PROSPECTING

search for economically exploitable mineral deposits. Up to the 20th century, prospecting was done by men roaming likely areas on foot and recognizing gold, iron, lead, or other valuable ores by sight. Certain types of mineral deposits are associated with certain types of rock and land forms; copper, lead, and zinc, for example, generally appear in igneous rock formed by cooling of masses of molten minerals at or near the Earth's surface. Geologists can sometimes infer the extent of deposits by mapping outcroppings; drilling is then used to confirm the estimates. In the 20th century, more sophisticated techniques developed as the result of the maturing of the physical sciences and the need to seek minerals beneath the surface. Some valuable minerals, such as iron and copper, are magnetic: first the compass and later the more sensitive magnetometer have been used to detect them. The gravimeter, an instrument that can detect minute changes in the Earth's gravitational field, can be used to detect certain minerals that have densities different from those of the surrounding formations. Some sulfide mineral deposits have undergone partial oxidation, and the resulting nonuniformity in chemical composition creates an electric potential that causes currents to flow in the surrounding ground; voltmeters can be used to detect them. Another electrical method involves implanting electrodes in the ground and tracing the current between them by means of a galvanometer; the current will seek out conductors in the ground. Seismic methods utilize information gained from the transmission of natural (earthquake) and artificial shock waves by different underground bodies. In systematic seismic exploration, a hole is drilled and an explosive charge set off in it; seismic waves, travelling to the boundaries between different rock layers and reflected from these layers, can be timed, and the types of rock deduced. Ores of uranium and thorium give off radiation that can be detected by suitable instruments such as the Geiger counter. Geochemical methods of prospecting involve chemical analysis for traces of metals in soils, vegetation, and stream water or silt. Methods of prospecting for oil and natural gas are similar to those used for minerals.