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| '''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 [[reflection (physics)|reflected]] [[seismic wave]]s. 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.[[File:Seismic Reflection Profile across the TX Passive Margin.jpg|thumb|Seismic Reflection Profile across the TX Passive Margin]] Reflection seismology is similar to [[sonar]] and [[acoustic location|echolocation]]. This article is about surface seismic surveys, for vertical seismic profiles, see [[vertical seismic profile|VSP]].
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| ==Outline of the method==
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| {{Unreferenced section|date=December 2010}}
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| [[Seismic wave]]s are mechanical perturbations that travel in the Earth at a speed governed by the [[acoustic impedance]] of the medium in which they are travelling. The acoustic (or seismic) impedance, ''Z'', is defined by the equation:
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| :<math>Z=V\rho \ </math>,
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| where ''V'' is the seismic [[wave velocity]] and ''ρ'' ([[Greek alphabet|Greek]] ''[[rho (letter)|rho]]'') is the [[density]] of the rock.
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| When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will [[Reflection (physics)|reflect]] off the interface and some will [[refraction|refract]] through the interface. At its most basic, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers (or [[geophone]]s) at the surface.<ref name="SheriffGeldart">Sheriff, R. E., Geldart, L. P., (1995), 2nd Edition. Exploration Seismology. Cambridge University Press.</ref> Knowing the travel times from the source to various receivers, and the velocity of the seismic waves, a geophysicist then attempts to reconstruct the pathways of the waves in order to build up an image of the subsurface.
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| 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 [[experiment]]ation and the physical laws that apply to the experiment, the experimenter wishes to develop an [[model (abstract)|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.
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| ===The reflection experiment===
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| The general principle of seismic reflection is to send [[elastic waves]] (using an energy source such as [[dynamite]] [[explosion]] or [[Vibroseis]]) into the Earth, where each layer within the Earth reflects a portion of the wave’s energy back and allows the rest to refract through. These reflected energy waves are recorded over a predetermined time period (called the record length) by receivers that detect the motion of the ground in which they are placed. On land, the typical receiver used is a small, portable instrument known as a [[geophone]], which converts ground motion into an [[analog signal|analogue]] electrical signal. In water, [[hydrophone]]s are used, which convert pressure changes into electrical signals. Each receiver’s response to a single shot is known as a “trace” and is recorded onto a [[magnetic tape]], then the shot location is moved along and the process is repeated. Typically, the recorded signals are subjected to significant amounts of [[signal processing]] before they are ready to be interpreted and this is an area of significant active research within industry and academia. In general, the more complex the geology of the area under study, the more sophisticated are the techniques required to remove noise and increase resolution. Modern seismic reflection surveys contain large amount of data and so require large amounts of computer processing, often performed on [[supercomputers]] or [[computer clusters]].
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| ===Reflection and transmission at normal incidence===
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| [[File:Normal reflection at an interface.png|thumb|P-wave reflects off an interface at normal incidence]]
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| When a seismic wave encounters a boundary between two materials with different acoustic impedances, some of the energy in the wave will be reflected at the boundary, while some of the energy will be transmitted through the boundary. The [[amplitude]] of the reflected wave is predicted by multiplying the amplitude of the incident wave by the seismic ''[[reflection coefficient]]'' <math>R</math>, determined by the [[Acoustic impedance|impedance]] contrast between the two materials.
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| For a wave that hits a boundary at [[surface normal|normal]] incidence (head-on), the expression for the reflection coefficient is simply
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| :<math>R=\frac{(Z_1 - Z_0)^2}{(Z_1 + Z_0)^2}</math>,
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| where <math>Z_0</math> and <math>Z_1</math> are the impedance of the first and second medium, respectively.
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| Similarly, the amplitude of the incident 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
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| :<math>T=\frac{4 Z_0 Z_1}{(Z_1 + Z_0)^2}</math>.
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| As the sum of the squares of amplitudes of the reflected and transmitted wave has to be equal to the square of amplitude of the incident wave, it is easy to show that
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| :<math>1-R=\frac{(Z_1 + Z_0)^2- (Z_1 - Z_0)^2}{(Z_1 + Z_0)^2}=\frac{4 Z_0 Z_1}{(Z_1 + Z_0)^2} = T</math>.
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| 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]].
