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Reflection seismology is a branch of seismology that uses reflected seismic waves to produce images of the Earth's subsurface. By noting the time it takes for a reflection to arrive at a receiver, it is possible to estimate of the depth of the feature that generated the reflection. In this way, reflection seismology is similar to sonar and echolocation.
Reflection seismology is extensively used in exploration for hydrocarbons (i.e., petroleum and natural gas). Reflection seismology is also used for basic research into the nature and origin of the rocks making up the Earth's crust.
There are two main branches of reflection seismology used to locate hydrocarbon deposits in the world. These are land based and marine. Although the ideas behind both are similar, the actual data acquistion processes are very different.
Land surveys require crews to deploy the hundreds or thousands of geophones necessary to record the data. The seismic source is usually either dynamite or vibrations created by the computer-coordinated shaking of large trucks. Most surveys today are conducted by laying out a two-dimensional array of geophones. This allows the interpreter to create a three-dimensional "picture" of the geology beneath the array, so these are called 3-D surveys. Older surveys used one-dimensional lines of geophones that only allowed the interpreter to make two-dimensional cross-sections.
Most marine surveys are conducted using vessels capable of towing one or more seismic cables (known as "streamers"). Modern 3D surveys use multiple streamers deployed in parallel, to record data suitable for the three-dimensional interpretation of the structures beneath the sea bed. A single vessel may tow anything up to 10+ streamers, each 6km+ in length, spaced 50-150m apart. Hydrophones are deployed at regular intervals within each streamer. These hydrophones are used to record sound signals, which are reflected back from structures within the rock. To calculate where a reflector is located fairly accurately, the surveyors need to know the position of both the sound source, and the receiver which records the signal. Precise navigation of the boat, and positioning of the cables, is required to ensure that the location of each hydrophone is known. The positioning accuracy required is achieved using a combination of acoustic networks, compasses and GPS receivers.
In a marine survey, the seismic source is usually an air gun that uses bubbles of compressed air to generate seismic waves.
Some marine surveys are conducted using sensors on the ocean bottom rather than on towed streamers. These surveys have the advantage of being able to record shear waves, which cannot travel through water.
Outline of the method
Seismic waves are a form of elastic wave that travel in the Earth. Any medium that can support wave propagation may be described as having an impedance. The seismic (or acoustic) impedance Z is defined by the equation
Z = Vρ,
When a seismic wave encounters a boundary between two different materials with different impedances, some of the energy of the wave will be reflected off the boundary, while some of it will be transmitted through the boundary.
A reflection experiment is carried out by initiating a seismic source (such as a dynamite explosion) and recording the reflected waves using one or more seismometers. On land, the typical seismometer used in a reflection experiment is a small, portable instrument known as a geophone, which converts ground motion into an analog electrical signal. In water, hydrophones, which convert pressure changes into electrical signals, are used. As the seismometers detect the arrival of the seismic waves, the signals are converted to digital form and recorded; early systems recorded the analog signals directly onto magnetic tape, photographic film, or paper. The signals may then be displayed by a computer as seismograms for interpretation by a seismologist. Typically, the recorded signals are subjected to significant amounts of signal processing and various imaging processes before they are ready to be interpreted. In general, the more complex the geology of the area under study, the more sophisticated are the techniques required to perform the data processing. Modern reflection seismic surveys require large amounts of computer processing, often performed on supercomputers or on computer clusters.
Interpretation of reflections
The time it takes for a reflection from a particular boundary to arrive at the geophone is called the travel time. If the seismic wave velocity in the rock is known, then the travel time may be used to estimate the depth to the reflector. For a simple vertically traveling wave, the travel time t from the surface to the reflector and back is given by the formula
, where d is the depth of the reflector and V is the wave velocity in the rock.
A series of apparently related reflections on several seismograms is often referred to as a reflection event. By correlating reflection events, a seismologist is able create an estimated cross-section of the geologic structure that generated the reflections. Interpretation of large surveys is usually performed with programs using high-end three dimensional computer graphics.
