EARTH EXPLORATION

the investigation of the surface of the Earth and of its interior. By the beginning of the 20th century most of the Earth's surface had been explored, at least superficially, except for the Arctic and Antarctic regions. Today the last of the unmarked areas on land maps have been filled in by radar and photographic mapping from aircraft and satellites. One of the last areas to be mapped was the Darin peninsula between the Panama Canal and Colombia. Heavy clouds, steady rain, and dense jungle vegetation made its exploration difficult, but airborne radar was able to penetrate the cloud cover to produce reliable, detailed maps of the area. In recent years data returned by Earth satellites have led to several notable discoveries, as, for example, drainage patterns in the Sahara, which are relics of a period when this region was not arid. Historically, exploration of the Earth's interior was confined to the near surface, and this was largely a matter of following downward those discoveries made at the surface. Most present-day scientific knowledge of the subject has been obtained through geophysical research conducted since World War II, and the deep Earth remains a major frontier in the late 20th century. Exploration of space and the ocean depths has been facilitated by the placement of sensors and related devices in these regions. Only a very limited portion of the subsurface regions of the Earth, however, can be studied in this way. Investigators can drill into only the uppermost crust, and the high cost of drilling severely limits the number of holes that can be dug. The deepest borehole so far drilled extends only to a depth of about 10 kilometres (6 miles). Because direct exploration is so restricted, investigators are forced to rely extensively on geophysical measurements (see below Methodology and instrumentation). Additional reading Texts on methodology and discoveries in Earth exploration include Arthur W. Rose, Herbert E. Hawkes, and John S. Webb, Geochemistry in Mineral Exploration, 2nd ed. (1979); Robert F. Legget and Allen W. Hatheway, Geology and Engineering, 3rd ed. (1988); Sampling of Soil and Rock (1971), published by the American Society for Testing and Materials; Martin H.P. Bott, Interior of the Earth: Its Structure, Constitution, and Evolution, 2nd ed. (1982); George D. Garland, Introduction to Geophysics: Mantle, Core, and Crust, 2nd ed. (1979); Robert E. Sheriff (compiler), Encyclopedic Dictionary of Exploration Geophysics, 3rd ed. (1991); Robert E. Sheriff, Geophysical Methods (1989); Robert E. Sheriff and L.P. Geldart, Exploration Seismology, 2 vol. (198283); W.M. Telford, L.P. Geldart, and Robert E. Sheriff, Applied Geophysics, 2nd ed. (1980); Chuji Tsuboi, Gravity (1983; originally published in Japanese, 1979); W.D. Parkinson, Introduction to Geomagnetism (1983); J.A. Jacobs, A Textbook on Geonomy (1974); and Floyd F. Sabins, Jr., Remote Sensing: Principles and Interpretation, 2nd ed. (1987). Robert E. Sheriff