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Reflection seismology
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Seismic reflection data
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses
the principles of seismology to estimate the properties of the Earth's subsurface from reflected
seismic waves. The method requires a controlled seismic source of energy, such as
dynamite/Tovex, a specialized air gun or vibrators, commonly known by their trademark name
Vibroseis. Vibrators are large trucks that shake a vibrating pad through a known frequency band.
By noting the time it takes for a reflection to arrive at a receiver, it is possible to estimate the
depth of the feature that generated the reflection. In this way, reflection seismology is similar to
sonar and echolocation.
Contents
[hide]
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1 Outline of the method
o 1.1 Reflection experiments
o 1.2 Reflection and transmission
o 1.3 Interpretation of reflections
2 Applications
o 2.1 Hydrocarbon exploration
 2.1.1 Land
 2.1.2 Marine (streamer)
 2.1.3 Marine (OBC)
o 2.2 Crustal studies
3 Environmental impact
4 History
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5 See also
6 Further reading
7 References
8 External links
[edit] 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 (see Acoustic impedance and
Electromagnetic impedance. The seismic (or acoustic) impedance Z is defined by the equation
Z= V ρ,
where V is the seismic wave velocity and ρ (Greek rho) is the density of the rock. 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.
In common with other geophysical methods, reflection seismology may be seen as a type of
inverse problem. That is, given a set of data collected by experimentation and the physical laws
that apply to the experiment, the experimenter wishes to develop an abstract model of the
physical system being studied. In the case of reflection seismology, the experimental data are
recorded seismograms, and the desired result is a model of the structure and physical properties
of the Earth's crust. In common with other types of inverse problems, the results obtained from
reflection seismology are usually not unique (more than one model adequately fits the data) and
may be sensitive to relatively small errors in data collection, processing, or analysis. For these
reasons, great care must be taken when interpreting the results of a reflection seismic survey.
[edit] Reflection experiments
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.
[edit] Reflection and transmission
When a seismic wave encounters a boundary between two materials with different impedances,
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 normalincidence transmission coefficient (the ratio of transmitted to incident pressure amplitudes) 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 known as mode conversion occurs.
Longitudinal waves (P-waves) are converted to transverse waves (S-waves) and vice versa. The
transmitted energy will be bent, or refracted, according to Snell's law. 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. The
reflection and transmission coefficients govern the signal strength (amplitude) at each reflector.
The coefficients at a given angle of incidence vary with (among many other things) the fluid
content of the rock. Practical use of non-normal incidence phenomena, known as AVO
(amplitude versus offset) has been facilitated by theoretical work to derive workable
approximations to the Zoeppritz equations, and by advances in computer processing capacity.
AVO studies attempt with some success to predict the fluid content (oil, gas, or water) of
potential reservoirs, to lower the risk of drilling unproductive wells and to identify new
petroleum reservoirs.
[edit] 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 called the Two-Way Time (TWT) and 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 can create an estimated crosssection of the geologic structure that generated the reflections. Interpretation of large surveys is
usually performed with programs using high-end three dimensional computer graphics.
[edit] Applications
Reflection seismology is extensively used in exploration for hydrocarbons (i.e., petroleum ,
natural gas) and such other resources as coal, ores, minerals, and geothermal energy. Reflection
seismology is also used for basic research into the nature and origin of the rocks making up the
Earth's crust. Reflection Seismology is also used in shallow application for engineering,
groundwater and environmental surveying. A method similar to reflection seismology which
uses electromagnetic instead of elastic waves is known as Ground-penetrating radar or GPR.
GPR is widely used for mapping shallow subsurface (up to a few meters deep).
[edit] Hydrocarbon exploration
Reflection seismology, or 'seismic' as it is more commonly referred to by the oil industry, is used
to map the subsurface structure of rock formations. Seismic technology is used by geologists and
geophysicists who interpret the data to map structural traps that could potentially contain
hydrocarbons. Seismic exploration is the primary method of exploring for hydrocarbon deposits,
on land, under the sea and in the transition zone (the interface area between the sea and land).
Although the technology of exploration activities has improved exponentially in the past 20
years, the basic principles for acquiring seismic data have remained the same.
In simple terms and for all of the exploration environments, the general principle is to send
sound energy waves (using an energy source like dynamite or Vibroseis) into the Earth, where
the different layers within the Earth's crust reflect back this energy. These reflected energy waves
are recorded over a predetermined time period (called the record length) by using hydrophones in
water and geophones on land. The reflected signals are output onto a storage medium, which is
usually magnetic tape. The general principle is similar to recording voice data using a
microphone onto a tape recorder for a set period of time. Once the data is recorded onto tape, it
can then be processed using specialist software which will result in processed seismic profiles
being produced. These profiles or data sets can then be interpreted for possible hydrocarbon
reserves.
