Reflection Seismology
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Reflection seismology (or seismic reflection) is a method of
exploration geophysics Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of ...
that uses the principles of
seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
to estimate the properties of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's subsurface from reflected
seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy ...
s. The method requires a controlled
seismic source A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a speci ...
of energy, such as dynamite or
Tovex Tovex (also known as Trenchrite, Seismogel, and Seismopac) is a water-gel explosive composed of ammonium nitrate and methylammonium nitrate that has several advantages over traditional dynamite, including lower toxicity and safer manufacture, tr ...
blast, a specialized air gun or a seismic vibrator. Reflection seismology is similar to
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
and echolocation. This article is about surface seismic surveys; for vertical seismic profiles, see VSP.


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). 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. Seismic reflection exploration grew out of the
seismic refraction Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic st ...
exploration method, which was used to find oil associated with
salt dome A salt dome is a type of structural dome formed when salt (or other evaporite minerals) intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered usin ...
s. 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 refraction seismic method in 1924. The 1924 discovery of the Orchard salt dome in Texas led to a boom in seismic refraction exploration along the Gulf Coast, but by 1930 the method had led to the discovery of most of the shallow
Louann Salt The Louann Salt is a widespread evaporite formation that formed in the Gulf of Mexico during the Callovian in the mid Jurassic. The Louann formed in a rift as the South American and North American Plates separated, from an embayment of the Pacific ...
domes, and the refraction seismic method faded. The Canadian inventor
Reginald Fessenden Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...
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 World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
. 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. John Clarence Karcher discovered seismic reflections independently while working for the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
Bureau of Standards (now the
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
) on methods of sound ranging to detect
artillery Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during siege ...
. In discussion with colleagues, the idea developed that these reflections could aid in exploration for
petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
. With several others, many affiliated with the
University of Oklahoma The University of Oklahoma (OU) is a Public university, public research university in Norman, Oklahoma. Founded in 1890, it had existed in Oklahoma Territory near Indian Territory for 17 years before the two Territories became the state of Oklahom ...
, Karcher helped to form the Geological Engineering Company, incorporated in
Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
in April 1920. The first field tests were conducted near
Oklahoma City, Oklahoma Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, it ranks 20th among United States cities in population, and ...
in 1921. Early reflection seismology was viewed with skepticism by many in the oil industry. An early advocate of the method commented: :"As one who personally tried to introduce the method into general consulting practice, the senior writer can definitely recall many times when reflections were not even considered on a par with the divining rod, for at least that device had a background of tradition." The Geological Engineering Company 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 Geophysical Service Inc. (often abbreviated GSI) was founded by John Clarence Karcher and Eugene McDermott in 1930 for the purpose of using refraction and reflection seismology to explore for petroleum deposits. History On December 6, 1941, t ...
(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 Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
. Early GSI employee
Henry Salvatori Henry Salvatori (March 28, 1901 – July 6, 1997) was an American geophysicist, businessman, philanthropist, and political activist. Salvatori founded Western Geophysical in 1933 and, after selling the company in 1960, pursued a second caree ...
left that company in 1933 to found another major seismic contractor,
Western Geophysical Western Geophysical was an international oil exploration company founded in California in 1933 by Henry Salvatori for the purpose of using reflection seismology to explore for petroleum. The company prospered and was sold by Salvatori to Litt ...
. Many other companies using reflection seismology in hydrocarbon exploration,
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
,
engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
studies, and other applications have been formed since the method was first invented. Major service companies in recent years have included CGG, ION Geophysical,
Petroleum Geo-Services PGS (Petroleum Geo-Services) is a technologically focused oilfield service company involved in providing geophysical services worldwide. Its seismic service offerings help oil companies find oil and gas reserves offshore. Product offerings span ...
, Polarcus, TGS and
WesternGeco WesternGeco is a geophysical services company. It is headquartered in Schlumberger House on the property of London Gatwick Airport in Crawley, West Sussex, in Greater London. __NOTOC__ Background The company provides reservoir imaging, moni ...
, but since the oil price crash of 2015, providers of seismic services have continued to struggle financially such as Polarcus, whilst companies that were seismic acquisition industry leaders just ten years ago such as CGG and WesternGeco have now removed themselves from the seismic acquisition environment entirely and restructured to focus upon their existing seismic data libraries, seismic data management and non-seismic related oilfield services.


