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The traditional method of finding oil before World War II was to map the surface geology and study the relationships of various rock units. This is known as field geology. Explorers concentrated their efforts on mapping visible structures and potential reservoir rocks in areas that had some indication of hydrocarbons. An example would be oil seeps (see the diagram to the left, from the U.S. Geological Service). Petroleum exploration onshore still begins with a look at the field geology, but the aim today is to understand and predict the types of rock that might be expected in the subsurface, in prospects outlined by sophisticated geophysical means.