Naturally enough, the three primary exploration environments for seismic exploration are land,
the transition zone and marine (shallow and deep water):
Land - The land environment is self explanatory, but can cover just about every type of terrain
that exists on Earth (such as jungle, desert, arctic tundra, swamp, forest, urban settings, mountain
regions and savannah).
Transition Zone (TZ) - The transition zone is considered to be the transition area between the
land and sea and can present unique challenges depending on the location. This may involve
setting source and receiver stations across river deltas, in swamps, across coral reefs, on beach
tidal areas and in the surf zone. TZ crews often work on land, in the transition zone and in the
shallow water marine environment on a single project.
Marine - The marine zone is either in shallow water areas (water depths of less than 30 to 40
metres would normally be considered shallow water areas for 3D marine seismic operations) or
in the deep water areas normally associated with the seas and oceans (such as the Gulf of
Mexico).
What parameters are used for each acquisition project depends on a significant number of
variables specific to a particular area. For example, in the marine environment the choice of a
tuned air gun array will depend on the known sub-sea geology, data from previous seismic
surveys, the depth at which the main features of geological interest exist within the Earth, the
desired frequency output of the source array, the amount of energy or power required and so on.
For the land environment, the source choice is normally between drilled dynamite shot holes or
mechanical vibrators. Again, the choice will depend on the specific geology and characteristics
of the prospect area but can also be influenced by non geophysical issues, such as terrain, safety
issues especially for explosive use and storage and local environmental concerns (such as
working in protected areas, working close to buildings and structures or in national parks etc).
[edit] Land
Desert land seismic camp
Land crews tend to be quite large entities, employing anywhere from a few hundred to a few
thousand people. They normally require substantial logistical support to cover not only the
seismic operation itself, but also to support the main camp (for catering, waste management and
disposal, camp accommodations, washing facilities, water supply, laundry etc), fly camps
(temporary camps set up away from the main camp on large land seismic operations, for example
where the distance is too far to drive back to the main camp with vibrator trucks), all of the crews
vehicles (maintenance, fuel, spares etc), security, possible helicopter operations, restocking of
the explosive magazine, medical support and many other logistical and support functions.
Outside of the camp personnel, the basic components of a seismic land crew are the surveyors,
layout and loading crew, shooters and recorders and the pick up crew. The general principle is
for the surveyors to survey in shot and receiver points on source and receiver lines (the latitude
and longitude coordinates of which are pre-determined by the client / contractor) using mobile
GPS stations. When a shot or receiver point is reached, this position will be staked out or marked
with the shot or receiver station number and line number.
Once sufficient lines of shot and receiver points have been surveyed in and shot holes have been
drilled to the appropriate depth, loaders put explosive charges into the shot holes on the source
lines (according to the project specification) and the receiver stations will be laid out with
geophone spreads on the receiver lines. When corresponding shot and receiver lines are ready,
the shooters prepare a single shot hole ready for firing, whilst the recording shack will be hooked
up to the geophone spread laid on the corresponding receiver line to record the reflected data.
Once a charge is ready to be shot, the recording shack initiates the shot hole firing sequence via a
radio link and records the seismic data from the whole geophone spread onto magnetic medium.
Once a shot is completed, the shooters move to the next shot hole and the shoot / record
sequence begins again.
Once lines have been shot, loaders continue to load shot holes on new source lines and the pick
up crews pick up and relay geophone spreads onto new receiver lines as required in the
acquisition plan. For vibrator crews, aka "Vibroseis" (vibrations are created by the computercoordinated vibration of hydraulically controlled plates on vibrator trucks), the vibrator trucks
move from shot hole to shot hole on the designated source line instead of the loaders and
shooters.
Receiver line on a desert land crew with recorder truck
Land surveys require crews to deploy the hundreds or thousands of geophones necessary to
record the data. Most surveys today are conducted by laying out a two-dimensional array of
geophones together with a two-dimensional pattern of source points. This allows the interpreter
to create a three-dimensional image of the geology beneath the array, so these are called 3D
surveys. Less expensive survey methods use one-dimensional lines of geophones that only
allowed the interpreter to make two-dimensional cross-sections.
