Seismic reflection profiling

Seismic reflection profiling is a method of applied exploration geophysics that allows scientists to determine the location of subsurface geological structures. It is accomplished by using one of the various methods of generating seismic waves. These waves are reflected to the surface from the subsurface and are received and analyzed. This analysis enables geologists to locate oil and, less frequently, mineral-bearing formations.

Body Seismic Waves

Seismic reflection profiling enables the earth scientists to determine what the earth's subsurface looks like without having to drill exploratory wells. This study of applied seismics is related to seismology, which is the study of earthquake waves. When an elastic body such as rock is stressed and suddenly breaks, the energy released is transferred through the material in the form of various types of waves. This is what happens during an earthquake. When stress builds up and the rock fractures, energy radiates out from the focus or zone of breakage in the form of an ever-enlarging sphere of wavefronts. As the sphere gets larger, the energy along any part of the wavefront is diminished or attenuated. This sphere will continue to enlarge and maintain its basic shape as long as the properties of the rock through which the waves are traveling remain the same.

The four types of seismic waves are classified into two major categories. Those waves that travel beneath the surface of the earth (that is, in a three-dimensional medium) are called body waves. Waves that travel at or near the surface of the earth (in a two-dimensional medium) are called surface waves. Body waves are caused by earthquakes that take place well beneath the surface of the earth. Surface waves are caused by near-surface earthquakes or by human-made explosions.

The two types of body waves are P, or primary, waves and S, or secondary, waves. The P waves are compressional waves and are the faster of the two types. P waves, for example, can be generated by pinching several coils of a “Slinky” toy together and releasing them. Sound waves are an example of this type of wave; sound will travel through any medium that is capable of being compressed. In a compressional wave, the particles of the conducting medium vibrate parallel to the direction of wave propagation, or travel. A compressional wave is set up by a vibration. In the case of sound waves, the vibration may be that of the human vocal chords; in the case of seismic compressional waves, it is the breaking of rock. The particles of the conducting material (air in the case of sound, rock in the case of seismic waves) nearest the point of vibration also begin to vibrate. These particles, as they move back and forth, strike additional particles and then return to their original position—the moving forward is known as compression, the moving back to the original position as rarefaction. This next group of particles moves forward, strikes another group of particles and then returns to their original positions. This repeated succession of compressions and rarefactions is how a compressional wave travels. The S waves, which are sometimes known as shear waves, are transverse waves: The particles in the conducting medium travel perpendicular to the direction in which the wave is traveling. S waves can be generated by tying a rope to a doorknob and rapidly moving the other end up and down with a flick of the wrist. Light and other forms of electromagnetic radiation are propagated by means of transverse waves. Unlike compressional waves, the secondary seismic waves will not travel through liquids.

Surface Waves

The two types of surface waves are named after the scientists who demonstrated their existence, Lord Rayleigh (English physicist John William Strutt) and English mathematician A. E. Love. Both of these types of seismic wave are of the S or transverse variety but travel at a slower speed than the body type of S wave. The Rayleigh wave travels along the surface in the vertical plane, not unlike the waves of the ocean. This type of wave differs from a transverse body wave in that instead of moving back and forth along a straight line perpendicular to the direction of wave motion, the particles in a Rayleigh wave move in an elliptical motion with the long axis of the ellipse usually vertical. Unlike other types of waves that may cause particles in their path to move back and forth a few times before coming to rest, the Rayleigh wave vibration lasts much longer. The particles will travel in their elliptical path many times before coming to rest.

The other type of seismic surface wave is the Love wave. It is a shear wave but, unlike the Rayleigh wave, the motion is parallel to the surface and at right angles to the direction of wave transmission. The vibration of a Love wave lasts much longer than that of a transverse body wave.

Wave Reflection and Refraction

By studying the paths of seismic waves generated by earthquakes, scientists have learned much about the interior of the earth. As stated earlier, waves expand in a spherically shaped shell from the focus, or center, as long as the rock properties remain the same. Should the rock properties change—for example, if the waves may encounter more or less dense material or perhaps a boundary or contact where rock properties change instantly rather than gradually—there is a significant change in the wave pattern. Some waves are reflected from that contact; others enter the new rock body and are refracted. For example, consider only the waves that a shallow earthquake will send straight down from the focus into the earth. Some of these waves will strike rock layers at various depths and will be reflected at various angles; some will return to the surface of the earth. Other waves will be refracted through the rock layers at different angles and will emerge elsewhere on the earth's surface.