Outline of the method

Seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy ...
s are mechanical perturbations that travel in the Earth at a speed governed by the
acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cu ...
of the medium in which they are travelling. The acoustic (or seismic) impedance, ''Z'', is defined by the equation: :Z=v\rho \ , where ''v'' is the seismic wave velocity and ''ρ'' (
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''
rho Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...
'') is the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of the rock. When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will
refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
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 (as
geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
s or
hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
s) at the surface.Sheriff, R. E., Geldart, L. P., (1995), 2nd Edition. Exploration Seismology. Cambridge University Press. 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. In common with other geophysical methods, reflection seismology may be seen as a type of
inverse problem An inverse problem in science is the process of calculating from a set of observations the causal factors that produced them: for example, calculating an image in X-ray computed tomography, source reconstruction in acoustics, or calculating the ...
. That is, given a set of data collected by
experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into Causality, cause-and-effect by demonstrating what outcome oc ...
ation and the physical laws that apply to the experiment, the experimenter wishes to develop an
abstract model A conceptual model is a representation of a system. It consists of concepts used to help people know, understand, or simulate a subject the model represents. In contrast, physical models are physical object such as a toy model that may be assemb ...
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.


The reflection experiment

The general principle of seismic reflection is to send elastic waves (using an energy source such as dynamite explosion or
Vibroseis A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specia ...
) 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 A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
, which converts
ground motion Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by seismic waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth a ...
into an analogue electrical signal. In water,
hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
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 data storage device, then the shot location is moved along and the process is repeated. Typically, the recorded signals are subjected to significant amounts of
signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
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 A supercomputer is a computer with a high level of performance as compared to a general-purpose computer. The performance of a supercomputer is commonly measured in floating-point operations per second (FLOPS) instead of million instructions p ...
or
computer clusters A computer cluster is a set of computers that work together so that they can be viewed as a single system. Unlike grid computers, computer clusters have each node set to perform the same task, controlled and scheduled by software. The comp ...
.


Reflection and transmission at normal incidence

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 The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of the reflected wave is predicted by multiplying the amplitude of the incident wave by the seismic ''
reflection coefficient In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected w ...
'' R, determined by the impedance contrast between the two materials. For a wave that hits a boundary at
normal Normal(s) or The Normal(s) may refer to: Film and television * ''Normal'' (2003 film), starring Jessica Lange and Tom Wilkinson * ''Normal'' (2007 film), starring Carrie-Anne Moss, Kevin Zegers, Callum Keith Rennie, and Andrew Airlie * ''Norma ...
incidence (head-on), the expression for the reflection coefficient is simply :R=\frac, where Z_1 and Z_2 are the impedance of the first and second medium, respectively. Similarly, the amplitude of the incident wave is multiplied by the ''
transmission coefficient The transmission coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A transmission coefficient describes the amplitude, intensity, or total power of a transmitte ...
'' T to predict the amplitude of the wave transmitted through the boundary. The formula for the normal-incidence transmission coefficient is :T=1+R=\frac. As the sum of the energies of the reflected and transmitted wave has to be equal to the energy of the incident wave, it is easy to show that :\frac+\frac=\frac=\frac. By observing changes in the strength of reflections, 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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
and wave velocity, by means of
seismic inversion In geophysics (primarily in oil-and-gas exploration/development), seismic inversion is the process of transforming seismic reflection data into a quantitative rock-property description of a reservoir. Seismic inversion may be pre- or post- stack, d ...
.