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| ===Reflection and transmission at non-normal incidence===
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| [[File:Reflection at an interface.png|thumb|Diagram showing the mode conversions that occur when a P-wave reflects off an interface at non-normal incidence]]
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| The situation becomes much more complicated in the case of non-normal incidence, due to mode conversion between [[P-waves]] and [[S-waves]], and is described by the [[Zoeppritz equations]]. In 1919, Karl Zoeppritz derived 4 equations that determine the amplitudes of [[Reflection (physics)|reflected]] and [[refraction|refracted]] waves at a planar interface for an incident P-wave as a function of the angle of incidence and six independent elastic parameters.<ref name=SheriffGeldart /> These equations have 4 unknowns and can be solved but they do not give an intuitive understanding for how the reflection amplitudes vary with the rock properties involved.<ref>Shuey, R. T. [1985] A simplification of the Zoeppritz equations. Geophysics, 50:609-614</ref>
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| The reflection and transmission coefficients, which govern the amplitude of each reflection, vary with angle of incidence and can be used to obtain information about (among many other things) the fluid content of the rock. Practical use of non-normal incidence phenomena, known as AVO (see [[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. The 3-term simplification of the Zoeppritz equations that is most commonly used was developed in 1985 and is known as the "Shuey equation". A further 2-term simplification is known as the "Shuey approximation", is valid for angles of incidence less than 30 degrees (usually the case in seismic surveys) and is given below:<ref name="Avseth">Avseth, P, T Mukerji and G Mavko (2005). Quantitative seismic interpretation. Cambridge University Press, Cambridge, p. 183</ref>
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| :<math>R(\theta ) = R(0) + G \sin^2 \theta </math>
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| where <math>R(0)</math> = reflection coefficient at zero-offset (normal incidence); <math>G</math> = AVO gradient, describing reflection behaviour at intermediate offsets and <math>(\theta)</math> = angle of incidence. This equation reduces to that of normal incidence at <math>(\theta)</math>=0.
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| ===Interpretation of reflections===
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| 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 <math>t</math> from the surface to the reflector and back is called the Two-Way Time (TWT) and is given by the formula
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| :<math>t = 2\frac{d}{V}</math>,
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| [[File:Snells Law.jpg|thumb|Snells Law]]
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| where <math>d</math> is the depth of the reflector and <math>V</math> is the wave velocity in the rock.
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| 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 [[geology|geologic]] structure that generated the reflections. Interpretation of large surveys is usually performed with programs using high-end three dimensional [[computer graphics]].
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| [[File:Snells Law.jpg|thumb|Snells Law]]
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| ===Sources of noise===
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| [[File:Types of noise on a seismic record.png|thumb|Sources of noise on a seismic record. Top-left: air wave; top-right: head wave; bottom-left: surface wave; bottom-right: multiple.]]
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| In addition to reflections off interfaces within the subsuface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded:
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| ====Air wave====
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| The airwave travels directly from the source to the receiver and is an example of [[coherent noise]]. It is easily recognisable because it travels at a speed of 330 m/s, the [[speed of sound]] in air.
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| ====Ground Roll / Rayleigh wave / Scholte Wave / Surface wave====
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| A [[Rayleigh wave]] typically propagates along a free surface of a solid, but the elastic constants and [[density]] of air are very low compared to those of rocks so the surface of the Earth is approximately a [[free surface]]. Low velocity, low frequency and high amplitude Rayleigh waves are frequently present on a seismic record and can obscure signal, degrading overall data quality. They are known within the industry as ‘Ground Roll’ and are an example of coherent noise that can be attenuated with a carefully designed seismic survey.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Ground Roll | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=ground%20roll | accessdate = September 8, 2013}}</ref> The [[Scholte wave]] is similar to
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| ground roll but occurs at the sea-floor (fluid/solid interface) and it can possibly obscure and mask deep reflections in marine seismic records.<ref>{{cite journal|last=Zheng|coauthors=Yingcai, et al.|title=Scholte waves generated by s eafloor topography|journal=arXiv preprint arXiv:1306.4383|year=2013}}</ref>
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| The velocity of these waves varies with wavelength, so they are said to be dispersive and the shape of the wavetrain varies with distance.<ref>Dobrin, M. B., 1951, Dispersion in seismic surface waves, Geophysics, 16, 63-80.</ref>
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| ====Refraction / Head wave / Conical wave====
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| A head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface. This motion causes a disturbance in the upper medium that is detected on the surface.<ref name=SheriffGeldart /> The same phenomenon is utilised in [[seismic refraction]].
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| ====Multiple reflection====
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| [[File:Multiple Reflection.jpg|thumb|Multiple reflection]]
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| An event on the seismic record that has incurred more than one reflection is called a ''multiple''. Multiples can be either short-path (peg-leg) or long-path, depending upon whether they interfere with primary reflections or not.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Multiples Reflection | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiple%20reflection | accessdate = September 8, 2013}}</ref><ref>{{cite journal | last = Pendrel | first = J. | title = Seismic Inversion—A Critical Tool in Reservoir Characterization | journal = [[Scandinavian Oil-Gas Magazine]] | number = 5/6 | year = 2006 | pages = 19-22}}</ref>
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| Multiples from the bottom of a body of water (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by [[seismic processing]].
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| ====Cultural noise====
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| Cultural noise includes noise from planes, helicopters and electrical pylons and all of these can be detected by the receivers.