[edit] Marine (streamer)
Seismic data collected by the USGS in the Gulf of Mexico
Deep water marine surveys are conducted using vessels capable of towing one or more seismic
cables known as "streamers" (see figure). 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 6 km+ in length, spaced
50–150 m 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 accurately calculate where subsurface features are located, navigators compute the
position of both the sound source and each hydrophone group which records the signal. The
positioning accuracy required is achieved using a combination of acoustic networks, compasses
and GPS receivers (often used with a radio correction applied call a differential GPS or DGPS).
[edit] Marine (OBC)
Shallow water marine surveys are conducted using sensors attached to an Ocean Bottom Cable
(OBC) laid out on the ocean bottom rather than in towed streamers. Due to operational
limitations, these types of surveys can be conducted up to depths of about 70 meters. One
operational advantage is that obstacles (such as platforms) do not affect the acquisition as much
as they do for streamer surveys. Most of the OBC surveys use dual component receivers,
combining a pressure sensor (hydrophone) and a vertical particle velocity sensor (vertical
geophone). OBC surveys can also use 4 component, i.e. those 2 components plus the 2 horizontal
velocity sensors. 4 component OBC surveys have the advantage of being able to record shear
waves, which do not travel through water. Multiple component OBC surveys hence lead to
improved imaging.
[edit] Crustal studies
The use of reflection seismology in studies of tectonics and the Earth's crust was pioneered by
groups such as the Consortium for Continental Reflection Profiling (COCORP) [1],[2].
[edit] Environmental impact
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. Higher environmental standards
have encouraged the development of lower impact seismic vehicles and acquisition
methodologies. Similarly modern seismic processing techniques allow seismic lines to deviate
around natural obstacles, or use pre-existing non-straight tracks and trails with less loss of data
quality than would once have been the case. The more recent use of inertial navigation
instruments for land survey instead of theodolites decreased the impact of seismic by allowing
the winding of survey lines between trees.
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.
[edit] History
Reflections of waves generated by earthquakes have been observed on seismograms since the
beginning of modern seismology. Seismologists have been able to develop familiar models of the
Earth's interior in part by observing these reflections from major boundaries deep within the
earth. However, the history of the use of human-generated seismic waves to map in detail the
geology of the Earth's crust is largely tied to commercial enterprise, particularly the petroleum
industry.
While Ludger Mintrop, a German mine surveyor first proposed seismological methods for
exploration and got his patent in 1916, the 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. Major
service companies today include CGGVeritas, ION Geophysical, and Petroleum Geo-Services.
Most major oil companies also have actively conducted research into seismic methods as well as
collected and processed seismic data using their own personnel and technology. Reflection
seismology has also found applications in non-commercial research by academic and
government scientists around the world.
[edit] See also
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Deconvolution
SEG Y - a popular file format for seismic reflection data
Depth conversion - the conversion of acoustic waves two-way travel time to actual depth
Seismic waves
Seismic refraction
Swell filter
[edit] Further reading
The following books cover important topics in reflection seismology. Most require some
knowledge of mathematics, geology, and/or physics at the university level or above.



Brown, Alistair R. (2004). Interpretation of three-dimensional seismic data, sixth ed.,
Society of Exploration Geophysicists and American Association of Petroleum Geologists.
ISBN 0891813640.
Biondi, B. (2006). 3d Seismic Imaging: Three Dimensional Seismic Imaging. Society of
Exploration Geophysicists. ISBN 0-07-011117-0.
Claerbout, Jon F. (1976). Fundamentals of geophysical data processing. McGraw-Hill.
ISBN 1560801379.
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Ikelle, Luc T. and Lasse Amundsen (2005). Introduction to Petroleum Seismology.
Society of Exploration Geophysicists. ISBN 1-56080-129-8.
Scales, John (1997). Theory of seismic imaging. Golden, Colorado: Samizdat Press.
Yilmaz, Öz (2001). Seismic data analysis. Society of Exploration Geophysicists. ISBN 156080-094-1.
Chapman, C. H. (2004), Fundamentals of Seismic Wave Propagation (Cambridge
University Press, Cambridge).
Further research in reflection seismology may be found particularly in books and journals of the
Society of Exploration Geophysicists, the American Geophysical Union, and the European
Association of Geoscientists and Engineers.
[edit] References
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Biography of Henry Salvatori
History of reflection seismology in Oklahoma
Milson, J., 2005, Field Geophysics, University College of London, Wiley Publications
[edit] External links
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Reflection Seismology Literature at Seismic Laboratory for Imaging and Modeling
Reflection Seismology Literature at Stanford Exploration Project
Website of the International Association of Geophysical Contractors
IAGC/OGP position paper on seismic surveys and marine mammals (PDF)
Retrieved from "http://en.wikipedia.org/wiki/Reflection_seismology"
Categories: Seismology measurement | Petroleum
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