Reflection and transmission at non-normal incidence

The situation becomes much more complicated in the case of non-normal incidence, due to mode conversion between
P-waves A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
and
S-waves __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...
, and is described by the Zoeppritz equations. In 1919, Karl Zoeppritz derived 4 equations that determine the amplitudes of reflected and
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
waves at a planar interface for an incident P-wave as a function of the angle of incidence and six independent elastic parameters. 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. 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 In geophysics and reflection seismology, amplitude versus offset (AVO) or amplitude variation with offset is the general term for referring to the dependency of the seismic attribute, amplitude, with the distance between the source and receiver (t ...
) 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:Avseth, P, T Mukerji and G Mavko (2005). Quantitative seismic interpretation. Cambridge University Press, Cambridge, p. 183 :R(\theta ) = R(0) + G \sin^2 \theta where R(0) = reflection coefficient at zero-offset (normal incidence); G = AVO gradient, describing reflection behaviour at intermediate offsets and (\theta) = angle of incidence. This equation reduces to that of normal incidence at (\theta)=0.


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, 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 Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
.


Sources of noise

In addition to reflections off interfaces within the subsurface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded:


Air wave

The airwave travels directly from the source to the receiver and is an example of coherent noise. It is easily recognizable because it travels at a speed of 330 m/s, the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
in air.


Ground roll / Rayleigh wave / Scholte wave / Surface wave

A
Rayleigh wave Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructi ...
typically propagates along a free surface of a solid, but the elastic constants and
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of air are very low compared to those of rocks so the surface of the Earth is approximately a
free surface In physics, a free surface is the surface of a fluid that is subject to zero parallel shear stress, such as the interface between two homogeneous fluids. An example of two such homogeneous fluids would be a body of water (liquid) and the air in ...
. 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. The
Scholte wave A Scholte wave is a surface wave (interface wave) propagating at an interface between a fluid and an elastic solid medium (such as an interface between water and sand). The wave is of maximum intensity at the interface and decreases exponentially a ...
is similar to ground roll but occurs at the sea-floor (fluid/solid interface) and it can possibly obscure and mask deep reflections in marine seismic records. The velocity of these waves varies with wavelength, so they are said to be dispersive and the shape of the wavetrain varies with distance.


Refraction / Head wave / Conical wave

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. The same phenomenon is utilised in
seismic refraction Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic st ...
.


Multiple reflection

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. Multiples from the bottom of a body of water and the air-water interface are common in marine seismic data, and are suppressed by seismic processing.


Cultural noise

Cultural noise includes noise from weather effects, planes, helicopters, electrical pylons, and ships (in the case of marine surveys), all of which can be detected by the receivers.


Electromagnetic noise

Particularly important in urban environments (i.e. power lines), it is hardly removable. Some particular sensor as microelectromechanical systems (MEMs) are used to decrease these interference when in such environments.


2D versus 3D

The original seismic reflection method involved acquisition along a two-dimensional vertical profile through the crust, now referred to as 2D data. This approach worked well with areas of relatively simple geological structure where dips are low. However, in areas of more complex structure, the 2D technique failed to properly image the subsurface due to out of plane reflections and other artefacts. Spatial aliasing is also an issue with 2D data due to the lack of resolution between the lines. Beginning with initial experiments in the 1960s, the seismic technique explored the possibility of full three-dimensional acquisition and processing. In the late 1970s the first large 3D datasets were acquired and by the 1980s and 1990s this method became widely used.


Applications

Reflection seismology is used extensively in a number of fields and its applications can be categorised into three groups, each defined by their depth of investigation: *Near-surface applications – an application that aims to understand geology at depths of up to approximately 1 km, typically used for
engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
and
environmental A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
surveys, as well as
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
and
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
exploration. A more recently developed application for seismic reflection is for geothermal energy surveys, although the depth of investigation can be up to 2 km deep in this case. *
Hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...
– 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 In reflection seismology, a seismic attribute is a quantity extracted or derived 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 geological or geo ...
analysis and other
exploration geophysics Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of ...
tools and used to help
geologists A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...
build a geological model of the area of interest. *Mineral exploration – The traditional approach to near-surface (<300 m) mineral exploration has been to employ geological mapping, geochemical analysis and the use of aerial and ground-based potential field methods, in particular for greenfield exploration, in the recent decades reflection seismic has become a valid method for exploration in hard-rock environments. *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. A method similar to reflection seismology which uses
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
instead of elastic waves, and has a smaller depth of penetration, is known as
Ground-penetrating radar Ground-penetrating radar (GPR) is a Geophysics, geophysical method that uses radar pulses to Geophysical imaging, image the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, ...
or GPR.