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| ==Applications==
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| Reflection seismology is used extensively in a number of fields and its applications can be categorised into three groups,<ref>{{Cite book| author=Yilmaz, Öz | year=2001 | title=Seismic data analysis | publisher=Society of Exploration Geophysicists | isbn=1-56080-094-1 |page= 1}}</ref> each defined by their depth of investigation:
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| *Near-surface applications – an application that aims to understand geology at depths of up to approximately 1 km, typically used for [[Engineering geology|engineering]] and [[Environmental geology|environmental]] surveys, as well as [[coal]]<ref>Gochioco, L. M., Seismic surveys for coal exploration and mine planning. The Leading Edge, April 1990, v. 9, p. 25-28</ref> and [[mineral]] exploration.<ref>{{cite journal | last = Milkereit | first = B. | last2 = Eaton | first2 = D. | last3 = Salisbury | first3 = M. | last4 = Adam | first4 = E. | last5 = Bohlen | first5 = Thomas | year = 2003 | title = 3D Seismic Imaging for Mineral Exploration | journal = Commission on Controlled-Source Seismology: Deep Seismic Methods | url = http://www.geophys.geos.vt.edu/hole/ccss/milkereitCCSS.pdf | format = PDF | accessdate = September 8, 2013}}</ref> A more recently developed application for seismic reflection is for [[geothermal energy]] surveys,<ref>{{cite web | title = The Role of Geophysics In Geothermal Exploration | website = Quantec Geoscience | url = http://www.quantecgeoscience.com/geothermal-exploration | accessdate = September 8, 2013}}</ref> although the depth of investigation can be up to 2 km deep in this case.<ref>{{cite journal | last = Louie | first = John N. | last2 = Pullammanappallil | first2 = S. K. | year = 2011 | title = Advanced seismic imaging for geothermal development | journal = New Zealand Geothermal Workshop 2011 Proceedings | url = http://crack.seismo.unr.edu/geothermal/Louie-NZGW11.pdf | format = PDF | accessdate = September 8, 2013}}</ref>
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| *[[Hydrocarbon exploration]] - used by the hydrocarbon industry to provide a high resolution map of acoustic impedance contrasts at depths of up to 10 km within the subsurface. This can be combined with [[seismic attribute]] analysis and other [[exploration geophysics]] tools and used to help [[geologists]] build a [[geological model]] of the area of interest.
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| *Crustal studies – investigation into the structure and origin of the [[Earth's crust]], through to the [[Moho discontinuity]] and beyond, at depths of up to 100 km.
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| A method similar to reflection seismology which uses [[electromagnetic spectrum|electromagnetic]] instead of elastic waves, and has a smaller depth of penetration, is known as [[Ground-penetrating radar]] or GPR.
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| ===Hydrocarbon exploration===
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| [[File:Workers performing seismic tests, Seismic Explorations, Inc..jpg|thumb|upright|Seismic testing in 1940.]]
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| Reflection seismology, more commonly referred to as “seismic reflection” or abbreviated to “seismic” within the hydrocarbon industry, is used by petroleum geologists and geophysicists to map and interpret potential [[petroleum reservoir]]s. The size and scale of seismic surveys has increased alongside the significant concurrent increases in computer power during the last 25 years. This has led the seismic industry from laboriously – and therefore rarely – acquiring small 3D surveys in the 1980s to now routinely acquiring large-scale high resolution 3D surveys. The goals and basic principles have remained the same, but the methods have slightly changed over the years.
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| The primary environments for seismic exploration are land, the transition zone and marine:
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| ''Land'' - The land environment covers almost every type of terrain that exists on Earth, each bringing its own logistical problems. Examples of this environment are jungle, desert, arctic tundra, forest, urban settings, mountain regions and savannah.
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| ''Transition Zone (TZ)'' - The transition zone is considered to be the area where the land meets the sea, presenting unique challenges because the water is too shallow for large seismic vessels but too deep for the use of traditional methods of acquisition on land. Examples of this environment are river deltas, swamps and marshes,<ref>{{cite web | website = Geokinetics | title = Transition Zone | url = http://www.geokinetics.com/Services/Acquisition/Transition-Zone-81.html | accessdate = September 8, 2013}}</ref> coral reefs, beach tidal areas and the surf zone. Transition zone seismic crews will often work on land, in the transition zone and in the shallow water marine environment on a single project in order to obtain a complete map of the subsurface.
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| ''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).
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| Seismic surveys are typically designed by [[National oil companies]] and [[International oil companies]] who hire service companies such as [[Breckenridge Exploration Co.]], [[CGGVeritas]], [[Petroleum Geo-Services]] and [[WesternGeco]] to acquire them. Another company is then hired to process the data, although this can often be the same company that acquired the survey. Finally the finished seismic volume is delivered the oil company so that it can be geologically interpreted.
