Seismics

Type of physical science: Classical physics

Field of study: Acoustics

"Seismics" refers to waves, such as acoustic or shock waves, that travel through the earth. The study of such waves is important in understanding earthquakes and in the search for mineral resources.

Overview

Seismics pertains to the study of waves that travel through the earth. Such seismic waves may be generated by an earthquake or by a man-made source, such as a dynamite blast, nuclear test, or the simple impact of a mass on the ground.

Seismic waves travel out in all directions when generated by a source, similar to the small ripplelike waves created when a stone is tossed in a pond of water. The concentric circles that spread from the point of impact of the stone consist of wave crests. Such a circular pattern of wavelets is termed a wave front and has many analogs--for example, the wave front from a sonic boom acoustic wave. The radius of the circular ripples represents the direction of travel of the wave outward from the source, or wave path. Some of the seismic waves will travel a path along the earth's surface and are termed "surface waves." Others, termed "body waves," travel along paths into the earth and eventually return to its surface.

Surface waves can be divided into two types, based on the way in which rock material moves when a surface wave passes through it. Rayleigh waves force rock material to move primarily in a vertical plane perpendicular to the earth's surface. There is also a small component of back-and-forth movement in the horizontal plane in the direction of travel of the wave as it moves outward from the source. The overall path of motion of the rock material is elliptical and counterclockwise. Love waves, on the other hand, set rock material in motion solely in a horizontal direction, parallel to the earth's surface and perpendicular to the direction of travel of the wave outward from the source. The motion of the rock material in the case of both Rayleigh and Love waves as they pass a particular point can be said to be repetitive or oscillatory as successive wave fronts pass through.

Body waves may be divided into two types, based on the motion of rock material in the path of the wave. The fastest traveling of all seismic waves is named the primary, or P wave. This wave is identical to an acoustic or shock wave; as a wave front encounters rock material, it successively compresses and stretches the material in the direction of travel of the wave outward from the source. Consequently, the material begins to oscillate in a back-and-forth motion parallel to the wave path direction. Since all matter may be compressed, these P waves are able to travel through liquids and gas as well. In the air, such waves become acoustic or sound waves. The secondary, or S wave, is similar to the Love wave in that it sets rock material in motion transverse or perpendicular to the wave path. Yet, since the S wave travels through the earth's interior, the motion of material may be in any direction at right angles to the wave path in contrast to the solely horizontal motion of material from the Love wave. The transverse motion of material in response to the S wave is also repetitive or oscillatory because of the passage of multiple wave fronts.

As primary and secondary body waves spread outward from a source into the earth's interior, they will reemerge eventually at the earth's surface. They may do this by traveling all the way through the earth and reemerging on the other side. They may encounter a boundary inside the earth and be deflected so that the change in wave path direction causes them to reemerge at the surface of the earth.

Deflections by boundaries are of two types: reflection and refraction. Reflection at a boundary is common to all types of waves. Some of the wave energy will penetrate the surface of the boundary, while the rest may bounce or reflect off it. The angle between the wave path of a reflected wave leaving the boundary and a line perpendicular to the boundary is equal to the angle of approach of the wave path. The incoming and reflected wave paths make a "V" pattern, with the point of the V at the boundary. The "V" is bisected by the perpendicular to the boundary, such as the shaft of an arrow. This knowledge allows scientists to predict where the reflected wave will reemerge at the surface of the earth.

A wave that penetrates the surface of a boundary is usually refracted. Refraction is the bending or deflection of the wave path as the wave crosses a boundary. This bending occurs because of a change in the physical properties of material across a boundary. A commonly observed example would be the refraction of light waves from air to water. If one places a straw in a glass of water and looks along the length of the straw, it appears bent at the water surface. This is caused by the bending of the light waves as they refract across the surface of the water. The same phenomenon can occur across the boundary of two rock layers with different physical properties.

The amount of refraction or bending of a wave path that occurs depends upon the angle of approach of the incoming wave as it approaches the boundary. This angle of approach is between the wave path and perpendicular to the boundary. The degree of bending of the wave path also depends on the velocity of the wave in the two materials. In 1621, Willebrord Snell originally developed a relation for the refraction of light waves, which included both velocity and angle of approach of the wave, known as the incident angle. For seismic waves, the corresponding relation is O_i/sinO_r=V_i/V_r, where O_i is the incident angle and O_r is the angle of the refracted wave. V_i represents the velocity in the incident medium, and V_r is the wave velocity in the refracting material.

Laboratory experiments designed to send seismic waves through rocks show that velocity is related directly to the physical properties of the rocks. The denser and more rigid a rock material is, the higher the velocity of a seismic wave as it passes through. For example, seismic wave velocities are much higher in granite than in clay. Liquids, since they have no rigidity, will not transmit S waves at all, and P waves pass through liquids with reduced velocities. S waves require rigidity to return energy to the wave path and therefore will not travel through either liquids or gases.

Applications

Much of what has been learned about the interior of the earth depends upon the study of body waves that reemerge at the earth's surface. Such study involves an analysis of the relation between velocity and physical properties of the rock materials as well as the path the wave has followed.