Hydrocarbon exploration

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 reservoirs. The size and scale of seismic surveys has increased alongside the significant increases in computer power since the late 20th century. This led the seismic industry from laboriously – and therefore rarely – acquiring small 3D surveys in the 1980s to 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. The primary environments for seismic
hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...
are land, the transition zone and marine: ''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. ''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, 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. ''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).


Seismic data acquisition

Seismic data acquisition is the first of the three distinct stages of seismic exploration, the other two being seismic data processing and seismic interpretation. Seismic surveys are typically designed by
National oil companies A national oil company (NOC) is an oil and gas company fully or in the majority-owned by a national government. According to the World Bank, NOCs accounted for 75% global oil production and controlled 90% of proven oil reserves in 2010. Due to thei ...
and International oil companies who hire service companies such as CGG,
Petroleum Geo-Services PGS (Petroleum Geo-Services) is a technologically focused oilfield service company involved in providing geophysical services worldwide. Its seismic service offerings help oil companies find oil and gas reserves offshore. Product offerings span ...
and
WesternGeco WesternGeco is a geophysical services company. It is headquartered in Schlumberger House on the property of London Gatwick Airport in Crawley, West Sussex, in Greater London. __NOTOC__ Background The company provides reservoir imaging, moni ...
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 to the oil company so that it can be geologically interpreted.


Land survey acquisition

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. There are a number of options available for a controlled seismic source in a land survey and particularly common choices are
Vibroseis A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specia ...
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. 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. Dynamite is an impulsive source that is regarded as the ideal geophysical source due to it producing an almost perfect
impulse function In mathematics, the Dirac delta distribution ( distribution), also known as the unit impulse, is a generalized function or distribution over the real numbers, whose value is zero everywhere except at zero, and whose integral over the entire ...
but it has obvious environmental drawbacks. For a long time, it was the only seismic source available until weight dropping was introduced around 1954, 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. 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). 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 resupply activities, medical support, camp and equipment maintenance tasks, security, personnel crew changes and waste management. Some operations may also operate smaller 'fly' camps that are set up remotely where the distance is too far to travel back to the main camp on a daily basis and these will also need logistical support on a frequent basis.


Marine survey acquisition (Towed Streamer)

Towed streamer marine seismic surveys are conducted using specialist seismic vessels that tow one or more cables known as streamers just below the surface typically between 5–15 metres depending upon the project specification that contain groups of hydrophones (or receiver groups) along their length (see diagram). Modern streamer vessels normally tow multiple streamers astern which can be secured to underwater wings, commonly known as doors or vanes that allow a number of streamers to be towed out wide to the port and starboard side of a vessel. Current streamer towing technology such as seen on the PGS operated Ramform series of vessels built between 2013 and 2017 has pushed the number of streamers up to 24 in total on these vessels. For vessels of this type of capacity, it is not uncommon for a streamer spread across the stern from 'door to door' to be in excess on one nautical mile. The precise configuration of the streamers on any project in terms of streamer length, streamer separation, hydrophone group length and the offset or distance between the source centre and the receivers will be dependent upon the geological area of interest below the sea floor that the client is trying to get data from. Streamer vessels also tow high energy sources, principally high pressure air gun arrays that operate at 2000psi that fire together to the create a tuned energy pulse into the seabed from which the reflected energy waves are recorded on the streamer receiver groups. Gun arrays are tuned, that is the frequency response of the resulting air bubble from the array when fired can be changed depending upon the combination and number of guns in a specific array and their individual volumes. Guns can be located individual on an array or can be combined to form clusters. Typically, source arrays have a volume of 2000 cubic inches to 7000 cubic inches, but this will depend upon the specific geology of the survey area. Marine seismic surveys generate a significant quantity of data due to the size of modern towed streamer vessels and their towing capabilities. 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 was accepted that this type of acquisition was useful for initial exploration but inadequate for development and production, in which
wells Wells most commonly refers to: * Wells, Somerset, a cathedral city in Somerset, England * Well, an excavation or structure created in the ground * Wells (name) Wells may also refer to: Places Canada *Wells, British Columbia England * 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). This successfully delivered increased illumination of the subsurface and a better signal to noise ratio. 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 field in 2004. 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. These are now the three common types of marine towed streamer seismic surveys.