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| ====Land survey acquisition====
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| {{See also|Vibroseis}}
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| [[File:Land Seismic Camp.jpg|thumb|Desert land seismic camp]]
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| [[File:Receiver line.jpg|thumb|left|Receiver line on a desert land crew with recorder truck]]
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| Land seismic surveys tend to be large entities, requiring hundreds of tons of equipment and employing anywhere from a few hundred to a few thousand people, deployed over vast areas for many months.<ref>{{cite journal |author= Jon Cocker|year= 2011|title= Land 3-D Seismic Survey Designed To Meet New Objectives|journal= E & P|volume= |issue= |pages= |publisher= Hart Energy|doi= |pmid= |pmc= |url= http://www.epmag.com/Technology-Contracts/Land-3-D-Seismic-Survey-Designed-Meet-Objectives_80767|accessdate= 12 March 2012}}s</ref> There are a number of options available for a controlled seismic source in a land survey and particularly common choices are [[Vibroseis]] and dynamite. Vibroseis is a non-impulsive source that is cheap and efficient but requires flat ground to operate on, making its use more difficult in undeveloped areas. The method comprises one or more heavy, all-terrain vehicles lowering a steel plate onto the ground, which is then vibrated with a specific frequency distribution and amplitude.<ref>{{cite book |title= Petroleum Geoscience|last1= Gluyas|first1= J|last2= Swarbrick|first2= R|authorlink= |coauthors= |year= 2004|publisher= Blackwell Publishing|location= |isbn= 978-0-632-03767-4|page=22 |pages= |url= |accessdate=}}</ref> It produces a low energy density, allowing it to be used in cities and other built-up areas where dynamite would cause significant damage, though the large weight attached to a Vibroseis truck can cause its own environmental damage.<ref>{{Cite book| author=Sheriff, R. E., Geldart, L. P. | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |pages= 209–210}}</ref> Dynamite is an impulsive source that is regarded as the ideal geophysical source due to it producing an almost perfect [[impulse function]] but it has obvious environmental drawbacks. For a long time, it was the only seismic source available until weight dropping was introduced around 1954,<ref>{{Cite book| author=Sheriff, R. E., Geldart, L. P. | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 200}}</ref> allowing geophysicists to make a trade-off between image quality and environmental damage. Compared to Vibroseis, dynamite is also operationally inefficient because each source point needs to be drilled and the dynamite placed in the hole.
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| A land seismic survey requires substantial logistical support. In addition to the day-to-day seismic operation itself, there must also be support for the main camp (for catering, waste management and laundry etc.), smaller camps (for example where the distance is too far to drive back to the main camp with vibrator trucks), vehicle and equipment maintenance, medical personnel and security.
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| Unlike in marine seismic surveys, land geometries are not limited to narrow paths of acquisition, meaning that a wide range of offsets and azimuths is usually acquired and the largest challenge is increasing the rate of acquisition. The rate of production is obviously controlled by how fast the source (Vibroseis in this case) can be fired and then move on to the next source location. Attempts have been made to use multiple seismic sources at the same time in order to increase survey efficiency and a successful example of this technique is Independent Simultaneous Sweeping (ISS).<ref>{{cite journal |author= Howe, D. ''et all''|year= 2008|title= Independent simultaneous sweeping - a method to increase the productivity of land seismic crews|journal= SEG Expanded Abstracts|volume= 27|issue= |pages= 2826|publisher= SEG|doi= |pmid= |pmc= |url= |accessdate= }}</ref>
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| ====Marine survey acquisition (streamer)====
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| [[File:Diagram of a marine seismic survey.png|thumb|Marine seismic survey using a towed streamer]]
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| [[File:NATS MAZ.png|thumb|Plan view of NATS and MAZ surveys]]
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| [[File:WATS.png|thumb|Plan view of a WATS/WAZ survey]]
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| [[File:Usgs-of02-368-line7.png|thumb|Seismic data collected by the [[United States Geological Survey|USGS]] in the [[Gulf of Mexico]]]]
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| Traditional marine seismic surveys are conducted using specially-equipped vessels that tow one or more cables containing a series of hydrophones at constant intervals (see diagram). The cables are known as ''streamers'', with 2D surveys using only 1 streamer and 3D surveys employing up to 12 or more (though 6 or 8 is more common). The streamers are deployed just beneath the surface of the water and are at a set distance away from the vessel. The seismic source, usually an airgun but other sources are available, is also deployed underneath the surface of the water and is located between the vessel and the first receiver. 2 sources are often used in order to achieve a fast rate of shooting. Marine seismic surveys generate a significant quantity of data,<ref>{{Cite book| author=Sheriff, R. E., Geldart, L. P. | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 260}}</ref> each streamer can be up to 6 or even 8 km long, containing hundreds of channels and the source is typically fired every 15 or 20 seconds.
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| A seismic vessel with 2 sources and towing a single streamer is known as a ''Narrow-Azimuth Towed Streamer'' (or NAZ or NATS). By the early 2000s, it had become accepted that this type of acquisition was useful for initial exploration but inadequate for development and production,<ref name="Barley, B. & Summers, T. 2007 450–458">{{cite journal |author= Barley, B. & Summers, T.|year= 2007|title= Multi-azimuth and wide-azimuth seismic: Shallow to deep water, exploration to production|journal= The Leading Edge|volume= 26|issue=4 |pages= 450–458|publisher= SEG|doi= 10.1190/1.2723209 |pmid= |pmc= |url= http://tle.geoscienceworld.org/content/26/4/450.abstract |accessdate= September 8, 2013}}</ref> in which [[Water well|wells]] had to be accurately positioned. This led to the development of the ''Multi-Azimuth Towed Streamer'' (MAZ) which tried to break the limitations of the linear acquisition pattern of a NATS survey by acquiring a combination of NATS surveys at different azimuths (see diagram).<ref>{{cite journal |author= Mike Howard|year= 2007|title= Marine seismic surveys with enhanced azimuth coverage: Lessons in survey design and acquisition|journal= The Leading Edge|volume= 26|issue= 4|pages= 480|publisher= SEG|doi= |pmid= |pmc= |url= http://www.slb.com/~/media/Files/westerngeco/resources/articles/2007/apr07_tle_marine.pdf |format= PDF |accessdate= September 8, 2013}}</ref> This successfully delivered increased illumination of the subsurface and a better signal to noise ratio.
