Magnetic and Electromagnetic Surveys
Geologists searching for petroleum have made good use of the fact that the earth has a strong magnetic field. Applying the principle that like rocks have like magnetic fields enables the geologist to compare slight differences in the magnetism generated by the minerals in the rocks. These differences suggest the locations of different formations. For example, igneous rock, frequently found as the basement rock under sedimentary layers, often contains minerals that are magnetic. Such rock seldom contains hydrocarbons, but, as described in chapter 1, it sometimes intrudes into the overlying sedimentary rock, creating arches and folds that may serve as hydrocarbon traps. Through magnetic surveys, geophysicists can get a fairly good picture of the configuration of the geological formations. A formation out of place with its surroundings may indicate the presence of a fault, which may be associated with hydrocarbon deposits.
The magnetometer detects slight variations in the earth's magnetic field. These variations mainly show how deep magnetized rocks are buried. A magnetometer can be towed behind a ship or an airplane to cover large areas. It transmits data to a device on board which records the information onto paper or magnetic tape.
A development of airborne magnetics is the micromagnetic technique for oil exploration. An airplane tows a micromagnetometer from a low altitude, normally about 300 feet (91 metres) above the ground. It detects micromagnetic anomalies, or deviations from the norm. A computer then processes the magnetic data tapes from the aircraft, and geologists use these analyses to predict fractures in the basement and the characteristics of the overlying sediments.
Magnetotellurics operates on the theory that rocks of differing composition have different electrical properties. Geologists record and measure the naturally occurring flow of electricity between rocks or across salt water, for example, and then analyze and interpret the information to reveal subsurface structures. Company geologists use magnetotellurics primarily in reconnaissance (exploratory) surveying, although improved data processing techniques have made it increasingly useful for development surveys. The amount of detail that the method yields depends on the distance between survey sites. Closely spaced sites result in a detailed survey, which is helpful in deciding where to drill new wells near an area of proven production. Regional surveys provide geologic cross sections, or cutaway views of the formations. Although techniques continue to improve, magnetic and electromagnetic surveying does not assure the detection of all traps that contain hydrocarbons. Such surveys are useful, however, for giving the geologist a general idea of where oil-bearing rocks are most likely to be found.
Geophysicists also make use of slight variations in the earth's gravitational field caused by the varying weight of rocks. Some rocks are denser than others; that is, a square yard (or metre) of dense rock weighs more than a square yard (or metre) of less dense rock, in the same way that a lead ball is denser than a cotton ball.
Very dense rocks close to the surface exert a gravitational force more powerful than that of a layer of very light rocks. Geophysicists applied this knowledge particularly in the early days of prospecting off the Gulf Coast (fig. 2.4). They could often locate salt domes by gravitational exploration because ordinary domes and anticlines are associated with maximum gravity, whereas salt domes are usually associated with minimum gravity.
The torsion balance, first marketed commercially in 1922, was one of the earliest gravitational instruments invented. It, as well as another early instrument—the pendulum—was rather difficult to use. Today, the most common instrument is the gravimeter or gravity meter, a sensitive weighing instrument for measuring variations in the gravitational field of the earth.
Although the basic principle of the gravitational method remains the same, new technology and instruments continue to improve data collection. A small, portable, highly accurate gravimeter is now available for land work.
Figure 2.4 Seismic exploration leads geophysicists from the deserts of Saudi
Arabia (A), to Utah's canyon lands (B), and into the freezing plains of Alaska (C). (Courtesy of American Petroleum Institute)
Also, gravity data can be collected onboard a ship with a great deal of accu racy, and from the air as well, but with less resolution. Interpreters now use computers to help analyze how gravity variations relate to geology. Gravit) maps and models help the geologist examine large areas of developmeni and provide guidelines for planning a seismic exploration program.
A seismic survey is usually the last exploration step before an oil company actually drills a prospective site. Unlike gravity, magnetic, and electromagnetic surveys that provide general information, seismic surveys give the explorationist more precise details on the formations beneath the surface.
Seismology works because the earth's crust has many layers with different thicknesses and densities. When energy from the surface, such as an explosion, strikes the layers, part of it travels through the layers and part of it is reflected back to the surface. It is like bouncing a rubber ball. If you drop the ball on the sidewalk, its bounce will be quite different than if you drop it onto a pile of sand.
In a similar way, each different layer in the earth "bounces" seismic energy back to the surface with its own particular characteristics.
Seismic surveys start with small artificially produced "earthquakes." Sensors called geophones pick up the reflected seismic waves and send them through cables to a recorder. The recorder, a seismograph, amplifies and records their characteristics to produce a seismogram (fig. 2.5). Seismograms generate a seismic section, which is a two-dimensional slice from the surface of the earth downward. The information from a seismic survey indicates Figure 2.5 A seismic section indicates the types of rock, their relative depth, and whether a trap is present boundaries between formations
The type of seismic section described above is now known in the industry as a 2D (two-dimensional) seismic section because a new technique has developed called 3D (three-dimensional) seismic surveying, or just 3D seismic for short. In this technique a company runs many seismic surveys close together to create a series of seismic sections of an area perhaps 2 or 3 miles (3 to 5 kilometres) square. Computer programs "paste" these sections together to form a cubic picture of the area. The advantage of 3D seismic is that, with the help of computers, an explorationist can slice the cube in any direction— north-south, east-west, horizontally, or on any other plane . Focusing on an area in this way provides much more reliable information about the geologic structures it contains.
3D seismic is becoming popular offshore, particularly in the Gulf of Mexico, where the areas surveyed tend to be larger than on land. An exploration company uses it where it already has a good idea that a large enough off accumulation exists to justify the expense.You may also hear the term 4D seismic, which refers to repeated 3D surveys (through time—the fourth dimension) to monitor changes in the formations, primarily changes in fluid levels.
The first seismometer, as inventor David Milne called it, was used in 1841 to measure and record the vibrations of the ground during earthquakes. A few years later, Italian L. Palmieri set up a similar instrument, which he called a seismograph, on Mount Vesuvius. From these simple beginnings seismic exploration evolved.