II. Read and translate the text

Drake’s original well was drilled close to a known surface seepage of crude oil, and for many years such seepages were the only reliable indicators of the presence of oil. As demand grew, however, new methods were devised for exploring the petroleum potential of underground rock formations. There are now three major exploration methods: (1) surface feature mapping, (2) seismograph observations, and (3) Earth gravity surveys. But there is still no accurate way to predict the actual presence of underground petroleum deposits. Exploration methods carried out on the surface can only indicate the presence or absence of underground formations that are favourable for petroleum accumulation.

Surface methods. A seepage of crude oil sometimes appears as a tarlike deposit in a low area such as the oil springs at Baku, on the Caspian Sea, described by Marco Polo. More often, it can be seen as a thin skim of oil on small creeks that pass through the seepage area. This latter phenomenon was responsible for the naming of Oil Creek (a place in Pennsylvania), where Drake's well was drilled. Such seepages of natural gas cannot usually be seen, though instruments can detect natural gas concentrations in air down to less than one part in 100,000. Exploration methods have been used in which air samples from areas immediately over or under the ground surface are tested. Such samples taken from a wide area can yield a map of gas concentration points. A similar method has been used to test for traces of petroleum gases dissolved in seawater in the hope that undersea seepages could be thus detected. These geochemical prospecting methods are not applicable to the large majority of underground reservoirs that do not have leakage to the surface.

A second class of surface prospecting methods is based on surface indications, of underground rock formations. In some cases, subsurface folds and faults in rock formations are repeated in the surface features. The presence of underground salt domes may be indicated by a low bulge in an otherwise flat ground surface. Since these features usually produce possible petroleum traps, drilling in the area may be productive.

Seismographic methods. most folds and faults in subsurface rocks are not apparent at the ground surface. Seismic methods that have been useful for mapping such underground formations involve the transmission and echo-producing properties of rock in a manner similar to underwater sonar methods. Sound waves travel at differing velocities depending on the types of rock formations and are reflected at interfaces between different types of rock. The sound wave source is usually a small explosion in a shallow drilled hole. Microphones are placed at various distances and in various directions from the source to record the arrival times of the echoes. This procedure is repeated at intervals over a large area. An experienced seismologist can then interpret the records to map the underground formation contours. Possible locations for petroleum accumulations may then be located for drilling.

Gravity meter surveys. The gravity meter has become a primary exploration tool. Although gravity at the Earth's surface is very nearly a constant, it is slightly greater where large rock formations lie closest to the surface. The gravity field, therefore, increases over the tops of anticlinal (arch-shaped) folds and other similar subsurface anomalies. A salt dome results in lowered gravity because salt is less dense than other rocks. Gravity surveys are usually carried out from the air. A sensitive instrument, the gravity meter, is suspended on a cable below the aircraft, while the recording devices are operated from inside. As the aircraft is flown over an area on a precise grid pattern, a map showing the gravity strengths is prepared from the recordings. These maps can show the presence of such possible accumulation spots as anticlinal folds, fault blocks, and salt domes, even though there are no surface indications of such. The gravity meter survey is faster and less expensive than the seismic method. It can locate only large rock features that exist relatively close to the surface, however, and is generally used as a preliminary survey tool to locate areas in which seismic work may profitably be concentrated.