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| The seismic properties of salt poses an additional problem for marine seismic surveys, it attenuates seismic waves and its structure contains overhangs that are difficult to image. This led to another variation on the NATS survey type, the ''wide-azimuth towed streamer'' (or WAZ or WATS) and was first tested on the [[Mad Dog (oil platform)|Mad Dog field]] in 2004.<ref>{{cite journal |author= Threadgold, I et al.|year= 2006|title= Implementing a wide azimuth towed streamer field trial: the what, why and mostly how of WATS in Southern Green Canyon|journal= SEG Expanded Abstracts |volume= 25|issue= |pages= 2091|publisher= SEG|doi= |pmid= |pmc= |url= http://www.onepetro.org/mslib/servlet/onepetropreview?id=SEG-2006-2901 |accessdate= September 8, 2013}}</ref> This type of survey involved 1 vessel solely towing a set of 8 streamers and 2 separate vessels towing seismic sources that were located at the start and end of the last receiver line (see diagram). This configuration was "tiled" 4 times, with the receiver vessel moving further away from the source vessels each time and eventually creating the effect of a survey with 4 times the number of streamers. The end result was a seismic dataset with a larger range of wider azimuths, delivering a breakthrough in seismic imaging.<ref name="Barley, B. & Summers, T. 2007 450–458"/> These are now the three common types of marine towed streamer seismic surveys.
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| ====Marine survey acquisition (Ocean-bottom & 4D)====
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| Marine survey acquisition is not just limited to seismic vessels; it is also possible to lay cables of geophones and hydrophones on the sea bed in a similar way to how cables are used in a land seismic survey, and use a separate source vessel. This method was originally developed out of operational necessity in order to enable seismic surveys to be conducted in areas with obstructions, such as [[Oil Production Platform|production platforms]], without having the compromise the resultant image quality.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Ocean Bottom Cable | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=ocean-bottom%20cable | accessdate = September 8, 2013}}</ref> Ocean bottom cables (OBC) are also extensively used in other areas that a seismic vessel cannot be used, for example in shallow marine (water depth <300m) and transition zone environments, and can be deployed by [[Remotely operated underwater vehicle|ROVs]] in deep water when [[repeatability]] is valued (see 4D, below). Conventional OBC surveys use dual-component receivers, combining a pressure sensor ([[hydrophone]]) and a vertical particle velocity sensor (vertical [[geophone]]), but more recent developments have expanded the method to use four-component sensors i.e. a hydrophone and three orthogonal geophones. Four-component sensors have the advantage of being able to also record [[seismic wave|shear waves]],<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Four-Component Seismic Data | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=four-component%20seismic%20data | accessdate = September 8, 2013}}</ref> which do not travel through water but can still contain valuable information.
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| In addition to the operational advantages, OBC also has geophysical advantages over a conventional NATS survey that arise from the increased fold and wider range of azimuths associated with the survey geometry.<ref>{{cite journal |last1= Stewart|first1= J.|last2= Shatilo|first2= A|year= 2004|title= A comparison of streamer and OBC seismic data at Beryl Alpha field, UK North Sea|journal= SEG Expanded Abstracts|volume= 23|issue= |pages= 841|publisher= SEG|doi= |url= |accessdate= }}</ref> However, much like a land survey, the wider azimuths and increased fold come at a cost and the ability for large-scale OBC surveys is severely limited.
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| 4D surveys are 3D seismic surveys repeated over a period of time in order to observe reservoir depletion during production and identify areas where there are barriers to flow that may not be easily detectable in conventional seismic. They are conventionally conducted using ocean-bottom cables because the cables can be accurately placed in their previous location after being removed. A number of 4D surveys have also been set up over fields in which ocean bottom cables have been purchased and permanently deployed. This method can be known as Life of Field Seismic (LoF).<ref>{{cite journal |author= Barley, B. & Summers, T.|year= 2007|title= Multi-azimuth and wide-azimuth seismic: Shallow to deep water, exploration to production|journal= The Leading Edge|volume= 26|issue=4 |pages= 456–457|publisher= SEG|doi= |pmid= |pmc= |url= http://tle.geoscienceworld.org/content/26/4/450.abstract |accessdate= September 8, 2013}}</ref>
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| In 2005, Ocean Bottom Nodes/Seismic (OBN / OBS) - an extension of the OBC method that uses battery-powered cableless receivers placed in deep water - was first trialled over the [[Atlantis Oil Field]] in a partnership between [[BP]] and [[Fairfield Industries]].<ref>{{cite journal |last1= Beaudoin|first1= G.|last2= |first2= |year= 2010|title= Imaging the invisible — BP's path to OBS nodes|journal= SEG Expanded Abstracts|volume= 29|issue= |pages= 3734|publisher= SEG|doi= |url= |accessdate= }}</ref> The placement of these nodes can be more flexible than the cables in OBC and they are easier to store and deploy due to their smaller size and lower weight. The world’s first 4D survey using nodes was acquired over the Atlantis field in 2009, with the nodes being placed by a ROV in a water depth of 1300-2200m to within 30m of where they were previously placed in 2005.<ref>{{cite journal |last1= Beaudoin|first1= G.|last2= Reasnor|first2= M.|year= 2010|title= Atlantis time-lapse ocean bottom node survey: a project team's journey from acquisition through processing|journal= SEG Expanded Abstracts|volume= |issue= 29|pages= 4155|publisher= SEG|doi= |url= |accessdate= }}</ref>
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| ====Seismic data processing====
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| {{See also|Deconvolution|Seismic migration}}
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| There are three main processes in seismic data processing: [[Deconvolution#Seismology|deconvolution]], [[common-midpoint]] (CMP) stacking and [[Seismic migration|migration]].<ref>{{Cite book| author=Yilmaz, Öz | year=2001 | title=Seismic data analysis | publisher=Society of Exploration Geophysicists | isbn=1-56080-094-1 |page= 4}}</ref>
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| ''Deconvolution'' is a process that tries to extract the reflectivity series of the Earth, under the assumption that a seismic trace is just the reflectivity series of the Earth convolved with distorting filters.<ref>{{Cite book| author=Sheriff, R. E., Geldart, L. P. | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 292}}</ref> This process improves temporal resolution by collapsing the seismic wavelet, but it is nonunique unless further information is available such as well logs, or further assumptions are made. ''Deconvolution'' operations can be cascaded, with each individual deconvolution designed to remove a particular type of distortion.