Seismic waves are an important tool in exploration for petroleum and natural gas. Wave velocity may be calculated by setting off dynamite in the bottom of a well borehole. By measuring the time of travel to the surface and knowing the distance, the velocity can be calculated. Elsewhere in the same area, with an energy source at the surface, the knowledge of a calculated velocity and the total travel time down to and back from a reflecting boundary allows the calculation of depth to the boundary, using the relation h = V(1/2)t, where t is the travel time for a vertically traveling body wave, V is the calculated velocity, and h is the depth to the reflecting boundary. Such a boundary could be, for example, the upper surface of a sandstone layer containing petroleum or natural gas.

Depth to a boundary of a hydrocarbon-bearing layer may also be obtained by study of refracted waves. For any boundary across which seismic waves refract, there will be one particular angle of approach that will result in the seismic body wave bending at 90 degrees to the perpendicular to the boundary and traveling along beneath the boundary parallel to it. Such an incident angle is called the critical angle. Such waves, as they travel along the boundary, will leak energy back to the surface at the critical angle. These waves are termed critically refracted and will emerge at the surface relatively close to the source.

Depth to a boundary may be obtained by studying critically refracted waves using a plot of travel time versus distance. Such a plot, known as a travel time chart, will consist of lines that connect the points representing the time and distance traveled by the critically refracted wave as it moves along from the source. The slope of such a line is a measure of the velocity of the critically refracted wave. Therefore, the velocity of the critically refracted wave can be calculated directly from the travel time chart. Depth to the boundary is related directly to travel time; as a result, knowledge of the velocity is an important step in determining depth.

Other key information in determining depth to a boundary from the critically refracted waves comes from studying the direct body wave, which travels directly from the source through the material above the boundary. In this approach, the direct wave is compared to the critically refracted wave. The situation is similar to watching two cars, which both begin at a freeway ramp. The first car travels the shorter distance along the parallel frontage road to the next ramp. The second car travels the longer route up the ramp along the freeway, and back down the second ramp. The car to arrive first will depend on distance and velocity. If the distance between ramps is relatively short, distance will determine the winner, which will be the car on the frontage road. As distance between ramps increases, the velocity of the second car on the freeway becomes important, and at some crossover distance the freeway car represents the critically refracted wave and the frontage car represents the direct wave. The distance between the frontage road and the freeway is analogous to the depth to the boundary. The crossover distance is a function not only of velocity but also of distance or, in this case, depth. The crossover distance may also be determined from the travel time chart. The crossover distance and velocities of the direct and critically refracted waves may be combined in an equation to determine depth (h): h=X_c(V_r-V_i)/(V_r+V_i) where X_c is the crossover distance, V_r is the wave velocity, and V_i is the velocity in the incident medium. Determination of velocity is an important step in the determination of depth to a boundary for both the reflected and the refracted wave. Velocity is also important in determining the physical properties of the materials in the earth's interior.

Body waves that follow paths close to the center of the earth encounter a boundary that refracts waves across it. P waves that cross the boundary show a decrease in velocity, while S waves do not reemerge on the other side of the earth. It is, in fact, understood that the S waves disappear, creating a shadow zone of no S wave arrivals on the opposite side of the earth from the source. This fact suggests that the material inside this boundary--at the core of the earth--must be liquid or gas. Since the material of which the earth is composed cannot exist as a gas at the crushing pressures created at such depths by the overlying material, then the outer part of this core region of the earth's interior must be a liquid. A study of the velocity of P waves that pass directly through the center of the earth shows that they arrive sooner than they should if the entire core of the earth were liquid. This indicates that there is a higher-velocity solid core within the liquid core at the earth's center.

Other changes in physical properties of materials inside the earth have been detected by the study of the velocity of body waves. One such change occurs at 640 kilometers beneath the surface of the earth, where an increase in velocity of seismic waves has been detected. This change has been termed the "640-kilometer discontinuity." It has been suggested that this discontinuity represents the point at which the crystal structures of rock materials collapse because of the tremendous pressure of overlying material. That would result in the formation of a new, denser and higher-velocity material, which would be more stable under great pressure.

The latest technology and advances in computers and computer science have enabled scientists to extract more data from body waves. Using very fast and very large-capacity computers, it has become possible to study in detail the paths and velocity of literally thousands of body waves. This approach has made possible the three-dimensional velocity mapping of large regions of the earth's interior. Termed seismic tomography, this technique is analogous to the CAT (computerized axial tomography) scan technique used in medical technology.

Another method to study the properties of rock materials is through analysis of surface wave velocities. Large shallow earthquakes produce abundant surface waves. The more slowly oscillating the surface wave is, the greater the depth at which material will be set in motion by it. The velocity of the surface wave, as with the body wave, is related directly to the physical properties of the rocks it passes through. For an earthquake such as the great Alaskan tremor in 1964, surface waves were generated that oscillated so slowly that they actually set material in motion at the very center of the earth.

