Seismographic technology and resource exploitation

Knowledge of the Earth’s interior has been greatly enhanced by seismic data. Without this knowledge, it would be impossible to obtain the oil, natural gas, coal, metals, and other earth resources that are a foundation of industrial society.

Background

The deepest mines and drill holes penetrate only a fraction of 1 percent of the thickness of the Earth. Although such samples of the interior provide important information, they are neither distributed evenly enough over the surface nor numerous enough to provide a complete picture. Therefore, information must come mostly from indirect evidence provided by instruments that probe the interior without actually going there. Modern electronics and computer technology have greatly improved the quality of instruments now used in resource exploration and exploitation. The most widely used instrument to discover hidden resources in the Earth is the seismograph.

Seismographs

A seismograph detects and records the seismic (sound) waves traveling beneath the Earth’s surface. These waves are of two principal types: compressional waves and shear waves. In compressional waves, particles are vibrated parallel to the direction of wave propagation, whereas in shear waves, the motion is perpendicular to the direction of wave travel. Seismographs can be manufactured to record both wave types.

Generally, a seismograph consists of a sensor (seismometer or seismic detector), an electronic amplifier, filters, and a recording system. In its wide range of uses, from resource exploration and exploitation to earthquake studies, a seismograph may be required to measure ground movements from as small as one millionth of a meter to as large as several meters, a range of more than ten orders of magnitude. The seismic detector (sensor) consists of a weight suspended from a frame by a delicate spring. The frame moves with the ground, but because of its inertia (mass), the weight tends to remain stationary. Attached to the weight is a coil of electrical wire. Ground motion moves the coil in a magnetic field created by a magnet attached to the frame of the seismograph. The relative motion between the coil and the magnet converts the mechanical ground motion into an electrical signal that passes through an amplifier. The amplified voltage controls a recording device that marks the ground motion on a moving sheet of paper. The recorded information is called a seismogram.

Sources of Seismic Waves

Our knowledge of the Earth’s deep interior has been mainly conveyed by earthquakes, most of which are caused by the sudden movement of rock masses. As these rocks grind together, energy is released that produces both compressional and shear waves. These waves spread throughout the Earth like the ripples made by a pebble tossed into a quiet pond of water. As the waves spread outward, some are reflected and some are refracted. The reflected waves travel downward to boundaries between rock layers, where they reflect (bounce or echo) back to the surface, while the refracted waves follow paths that bend at each rock boundary. Eventually, these seismic waves reach the Earth’s surface, where they are detected by a seismograph.

Earthquakes release the large amounts of energy needed to probe the deep layers (mantle and core) of the Earth. Other methods can produce seismic waves that can be focused on the geologic features closer to the Earth’s surface. These waves can be generated by artificial explosions, as of a charge of dynamite, or by dropping a weight or pounding the ground with a sledgehammer. To eliminate the environmental risks associated with using explosives, seismologists may use a system called vibroseis (pronounced VI-broh-size). In this system, a huge vibrator mounted on a special truck repeatedly strikes the Earth to produce sound waves.

Geophysics

The branch of Earth science dealing with the analysis of seismographic data is geophysics, the science that applies physics to the study of the Earth and its environment. Geophysicists can use the speed of seismic waves recorded by a seismograph to determine the depth and structure of many rock formations, since the speed varies according to the physical properties of the rock through which the wave travels. Seismology is the field of geophysics that deals with the study of seismic waves produced by earthquakes or other sources, such as vibroseis. These studies have helped determine the location of many natural resources in the Earth and have led to a better understanding of earthquakes and other processes that shape the Earth.

Seismic Exploration with the Seismic Reflection Method

In a seismic survey, geophysicists typically arrange seismic detectors along a straight line (profile) and then generate sound waves by vibroseis or an explosion. A seismograph records how long it takes the sound waves to travel to a rock layer, reflect, and return to the surface. The equipment is then moved a short distance along the line, and the experiment is repeated. This procedure is known as the seismic reflection profiling method. Beginning in the mid-1980’s, the advent of high-resolution seismic detectors and digital engineering seismographs led to applications of reflection seismology to environmental, groundwater, and engineering problems, as well as to oil, gas, and exploration and exploitation.

Using a variety of computer programs, researchers process recorded data (seismograms) to generate cross-sectional images of the Earth to a depth of 3 kilometers or greater. Such cross-sections present an image of the rock layers beneath the seismic line. Based on the characteristic geometries for oil and gas traps and mineral deposits, these images outlining rock structures are used to predict where oil, natural gas, coal, and other resources, such as and mineral deposits, are most likely to exist in the subsurface. Geophysicists and geologists cannot tell whether oil or other resources will be found for certain, but using processed seismic data as a basis for deciding where to drill makes it much more likely that the resource will be found.

By using highly sensitive seismographs, geophysicists can detect the changes that occur in the amplitude (height) of the recorded sound waves. Sound waves change in amplitude when they are reflected from rocks that contain gas and other fluids. Such changes appear as irregularities, called bright spots, on the recorded sound wave patterns, and they indicate the presence of fluids in underground and underwater rock formations. In addition, with carefully planned seismic surveys that collect both compressional and shear wave data, the fine details of seismic records can be used to infer the types of rocks in the subsurface.

Exploitation of Oil and Other Resources

Historically, most applications of seismic technology have been limited to exploration. However, seismic reflection data can be used not only to explore for new oil and gas reservoirs and other resources but also to exploit existing reservoirs and resource deposits by more extensively mapping these locations in order to yield optimum production. Many companies now use teams of geophysicists, geologists, and engineers to plan the acquisition of data from the best sources and analyze and integrate all the information into a consistent description of the and/or deposit. This team approach requires that each member understand the technology involved in obtaining reliable, accurate data so that the best possible information is used to estimate reservoir and/or deposit properties.

The geologic detail needed to develop most hydrocarbon reservoirs substantially exceeds the detail required to find them. For effective planning and drilling, a complete understanding of the lateral extent, thickness, and depth of the reservoir is absolutely essential. This understanding can be achieved with only detailed seismic interpretation of three-dimensional seismic surveys. In three-dimensional seismic reflection surveying a common practice is to place seismic detectors at equal intervals and collect data from a grid of profiles (lines) covering the area of interest. As more wells are drilled in the area, the three-dimensional data evolve into a continuously utilized and updated management tool that affects reservoir planning and evaluation for years after the original seismic survey.

Ultimately, knowledge of subsurface comes only from drilling the targets that have been determined to be most likely to contain the resources sought. Since drilling deep drill holes is very expensive, applied seismic technology is a key to cost-effective exploration and exploitation of oil, natural gas, and many other natural resources.

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