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| ''CMP stacking'' is a robust process that uses the fact that a particular location in the subsurface will have been sampled numerous times and at different offsets. This allows a geophysicist to construct a group of traces with a range of offsets that all sample the same subsurface location, known as a ''Common Midpoint Gather''.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Common-midpoint | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=CMP | accessdate = September 8, 2013}}</ref> The average amplitude is then calculated along a time sample, resulting in significantly lowering the random noise but also losing all valuable information about the relationship between seismic amplitude and offset. Less significant processes that are applied shortly before the ''CMP stack'' are ''[[Normal moveout|Normal moveout correction]]'' and ''statics correction''. Unlike marine seismic data, land seismic data has to be corrected for the elevation differences between the shot and receiver locations. This correction is in the form of a vertical time shift to a flat datum and is known as a ''statics correction'', but will need further correcting later in the processing sequence because the velocity of the near-surface is not accurately known. This further correction is known as a ''residual statics correction.''
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| ''Seismic migration'' is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface.
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| ====Seismic interpretation====
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| {{See also|Geologic modelling}}
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| The goal of seismic interpretation is to obtain a coherent geological story from the map of processed seismic reflections.<ref>{{cite book |title= Petroleum Geoscience|last1= Gluyas|first1= J|last2= Swarbrick|first2= R|authorlink= |coauthors= |year= 2004|publisher= Blackwell Publishing|location= |isbn= 978-0-632-03767-4|page=24 |pages= |url= |accessdate=}}</ref> At its most simple level, seismic interpretation involves tracing and correlating along continuous reflectors throughout the 2D or 3D dataset and using these as the basis for the geological interpretation. The aim of this is to produce structural maps that reflect the spatial variation in depth of certain geological layers. Using theses maps hydrocarbon traps can be identified and models of the subsurface can be created that allow volume calculations to be made. However, a seismic dataset rarely gives a picture clear enough to do this. This is mainly because of the vertical and horizontal seismic resolution<ref>[http://www.epgeology.com/geophysics-seismics-f22/seismic-interpretation-basics-t194.html Basics of Seismic Interpretation]</ref> but often noise and processing difficulties also result in a lower quality picture. Due to this, there is always a degree of uncertainty in a seismic interpretation and a particular dataset could have more than one solution that fits the data. In such a case, more data will be needed to constrain the solution, for example in the form of further seismic acquisition, [[borehole logging]] or [[gravimetry|gravity]] and [[Aeromagnetic survey|magnetic survey data]]. Similarly to the mentality of a seismic processor, a seismic interpreter is generally encouraged to be optimistic in order encourage further work rather than the abandonment of the survey area.<ref>{{Cite book| author=Sheriff, R. E., Geldart, L. P. | year=1995 | title=Exploration Seismology |edition=2nd | publisher=Cambridge University Press | isbn=0-521-46826-4 |page= 349}}</ref> Seismic interpretation is completed by both [[geologists]] and [[geophysicists]], with most seismic interpreters having an understanding of both fields.
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| In hydrocarbon exploration, the features that the interpreter is particularly trying to delineate are the parts that make up a [[petroleum reservoir]] - the [[source rock]], the reservoir rock, the seal and [[structural trap|trap]].
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| ====Seismic attribute analysis====
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| {{See also|Seismic attribute}}
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| Seismic attribute analysis involves extracting or deriving a quantity from seismic data that can be analysed in order to enhance information that might be more subtle in a traditional seismic image, leading to a better [[geology|geological]] or [[geophysics|geophysical]] interpretation of the data.<ref>{{cite web | title = Petrel Seismic Attribute Analysis | website = [[Schlumberger]] | url = http://www.slb.com/services/software/geo/petrel/seismic/seismic_multitrace_attributes.aspx | accessdate = September 8, 2013}}</ref> Examples of attributes that can be analysed include mean amplitude, which can lead to the delineation of [[bright spot]]s and [[dim spot]]s, [[Coherence (signal processing)|coherency]] and [[amplitude versus offset]]. Attributes that can show the presence of hydrocarbons are called [[direct hydrocarbon indicator]]s.