The study of reflection and refraction of seismic waves has revealed that the earth is divided into three major zones: the crust, the mantle, and the core. It has been found that secondary or shear waves will not pass through the core but are reflected from it, while the primary waves pass through easily. Based on this evidence, scientists have concluded that a portion of the earth's core is a liquid. In addition to being able to find discontinuities or rock boundaries by studying the paths of seismic waves, from the study of the velocities of these waves, scientists can determine the density of the rock through which the waves pass.

During World War I, both the Allies and the Germans made some progress in using seismic detectors to locate the positions of large field artillery pieces. Some scientists who were involved with this study during the war became active in seismic prospecting development in the United States during the early 1920s. The method of seismic reflection was first used in an attempt to find oil-bearing rock in the area of the Texas Gulf Coast in the late 1920s. These early attempts were not very successful. The same techniques were being successfully employed in Oklahoma, however, and with this experience, seismic reflection prospecting became well established by 1931.

Later improvements in instrumentation and in field techniques enabled reflection profiling to be used under a wide range of geologic conditions. It is now used successfully in all oil-producing regions of the world.

Seismic Surveying

One of the methods that geologists and geophysicists use to study the earth's subsurface is seismic reflection profiling. The great advantage of this technique is that the earth scientists can have a very accurate picture of subsurface geologic strata without going to the considerable expense of drilling numerous core samples or exploratory wells. The petroleum industry, more than any other concern, uses seismic reflection techniques. Seismic surveying is used to a lesser extent for construction site evaluation, groundwater exploration, and mineral exploration.

Since the early days of seismic surveying, explosives such as dynamite have played an important role. In modern times, however, dynamite has been replaced by safer types of explosives such as ammonium nitrate. The size of the charge can be varied depending on the nature of the survey. For most surveys, the explosive is placed in a hole called the shot hole. The holes may vary in depth from about a meter to a few hundred meters. These holes are drilled by a small drilling rig attached to a truck. There are always certain disadvantages to the use of explosives. First is the always-present potential for destructive side effects, and second is the inconvenience and cost of drilling shot holes. In addition, an explosion introduces an uncontrolled range of wave frequencies.

Surveying Techniques

In practice, seismic surveying consists of placing receivers, known as geophones, at various intervals (usually about 30–60 meters apart) and using them to detect vibrations that have been reflected back to the surface from subsurface geological features. Those vibrations may be caused by an explosive charge (impulsive technique), the explosion of a mixture of gases inside a closed steel chamber in contact with the ground (dinoseis technique), the repeated dropping of a 2-ton mass from a height of 2-3 meters (thumper technique), or a vibrator located on the surface (vibroseis technique).

When the dinoseis method is used, several canisters of gas (usually three to six) are fired simultaneously, which forces more of an energetic reaction from the surface. Like the conventional explosives technique, the dinoseis method can be used to reach and identify the location of deep structures. The thumper technique does not work well except for small engineering surveys because this method produces very little energy in the form of waves that penetrate into the surface. Much of the energy generated by dropping the weight is dissipated by surface waves. The vibroseis technique was developed in the 1950s as an alternative to the use of an impulse source. In the vibroseis system, energy is produced by a vibrating pad that is pressed firmly to the ground. The pad is attached to the underside of a truck by means of hydraulic jacks; when these jacks are employed, most of the weight of the truck forces the pad against the ground. Unlike the case of the impulsive source, the frequency of the vibrations can be controlled. Typically, the frequency is varied or swept from fifteen cycles per second to ninety cycles per second over a period of several seconds. The reverse is also sometimes used, with frequencies being changed from higher to lower.

Geophones

The receiver used to detect vibrations from any one of these methods is known as a geophone. This device consists of a piece of magnetized metal attached to a container and surrounded by a suspended coil of wire. When the ground vibrates, the coil picks up these vibrations and oscillates up and down around the magnet. This action induces an electric current in the coil, which is detected: The greater the vibration of the ground, the greater the current generated. As the ground vibrates, the geophone produces a continuously varying signal.