Reflection and transmission
When a seismic wave encounters a boundary between two materials, some of the energy in the wave will be reflected at the boundary, while some of the energy will continue through the boundary. The amplitude of the reflected wave is predicted by multiplying the amplitude of the incoming wave by the seismic reflection coefficient R, determined by the impedance contrast between the two materials.
For a wave that hits a boundary at normal incidence (head-on), the expression for the reflection coefficient is simply
where Z0 and Z1 are the impedance of the first and second medium, respectively.
Similarly, the amplitude of the incoming wave is multiplied by the transmission coefficient to predict the amplitude of the wave transmitted through the boundary. The formula for the normal-incidence transmission coefficient is
From this, it is easy to show that
By observing changes in the strength of reflectors, seismologists can infer changes in the seismic impedances. In turn, they use this information to infer changes in the properties of the rocks at the interface, such as density and elastic modulus.
For non-normal incidence (at an angle), a phenomenon know as mode conversion occurs. Longitudinal waves (P-waves) are converted to transverse waves (S-waves ) and vice versa. In this case, the expressions for the reflection and transmission coefficients are found by applying appropriate boundary conditions to the wave equation, a topic beyond the scope of this article. The resulting formulas, first determined at the beginning of the 20th century, are known as the Zoeppritz equations.
As with all human activities, reflection seismic experiments may impact the Earth's natural environment. On land, conducting a seismic survey may require the building of roads in order to transport equipment and personnel. Even if roads are not required, vegetation may need to be cleared for the deployment of geophones. If the survey is in a relatively undeveloped area, significant habitat disturbance may result. Many land crews now use helicopters instead of land vehicles in remote areas. Most countries require that seismic surveys are conducted according to environmental standards established by government regulation.
The main environmental concern for marine surveys is the potential of seismic sources to disturb animal life, especially cetaceans such as whales, porpoises, and dolphins. These animals have sensitive hearing, and some scientists believe the underwater sound waves created by air guns might disturb the animals or even damage their ears. Research is ongoing into these questions. Companies acquiring marine seismic surveys often adopt voluntary standards for adapting or ceasing operations in the presence of certain animals.
Seismic surveys may also have a positive impact by reducing the number of unsuccessful wells drilled while exploring for hydrocarbon deposits and by increasing the amount of hydrocarbons produced from existing wells.
The history of reflection seismology is largely tied to that of the petroleum industry. Canadian inventor Reginald Fessenden was the first to conceive of using reflected seismic waves to infer geology. He filed patents on the method in 1917 while working on methods of detecting submarines during World War I. Due to the war, he was unable to follow up on the idea. However, John Clarence Karcher discovered seismic reflections independently while working for the United States Bureau of Standards (now the National Institute of Standards and Technology) on methods of sound ranging to detect artillery. In discussion with colleagues, the idea developed that these reflections could aid in exploration for petroleum. With several others, many affiliated with the University of Oklahoma, Karcher helped to form the Geological Engineering Company, incorporated in Oklahoma in April, 1920. The first field tests were conducted near Oklahoma City, Oklahoma in 1921.
The company soon folded due to a drop in the price of oil. In 1925, oil prices had rebounded, and Karcher helped to form Geophysical Research Corporation (GRC) as part of the oil company Amerada . In 1930, Karcher left GRC and helped to found Geophysical Service Incorporated (GSI). GSI was one of the most successful seismic contracting companies for over 50 years and was the parent of an even more successful company, Texas Instruments. Early GSI employee Henry Salvatori left that company in 1933 to found another major seismic contractor, Western Geophysical. As of 2005, after several mergers and acquisitions, the heritages of GSI and Western Geophysical still exist, along with several pioneering European companies such as GECO, Seismos, and Prakla, as part of the seismic contracting company WesternGeco. Many other companies using reflection seismology in hydrocarbon exploration, hydrology, engineering studies, and other applications have been formed since the method was first invented. Reflection seismology has also found applications in non-commercial research by academic and government scientists around the world.
Much research in reflection seismology may be found in books and journals of the Society of Exploration Geophysicists, the American Geophysical Union, and the European Association of Geoscientists and Engineers .
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