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| ===Crustal studies===
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| The use of reflection seismology in studies of [[tectonics]] and the Earth's crust was pioneered in the 1970s by groups such as the Consortium for Continental Reflection Profiling (COCORP), who inspired deep seismic exploration in other countries such as BIRPS in Great Britain and ECORS in France.<ref>{{cite web |url= http://www.geo.cornell.edu/geology/cocorp/COCORP.html|title= Consortium for Continental Reflection Profiling|author= |date= |work= |publisher= |accessdate=6 March 2012}}</ref> The British Institutions Reflection Profiling Syndicate (BIRPS) was started up as a result of oil hydrocarbon exploration in the North Sea. It became clear that there was a lack of understanding of the tectonic processes that had formed the geological structures and [[sedimentary basin]]s which were being explored. The effort produced some significant results and showed that it is possible to profile features such as [[thrust fault]]s that penetrate through the crust to the [[upper mantle]] with marine seismic surveys.<ref>{{cite web |url= http://www.earthscrust.org/science/startups/birps-su.html|title= BIRPS|author= Crustal Architecture and Images|date= |work= |publisher= |accessdate=6 March 2012}}</ref>
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| ==Environmental impact==
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| As with all human activities, seismic reflection surveys may have some [[Human impact on the environment|impact on the Earth's natural environment]] and both the hydrocarbon industry and environmental groups partake in research to investigate these effects.
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| ===Land===
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| On land, conducting a seismic survey may require the building of [[road]]s, for transporting equipment and personnel, and vegetation may need to be cleared for the deployment of equipment. If the survey is in a relatively undeveloped area, significant [[habitat (ecology)|habitat]] disturbance may occur and many governments require seismic companies to follow strict rules regarding destruction of the environment, for example the use dynamite as a seismic source may be disallowed. Seismic processing techniques allow for seismic lines to deviate around natural obstacles, or use pre-existing non-straight tracks and trails. With careful planning, this can greatly reduce the environmental impact of a land seismic survey. 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.
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| ===Marine===
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| The main environmental concern for marine seismic surveys is the potential of noise associated with the high-energy seismic source to disturb animal life, especially [[cetacean]]s such as [[whale]]s, [[porpoise]]s, and [[dolphin]]s, as these mammals use sound as their primary method of communication with one another.<ref>{{cite book |title= Marine Mammals and Noise|last= Richardson|first= W. John|authorlink= |coauthors= ''et al''|year= 1995|publisher= Academic Press|location= |isbn= 978-0-12-588441-9|page= |pages= 1|url= |accessdate=}}</ref> High-level and long-duration sound can cause physical damage, such as hearing loss, whereas lower-level noise could cause temporary threshold shifts in hearing, obscuring sounds that are vital to marine life.<ref>{{cite journal |author= Gausland, I.|year= 2000|title= Impact of seismic surveys on marine life|journal= The Leading Edge|volume= |issue= |pages= 904|publisher= SEG|doi= |pmid= |pmc= |url= http://www.anp.gov.br/brasil-rounds/round9/round9/guias_R9/sismica_R9/Bibliografia/Gausland%202000%20-%20Impact%20of%20seismic%20surveys%20on%20marine%20life.PDF|accessdate= 8 March 2012}}</ref>
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| A study has shown<ref>{{cite journal |author= McCauley, R.D. ''et al''|year= 2000|title= Marine seismic surveys: A study of environmental implications|journal= APPEA|volume= |issue= |pages= 692–708|publisher= |doi= |pmid= |pmc= |url= http://www.anp.gov.br/meio/guias/sismica/biblio/McCauleye2000.PDF|accessdate= 8 March 2012}}</ref> that migrating [[humpback whales]] will leave a minimum 3 km gap between themselves and an operating seismic vessel, with resting humpback whale pods containing cows exhibiting increased sensitivity and leaving an increased gap of 7–12 km. Conversely, the study found that male humpback whales were attracted to a single operating airgun as they were believed to have confused the low-frequency sound with that of [[Whale surfacing behaviour|whale breaching behaviour]]. In addition to whales, [[sea turtles]], [[fish]] and [[squid]] all showed alarm and avoidance behaviour in the presence of an approaching seismic source. It is difficult to compare reports on the effects of seismic survey noise on marine life because methods and units are often inadequately documented.
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| The [[Gray whale]] will avoid its regular migratory and feeding grounds by >30 km in areas of seismic testing{{citation needed|date=July 2012}}. 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.
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| Offering another point of view, a joint paper from the International Association of Geophysical Contractors (IAGC) and the International Association of Oil and Gas Producers (OGP) argue that the noise created by marine seismic surveys is comparable to natural sources of seismic noise, stating:<ref>Scientific Surveys and Marine Mammals - Joint OGP/IAGC Position Paper, December 2008 - http://www.ogp.org.uk/pubs/358.pdf</ref>
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| <blockquote>"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."</blockquote>
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| ==History==
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| 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).<ref>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: [http://www.croatianhistory.net/etf/orlic.pdf]</ref> 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.