Context

The important theoretical basis of seismics was provided by the work of scientists Snell, Lord Rayleigh, and Hough Love. Snell developed the first important relationships describing the behavior of waves at boundaries. Rayleigh and Love developed the mathematical relationships for the propagation of the seismic waves that bear their names. Important contributions were also made by other scientists who gathered and analyzed experimental data from seismic waves. Robert Mallet, in 1850, was the first to measure seismic wave velocities from explosive sources. Richard Dixon Oldham, studying early earthquake records, proposed the existence of a core to the earth, later corroborated by the work of Beno Gutenberg.

The perfection of the recording of reflected seismic waves by J. C. Karcher in 1921 was a great leap forward in the application of seismic waves to petroleum exploration. The subsequent development of this tool has so expanded that the use of reflected seismic waves is the most important tool in minerals exploration, particularly in the search for oil and natural gas deposits.

The recording of reflected seismic waves is also the leading means of analysis and study of the structure of the earth's crust. This method has been applied very successfully in the United States, where it has led to the discovery of large buried chambers filled with molten lava in New Mexico and California. This technique has also detected and outlined the geometry of giant continental welds, or sutures, where pieces of continental crust have collided, such as beneath the Appalachian mountains of the Eastern United States.

The ability of continents to collide or move at all was doubted for decades, once continental drift had been proposed by Alfred Lothar Wegener in 1910. This doubt resulted from the fact that no explanation could be suggested for the movement of the continents since there was very little known about the interior of the earth. A study of seismic body waves led to the recognition of a low-velocity zone of weak or plastic rocks beneath the earth's crust. It was recognized immediately that it was on this low-velocity zone that the earth's crust was able to slip or move, carrying the continents along in passive fashion, like passengers on a raft.

The application of techniques developed from the study of seismic waves is the most important source of data known about the interior of the earth. These techniques have been applied both in minerals exploration and in understanding the earth's interior structure and its relationship to the evolution of Earth.

Principal terms:

BODY WAVES: seismic waves that travel through materials; they are of two types, compressional, or P waves, and transverse, or S waves

CRITICAL ANGLE: the angle of approach of a body wave to a boundary that will cause the body wave to bend or refract to travel parallel to the boundary after crossing it

OSCILLATION: a repetitive pattern; rock material will repeat its pattern of motion as seismic waves pass through it

P WAVE: a seismic body wave that compresses and extends material in its path parallel to the direction of travel of the wave outward from the source

REFLECTION: the return or bouncing of a wave off a boundary, such as the boundary between two rock layers of different physical properties

REFRACTION: the bending of the path of a seismic wave when it crosses the boundary between two materials of different physical properties

RIGIDITY: the tendency for material to resist a change in shape; rocks have strong differences in rigidity, while liquids and gases do not possess rigidity

S WAVE: a seismic body wave that sets material in motion perpendicular to the direction of travel of the wave outward from the source

SNELL'S LAW: an equation that relates seismic velocity to the angle of approach of a wave to a boundary; this relationship allows one to predict the angle of refraction, or bending, of a seismic wave across a boundary

SURFACE WAVES: seismic waves that travel along the surface of the earth

Bibliography

Bolt, Bruce A. EARTHQUAKES. Rev. ed. New York: W. H. Freeman, 1988. This book is intended as a primer for the nonscientist and requires no previous background. Contains a brief summary of plate tectonics and a good introduction to seismic waves.

Golden, Frederic. THE TREMBLING EARTH. New York: Charles Scribner's Sons, 1983. An introduction to earthquakes in nontechnical terms for the pre-college reader. Includes an excellent section on the application of the study of earthquake waves to understand the earth's interior.

Karcher, J. C. "The Reflection Seismograph." In GEOPHYSICAL: THE LEADING EDGE OF EXPLORATION. Vol. 6. Tulsa: Society of Exploration Geophysicists, 1987. A fascinating firsthand account by the inventor of the reflection seismograph and its application to exploration for petroleum and natural gas.

Kock, W. E. SOUND WAVES AND LIGHT WAVES. Garden City, N.Y.: Doubleday, 1965. An excellent chapter on wave motion treats wave velocity, compressional (P) waves, and transverse (S) waves. Well suited to the general reader, including pre-college audiences.

Phillips, O. M. THE HEART OF THE EARTH. San Francisco: Freeman, Cooper, 1968. Well-written book on the earth's interior. Contains an excellent chapter on earthquakes and seismic waves. Contains minimal mathematics and is easily understood by the advanced high school student.

Richter, Charles Francis. ELEMENTARY SEISMOLOGY. San Francisco: W. H. Freeman, 1958. The classic introductory college textbook written at the freshman level. Mathematics is kept to a minimum and is consistent with a nonscientist's background.

Smith, P. J. TOPICS IN GEOPHYSICS. Cambridge, Mass.: MIT Press, 1973. An outstanding introduction to the science of geophysics. Contains one of the best explanations of the phenomena of critical refraction.

Waldron, R. A. WAVES AND OSCILLATIONS. Princeton, N.J.: Van Nostrand, 1964. General approach to the study of waves at the college level for nonscientists. Very understandable and thorough treatment of reflection of waves.

Reflection and Refraction