Marine survey acquisition (Ocean Bottom Seismic (OBS))

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 production platforms, without having the compromise the resultant image quality. 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 A remotely operated underwater vehicle (technically ROUV or just ROV) is a tethered underwater mobile device, commonly called ''underwater robot''. Definition This meaning is different from remote control vehicles operating on land or in the a ...
s (ROVs) in deep water when repeatability is valued (see 4D, below). Conventional OBC surveys use dual-component receivers, combining a pressure sensor (
hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
) and a vertical particle velocity sensor (vertical
geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
), 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
shear waves In physics, a transverse wave is a wave whose oscillations are perpendicular to the direction of the wave's advance. This is in contrast to a longitudinal wave which travels in the direction of its oscillations. Water waves are an example of t ...
, which do not travel through water but can still contain valuable information. 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. 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. In 2005, ocean bottom nodes (OBN) – an extension of the OBC method that uses battery-powered cableless receivers placed in deep water – was first trialled over the
Atlantis Oil Field The Atlantis oil field is the third largest oil field in the Gulf of Mexico. The field was discovered in 1998 and is located at the Green Canyon blocks 699, 700, 742, 743, and 744 in United States federal waters in the Gulf of Mexico about from ...
in a partnership between BP and
Fairfield Geotechnologies Fairfield Geotechnologies is a seismic service company focusing on data licensing and data processing as well as imaging, data analytics and data interpretation. It is headquartered in Houston, Texas. History The company was founded in 1976 and i ...
. 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.


Marine survey acquisition (Ocean Bottom Nodes (OBN))