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| The [[Canada|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 first commercial discovery of oil using the seismic method in 1924.<ref>Sheriff, R. E., and Geldart, L. P., 1995, Exploration Seismology, Second Edition, Cambridge University Press, pp. 3-6.</ref> [[J. Clarence Karcher|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.
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| 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 [[Hess Corporation|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]].
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| 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 [[WesternGeco]], [[CGG (company)|CGG]], [[ION Geophysical]], [[Petroleum Geo-Services]] and [[Polarcus]]. 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.
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| ==See also==
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| *[[Deconvolution]]
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| *[[SEG Y]], a popular file format for seismic reflection data
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| *[[Depth conversion]], the conversion of acoustic waves two-way travel time to actual depth
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| *[[Seismic waves]]
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| *[[Seismic refraction]]
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| *[[Swell filter]]
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| *[[Passive seismic]]
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| *[[Seismic migration]]
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| *[[Synthetic seismogram]]
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| *[[Seismic Unix]], open source software for processing of seismic reflection data
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| *[[Exploration geophysics]]
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| ==Further reading==
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| 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.
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| * {{Cite book| last=Brown |first=Alistair R. |year=2004 |title=Interpretation of three-dimensional seismic data |publisher=Society of Exploration Geophysicists and American Association of Petroleum Geologists |edition=sixth ed.| isbn=0-89181-364-0}}
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| * {{Cite book| last=Biondi | first=B. | year=2006 | title=3d Seismic Imaging: Three Dimensional Seismic Imaging| publisher=Society of Exploration Geophysicists | isbn=0-07-011117-0 |url=http://sepwww.stanford.edu/data/media/public/sep//biondo/3DSI_frame.html}}
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| * {{Cite book| last=Claerbout | first=Jon F. | year=1976 | title=Fundamentals of geophysical data processing| publisher=McGraw-Hill | isbn=1-56080-137-9 |url=http://sepwww.stanford.edu/data/media/public/sep//prof/fgdp/toc_html/}}
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| * {{Cite book| author=Ikelle, Luc T. and Lasse Amundsen | year=2005 | title=Introduction to Petroleum Seismology | publisher=Society of Exploration Geophysicists | isbn=1-56080-129-8 }}
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| * {{Cite book| last=Scales |first=John | year=1997 | title=Theory of seismic imaging| publisher=Samizdat Press |location=Golden, Colorado | url=http://samizdat.mines.edu/imaging/}}
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| * {{Cite book| author=Yilmaz, Öz | year=2001 | title=Seismic data analysis | publisher=Society of Exploration Geophysicists | isbn=1-56080-094-1}}
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| *{{Cite book| author=Milsom, J., University College of London | year=2005 | title=Field Geophysics | publisher=Wiley Publications| isbn=978-0-470-84347-5}}
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| *{{Cite book| author=Chapman, C.H.. | year=2004 | title=Fundamentals of Seismic Wave Propagation | publisher=Cambridge University Press| isbn=978-0-521-81538-3}}
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| 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]].
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| ==References==
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| {{Reflist|2}}
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| ==External links==
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| * [http://www.mssu.edu/seg-vm/bio_henry_salvatori.html Biography of Henry Salvatori]
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| * [http://gst.seg.org/TL/2001/05/TidBits.shtml History of reflection seismology in Oklahoma]
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| * [http://slim.eos.ubc.ca Reflection Seismology Literature at Seismic Laboratory for Imaging and Modeling]
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| * [http://sepwww.stanford.edu/doku.php?id=sep:about:about Reflection Seismology Literature at Stanford Exploration Project]
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| * [http://www.iagc.org Website of the International Association of Geophysical Contractors]
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| * [http://www.ogp.org.uk/pubs/358.pdf IAGC/OGP position paper on seismic surveys and marine mammals] (PDF)
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| * [http://radexpro.com/site/files/tutorials/EN_tutorCDP.pdf Tutorial on seismic reflection data processing]
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| * A [http://delph.ixblue.com/readings/ guide to marine High-Resolution Seismic / Sub-Bottom Profiler acquisition and processing] and [http://delph.ixblue.com/seismic/gallery/ image galleries]
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| * Overview of IAGC Marine Geophysical Operations Educational CD - 1 http://www.youtube.com/watch?v=2XlI6rp5F48
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| * [http://www.earthscrust.org/science/ Useful website with detail about seismic crustal studies]
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| * [http://www.seismicsurvey.com.au/ Information about using Seismic Survey in oil and gas exploration in Australia]
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| *[http://www.youtube.com/watch?v=lRsAO7Y2k60&list=PL369AB2240FF8D16B&index=1 Illustration for seismic acquisition on youtube]
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| <br>
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| {{Petroleum industry}}
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| {{Use dmy dates|date=September 2010}}
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| {{DEFAULTSORT:Reflection Seismology}}
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| [[Category:Seismology measurement]]
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| [[Category:Petroleum geology]]
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