Reflection seismology

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 a seismic vibrator, commonly known by the trademark name Vibroseis. Reflection seismology is similar to sonar and echolocation.

Contents

Outline of the method

Seismic waves are mechanical perturbations 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 \rho (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.

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.

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

R=\frac{Z_1 - Z_0}{Z_1 %2B Z_0},

where Z_0 and Z_1 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 (the ratio of transmitted to incident pressure amplitudes) is

T=\frac{2 Z_0}{Z_1 %2B Z_0}.

As the sum of the amplitudes of the reflected and transmitted wave has to be equal to the amplitude of the incident wave, it is easy to show that

1-R=\frac{Z_1 %2B Z_0- Z_1%2B Z_0}{Z_1 %2B Z_0}=\frac{Z_0 %2B Z_0}{Z_1 %2B Z_0} = T.

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.

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

t = 2\frac{d}{V},

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 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.

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).

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. 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.).

Land

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 crew's 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 computer-coordinated 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.

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.

Marine (streamer)

Deep water marine seismic surveys are conducted using purpose built vessels capable of towing one or more seismic cables or "streamers" (see figure) just below the sea surface, along with an energy source towed just below the surface and between the stern of the vessel and the head of the streamers. Modern 3D surveys use multiple streamers deployed in parallel and often multiple energy sources (commonly two), 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 built into the streamers at regular intervals; these record and digitize the energy waves which are reflected back from sub-sea structures. To accurately calculate where subsurface features are located, navigators compute the position of both the energy 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).

A modification on this basic technique can also be used to record sub surface data directly underneath offshore structures, predominantly exploration and production platforms and other permanent offshore structures such as FPSO's (floating production, storage and offloading unit) which cannot be moved to facilitate a survey vessel; this technique is known as undershooting. This requires a separate source vessel and a streamer vessel to pass either side of the obstruction, firing the energy source on the source vessel and recording the reflected data on the towed streamers on the streamer vessel. Both vessels are linked by a data telemetry system to co-ordinate and synchronise the firing and recording operations. By varying the distance from the source and streamer vessel to the obstruction (changing the offset), a wide swathe of data can be collected from underneath the obstruction without any disturbance to the permanent offshore operation.

Marine (OBC)

Marine surveys can also be 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, most of these types of surveys are conducted in water depths less than 70 meters, however OBC crews in recent years have acquired 3D surveys in depths up to 2000 meters. One operational advantage is that obstacles (such as platforms) do not limit 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 four component, i.e. a hydrophone components plus the three orthogonal velocity sensors. Four component OBC surveys have the advantage of being able to also record shear waves, which do not travel through water. Multiple component OBC surveys hence can lead to improved subsurface imaging. Ocean Bottom Cable surveys can also cost significantly more than conventional streamer surveys over the same area. This additional cost is usually only justified when the improved imaging is required for accurate reservoir delineation, or when surface obstacles prevent a conventional streamer survey from being acquired in the area.

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) [2],[3].

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. Surveys involves towing an array of 15-45 pneumatic air guns below the ocean surface behind the survey vessel and emit sound pulses of a “predominantly low frequency (10–300 Hz), high intensity (215-250 dB). 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. Seismic surveying can damage the reproductive processes, auditory functions and other damaging effects to highly lucrative marine species (lobster, crab) and it poses potentially fatal effects to marine mammals. Seismic testing is not fully responsible for whales running ashore or becoming stranded, but there is evidence that it plays a major role. Studies of seismic effects on several whale species such as Gray, Bowhead, Blue, Humpback and Sperm whales indicated substantial effects in behavior, breathing, feeding and diving patterns. Dr. Bernd Würsig, a professor for marine biology at Texas A&M University in Galveston, Texas states that the Gray whale will avoid its regular migratory and feeding grounds by >30 km in areas of seismic testing. Similarly the breathing of gray whales was shown to be more rapid, indicating discomfort and panic in the whale. It is circumstantial evidence such as this that has led researchers to believe that avoidance and panic might be responsible for increased whale beachings although research is ongoing into these questions.

However, the research carried out by both the E&P (exploration and production) sector and by environmental groups needs to be considered carefully in terms of impartiality as both may reference research or publish data that only promotes their own aims and goals. For example, the following quote comes from a position paper published by an E&P representative group which would appear to contradict the conclusions stated above. The quote from the executive summary states that:

"The sound produced during seismic surveys is comparable in magnitude to many naturally occurring and other man-made sound sources. Furthermore, the specific characteristics of seismic sounds and the operational procedures employed during seismic surveys are such that the resulting risks to marine mammals are expected to be exceptionally low. In fact, three decades of world-wide seismic surveying activity and a variety of research projects have shown no evidence which would suggest that sound from E&P seismic activities has resulted in any physical or auditory injury to any marine mammal species." [1]

History

Reflections and refractions of seismic waves at geologic interfaces within the Earth were first observed on recordings of earthquake-generated seismic waves. The basic model of the Earth's deep interior is based on observations of earthquake-generated seismic waves transmitted through the Earth’s interior (e.g., Mohorovičić, 1910).[2] The use of human-generated seismic waves to map in detail the geology of the upper few kilometers of the Earth's crust followed shortly thereafter and has developed mainly due to commercial enterprise, particularly the petroleum industry.

The Canadian inventor Reginald Fessenden was the first to conceive of using reflected seismic waves to infer geology. His work was initially on the propagation of acoustic waves in water, motivated by the sinking of the Titanic by an iceberg in 1912. He also worked on methods of detecting submarines during World War I. He applied for the first patent on a seismic exploration method in 1914, which was issued in 1917. Due to the war, he was unable to follow up on the idea. Meanwhile, Ludger Mintrop, a German mine surveyor, devised a mechanical seismograph in 1914 that he successfully used to detect salt domes in Germany. He applied for a German patent in 1919 that was issued in 1926. In 1921 he founded the company Seismos, which was hired to conduct seismic exploration in Texas and Mexico, resulting in the discovery of the first commercial discovery of oil using the seismic method in 1924.[3] 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, Petroleum Geo-Services and Fugro. 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.

See also

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.

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.

References

  1. ^ Scientific Surveys and Marine Mammals - Joint OGP/IAGC Position Paper, December 2008 - http://www.ogp.org.uk/pubs/358.pdf
  2. ^ Grusic, V., and Orlic, M., Early Observations of Rotor Clouds by Andrija Mohorovičić, Bulletin of the American Meteorlogical Society, May 2007, pp. 693-700, accessed 4 January 2010: [1]
  3. ^ Sheriff, R. E., and Geldart, L. P., 1995, Exploration Seismology, Second Edition, Cambridge University Press, pp. 3-6.

External links