The development of node technology came as a direct development from that of ocean bottom cable technology, i.e. that ability to place a hydrophone in direct contact with the seafloor to eliminate the seafloor to hydrophone sea water space that exists with towed streamer technology. The ocean bottom hydrophone concept itself is not new and has been used for many years in scientific research, but its rapid use as a data acquisition methodology in oil and gas exploration is relatively recent. Nodes are self-contained 4-component units which include a hydrophone and three horizontal and vertical axis orientation sensors. Their physical dimensions vary depending on the design requirement and the manufacturer, but in general nodes tend to weigh in excess of 10 kilograms per unit to counteract buoyancy issues and to lessen the chance of movement on the seabed due to currents or tides. Nodes are usable in areas where streamer vessels may not be able to safely enter and so for the safe navigation of node vessels and prior to the deployment of nodes, a
bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...
seabed survey is normally conducted of the survey area using side-scan technology to map the seabed
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...
in detail. This will identify any possible hazards that could impact the safe navigation of node and source vessels and also to identify any issues for node deployment including subsea obstructions, wrecks, oilfield infrastructure or sudden changes in water depths from underwater cliffs, canyons or other locations where nodes may not be stable or not make a good connection to the seabed. Unlike OBC operations, a node vessel does not connect to a node line, whereas ocean bottom cables need to be physically attached to a recorder vessel to record data in real-time. With nodes, until the nodes are recovered and the data reaped from them (reaping is the industry term used to remove data from a recovered node when it is placed within a computerised system that copies the hard drive data from the node), there is an assumption that the data will be recorded as there is no real-time quality control element to a node's operating status as they are self-contained and not connected to any system once they are deployed. The technology is now well-established and very reliable and once a node and its battery system has passed all of its set up criteria there is a high degree of confidence that a node will work as specified. Technical downtime during node projects, i.e. individual node failures during deployment are usually in single figures as a percentage of the total nodes deployed. Nodes are powered by either rechargeable internal
Lithium-ion battery A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also s ...
packs or replaceable non-rechargeable batteries - the design and the specification of the node will determine what battery technology is used. The battery life of a node unit is a critical consideration in the design of a node project; this is because once the battery runs out on a node, the data that has been collected is no longer stored on the solid-state hard drive and all data recorded since it was deployed on the sea floor will be lost. Therefore, a node with a 30-day battery life must be deployed, record data, be recovered and reaped within that 30-day period. This also ties in with the number of nodes that are to be deployed as this is closely related to battery life too. If too many nodes are deployed and the OBN crew’s resources are not sufficient to recover these in time or external factors such as adverse weather limits recovery operations, batteries can expire and data can be lost. Disposable or non-rechargeable batteries can also create a significant waste management issue as batteries must be transported to and from an operation and the drained batteries disposed of by a licensed contractor ashore. Another important consideration is that of synchronising the timing of individual node clock units with an internal clock drift correction. Any error in synchronising nodes properly before they are deployed can create unusable data. Because node acquisition is often multi-directional and from a number of sources simultaneously across a 24-hour time frame, for accurate data processing it is vital that all of the nodes are working to the same clock time. The node type and specification will determine the node handling system design and the deployment and recovery modes. At present there are two mainstream approaches; node on a rope and ROV operations. Node on a rope This method requires the node to be attached to a steel wire or a high specification rope. Each node will be evenly spaced along the rope which will have special fittings to securely connect the node to the rope, for example every 50 metres depending upon the prospect design. This rope is then laid by a specialist node vessel using an node handling system, usually with
dynamic positioning Dynamic positioning (DP) is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyrocompass ...
along a pre-defined node line. The nodes are ‘landed’ onto pre-plotted positions with an agreed and acceptable error radius, for example, a node must be placed within a 12.5 metre radius from the navigation pre-plot positions. They are often accompanied by pingers, small transponders that can be detected by an underwater acoustic positioning transducer which allows a pinging vessel or the node vessel itself to establish a definite sea floor position for each node on deployment. Depending on the contract, pingers can be located on every node or every third node, for example. Pinging and pinging equipment is the industry shorthand for the use of USBL or Ultra-short baseline acoustic positioning systems which are interfaced with vessel based
differential GPS Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). A DGPS for GPS can increase accuracy by about a thousandfold, from approximately to . DGPSs c ...
or Differential Global Positioning System navigation equipment. Node lines are usually recovered by anchor or grapple hook dragging to recover the node line back onboard the vessel. Handling systems on node vessels are used to store, deploy and recover nodes and their specific design will depend upon the node design. Small nodes can include a manual handling element whereas larger nodes are automatically handled by robotic systems for moving, storing, recharging and reaping nodes. Node vessels also use systems such as spoolers to manage rope lines and rope bins to store the many kilometres of ropes often carried onboard node on a rope vessels. Node on a rope is normally used where there is shallow water within the prospect, for example less than 100 metres or a transition zone area close to a beach. For deeper water operations, a dynamic positioning vessel is used to ensure accurate deployment of nodes, but these larger vessels have a limitation as to how far into shore they can safely navigate into; the usual cutoff will be between 15 to 20 metres water depth depending on the vessel and its in-water equipment. Specialist shallow water boats can then be used for deploying and recovering nodes in water depths as shallow as 1 to 3 metres. These shallow water nodes can then be used to tie-in with
geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
s on the shore to provide a consistent seismic line transition from water to land. There are some issues with this approach which make them vulnerable to damage or loss on a project and these all must be risk assessed and mitigated. Since nodes connected together on a rope sit on the sea floor unattended: they can be moved due to strong currents, the ropes can snag on seabed obstructions, they can be dragged by third party vessel anchors and caught by trawling fishing vessels. The threat of these types of potential hazards to this equipment should normally be identified and assessed during the project planning phase, especially in oilfield locations where well heads, pipelines and other subsea structures exist and where any contact with these must be avoided, normally achieved by adopting exclusion zones. Since it is possible for node lines to be moved after deployment, the issue of node position on recovery is critical and therefore positioning during both deployment and recovery is a standard navigation quality control check. In some case, node lines may need to be recovered and re-laid if the nodes have moved outside of the contract specifications. ROV deployment This method utilises ROV (
remotely operated underwater vehicle A remotely operated underwater vehicle (technically ROUV or just ROV) is a tethered underwater mobile device, commonly called ''underwater robot''. Definition This meaning is different from remote control vehicles operating on land or in the a ...
) technology to handle and place nodes at their pre-plotted positions. This type of deployment and recovery method uses a basket full of nodes which is lowered into the water. An ROV will connect with the compatible node basket and remove individual nodes from a tray in a pre-defined order. Each node will be placed on its allocated pre-plot position. On recovery, the process works in reverse; the node to be recovered is picked up by the ROV, placed into the node basket tray until the basket is full when it is lifted back to the surface. The basket is recovered onto the node vessel, the nodes are removed from the basket and reaped. ROV operations are normally used for deep water node projects, often in water depths to 3000 metres in the open ocean. However, there are some issues with ROV operations that need to be considered. ROV operations tend to be complex, especially deep water ROV operations and so the periodic maintenance demands may impact upon production. Umbilical’s and other high technology spares for ROV’s can be extremely expensive and repairs to ROVs which require onshore or third-party specialist support will stop a node project. Due to extreme water depths, the node deployment and recovery production rate will be much lower due to the time for node basket transit from surface to seafloor and there will almost certainly be weather or sea condition limitation for ROV operations in open ocean areas. The
logistics Logistics is generally the detailed organization and implementation of a complex operation. In a general business sense, logistics manages the flow of goods between the point of origin and the point of consumption to meet the requirements of ...
for supporting operations far from shore can also be problematic for regular resupply, bunkering and crew change activities.