The geophone signal is transmitted to the recording systems by means of the seismic cable. The recording device produces a record of the vibration of the ground, called a seismogram. In some seismic recorders, the signal coming in from the geophones is first amplified and then sent to galvanometers, which are devices that detect small currents. Each geophone sends a signal to a different galvanometer. This device contains a suspended coil that rotates in response to an electrical current. Attached to each coil is a tiny concave mirror that reflects light to a photosensitive paper. As the coil and mirror rotate back and forth and the paper slowly advances, an irregular line is projected on the paper. This line shows the vibrations of the ground. Marks are also placed on the chart by a timing device. It is common practice to record the signals from six, twelve, twenty-four, forty-eight, or even ninety-six different geophones simultaneously. Each geophone-amplifier-galvanometer of the system is known as a channel. With the growth of the electronics and computer industries, new types of recording devices have been developed. A modern digital seismic recording system records the incoming vibrations on magnetic tape, which can later generate the seismogram display on electrostatic paper. As geophones detect the arrival of seismic waves, the signals can be digitized for further signal processing by computers.

Seismic Crew

In seismic exploration studies, the equipment that has been described is operated by a unit known as a seismic crew. Before any actual seismic work is done, the necessary permits must be obtained. Next, the land surface must be surveyed and a decision made on where the actual tests will be conducted. If explosives are to be used, shot holes must be drilled. Two or more “shooters” handle the explosives. If vibroseis is used, one to five technicians are required to operate the large vibrator truck. A ground crew consisting of a foreman and several crew members place the geophones in their proper positions and lay out and connect the seismic cables. Depending on the size of the operation, the ground crew may consist of from about six to twenty-four people. Many more may be added to the crew when operating in a rugged terrain.

Until the 1960s, seismic crews were accompanied by two or more trained seismologists who interpreted the data. Now the interpreters are found primarily at data-processing centers. Digital tapes of the field data are delivered to the centers on a regular basis. Cornell University's Consortium for Continental Reflection Profiling (COCORP) pioneered the use of multichannel seismic reflection profiling for exploring the continental lithosphere. COCORP has collected more than 11,000 kilometers of profiling at thirty sites in the United States.

Data Interpretation

Although the actual method of interpretation of the data is quite technical, the basic principle is rather simple. A signal (vibrations) from an explosive or another source is transmitted through the ground. When this seismic energy encounters discontinuities of various types in the earth, part of that energy is reflected back to the surface, where it is detected by geophones. The signal is then recorded. The time at which the signal was sent is known, and the time at which the reflected signal was received at the geophone can be determined. If the speed of the waves through the various types of rock is known, the depth to the reflecting boundary is a matter of velocity of the wave multiplied by the time of travel.

Modern Uses for Seismic Reflection Profiling

The goal of seismic reflection is to reveal as clearly as possible the subsurface structure of the earth. The greatest use of seismic reflection continues to be in the oil exploration industry. An important advantage of this technique is that reflections are obtained from boundaries at several different depths. The depth of any reflection can be determined if the velocity of waves in that particular type of rock is known or can be determined by another method. The exact depth to oil- or mineral-bearing strata can therefore be determined without the costly drilling of test wells or drilling cores. The extent of the rock strata in question can be determined by moving the survey equipment and redoing the test as many times as necessary. In addition to its use in the oil industry, modern seismic reflection techniques may be used to determine the depth to groundwater in a given location, mapping the surface of bedrock, or estimating stratigraphy. Computational advances in data processing and analysis have significantly improved the resolution and interpretation of seismic reflection data. These developments have expanded the technique’s applications from traditional petroleum exploration into environmental assessment, groundwater studies, and geotechnical engineering.

Principal Terms

attenuated: becoming less intense as the distance from the source increases

discontinuity: the sudden change in physical properties of rock with increased depth

electromagnetic radiation: forms of energy, such as light and radio waves, that consist of electric and magnetic fields that move through space

focus: the source of earthquake waves; the actual point of rock breakage

propagated: conducted through a medium

strata: rock layers produced by sediment deposition in layers or beds

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