand subsurface geology. At its simplest, this means mapping subsurface structure to find structures where oil and gas may be trapped, or mapping faults that may be barriers to oil flow in a producing field. It would be good to have a map of the quality of the reservoir as well (e.g. its thickness and porosity), partly to estimate the volume of oil that may be present in a given trap, and partly to plan how best to get the oil or gas out of the ground. It would be better still to see where oil and gas are actually present in the subsurface, reducing the risk of drilling an unsuccessful exploration well, or even following the way that oil flows through the reservoir during production to make sure we don't leave any more of it than we can help behind in the ground. Ideally, we would like to get all this information cheaply, which in the offshore case means using as few boreholes as possible.

One traditional way of understanding the subsurface is from geological mapping at the surface. In many areas, however, structure and stratigraphy at depths of thousands of feet cannot be extrapolated from geological observation at the surface. Geological knowledge then depends on boreholes. They will give very detailed information at the points on the map where they are drilled.