Time lapse acquisition (4D)

Time lapse or 4D surveys are 3D seismic surveys repeated after a period of time, the 4D term refers to the fourth dimension which in this case is time. Time lapse surveys are acquired in order to observe reservoir changes during production and identify areas where there are barriers to flow that may not be detectable in conventional seismic. Time lapse surveys consist of a baseline survey and a monitor or repeat survey, acquired after the field has been in production. Most of these surveys have been repeated NATS surveys as they are cheaper to acquire and most fields historically already had a NATS baseline survey. Some of these surveys are collected using ocean-bottom cables because the cables can be accurately placed in their previous location after being removed. Better repetition of the exact source and receiver location leads to improved repeatability and better signal to noise ratios. 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 (LoFS) or permanent reservoir monitoring (PRM). 4D seismic surveys using towed streamer technology can be very challenging as the aim of a 4D survey it to repeat the original or baseline survey as accurately as possible. Weather, tides, current and even the time of year can have a significant impact upon how accurately such a survey can achieve that repeatability goal. OBN has proven to be another very good way to accurately repeat a seismic acquisition. The world's first 4D survey using nodes was acquired over the Atlantis Oil Field in 2009, with the nodes being placed by a ROV in a water depth of 1300–2200 metres to within a few metres of where they were previously placed in 2005.


Seismic data processing

There are three main processes in seismic data processing:
deconvolution In mathematics, deconvolution is the operation inverse to convolution. Both operations are used in signal processing and image processing. For example, it may be possible to recover the original signal after a filter (convolution) by using a deco ...
, common-midpoint (CMP) stacking and
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
. ''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. 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. ''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''. 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 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.'' ''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.


Seismic interpretation

The goal of seismic interpretation is to obtain a coherent geological story from the map of processed seismic reflections. 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 these 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 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
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
and 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. Seismic interpretation is completed by both
geologists A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...
and
geophysicists Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...
, with most seismic interpreters having an understanding of both fields. 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 In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...
, the reservoir rock, the seal and
trap A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research. Trap or TRAP may also refer to: Art and entertainment Films and television * ''Trap'' (2015 film), Fil ...
.


Seismic attribute analysis

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
geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...
or
geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' some ...
interpretation of the data. Examples of attributes that can be analysed include mean amplitude, which can lead to the delineation of
bright spot In reflection seismology, a bright spot is a local high amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator. It is used by geophysicists in hydrocarbon expl ...
s and
dim spot In reflection seismology, a dim spot is a local low-amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator. It primarily results from the decrease in acoustic im ...
s, coherency and
amplitude versus offset In geophysics and reflection seismology, amplitude versus offset (AVO) or amplitude variation with offset is the general term for referring to the dependency of the seismic attribute, amplitude, with the distance between the source and receiver (t ...
. Attributes that can show the presence of hydrocarbons are called
direct hydrocarbon indicator A hydrocarbon indicator (HCI) or direct hydrocarbon indicator (DHI), is an anomalous seismic attribute value or pattern that could be explained by the presence of hydrocarbons in an oil or gas reservoir. DHIs are particularly useful in hydrocarb ...
s.


Crustal studies

The use of reflection seismology in studies of
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
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. The
British Institutions Reflection Profiling Syndicate The British Institutions Reflection Profiling Syndicate, better known by its acronym BIRPS, was set up to acquire deep seismic reflection profiles around the United Kingdom Continental Shelf (UKCS). It was formed, initially as BURPS, the British Un ...
(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 Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subside ...
s which were being explored. The effort produced some significant results and showed that it is possible to profile features such as
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
s that penetrate through the crust to the
upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...
with marine seismic surveys.


Environmental impact

As with all human activities, seismic reflection surveys have some impact on the Earth's natural environment and both the hydrocarbon industry and environmental groups partake in research to investigate these effects.


Land

On land, conducting a seismic survey may require the building of
road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...
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 In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
disturbance may occur and many governments require seismic companies to follow strict rules regarding destruction of the environment; for example, the use of 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. The potential impact of any seismic survey on land needs to be assessed at the planning stage and managed effectively. Well regulated environments would generally require Environmental and Social Impact Assessment (ESIA) or
Environmental Impact Assessment Environmental Impact assessment (EIA) is the assessment of the environmental consequences of a plan, policy, program, or actual projects prior to the decision to move forward with the proposed action. In this context, the term "environmental imp ...
(EIA) reports prior to any work starting. Project planning also needs to recognise that once a project has completed, what impact if any, will be left behind. It’s the contractors and clients responsibility to manage the remediation plan as per the contract and as per the laws where the project has been conducted. Depending upon the size of a project, land seismic operations can have a significant local impact and a sizeable physical footprint, especially where storage facilities, camp utilities, waste management facilities (including black and grey water management), general and seismic vehicle parking areas, workshops and maintenance facilities and living accommodation are required. Contact with local people can cause potential disruptions to their normal lives such as increased noise, 24-hour operations and increased traffic and these have to be assessed and mitigated. Archeological considerations are also important and project planning must accommodate legal, cultural and social requirements that will have to be considered. Specialist techniques can be used to assessed safe working distances from buildings and archeological structures to minimise their impact and prevent damage.


Marine

The main environmental concern for marine seismic surveys is the potential for noise associated with the high-energy seismic source to disturb or injure animal life, especially cetaceans such as
whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s, porpoises, and
dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the ...
s, as these mammals use sound as their primary method of communication with one another. High-level and long-duration sound can cause physical damage, such as hearing loss, whereas lower-level noise can cause temporary threshold shifts in hearing, obscuring sounds that are vital to marine life, or behavioural disturbance. A study has shown that migrating humpback whales will leave a minimum 3 km gap between themselves and an operating seismic vessel, with resting humpback whale pods with 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 breaching behaviour. In addition to whales, sea turtles,
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
and
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
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. 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. Even so, airguns are shut down only when cetaceans are seen at very close range, usually under 1 km 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 (IOGP) argue that the noise created by marine seismic surveys is comparable to natural sources of seismic noise, stating: The UK government organisation, the
Joint Nature Conservation Committee The Joint Nature Conservation Committee (JNCC) is the public body that advises the UK Government and devolved administrations on UK-wide and international nature conservation. Originally established under the Environmental Protection Act 1990, J ...
(more commonly known as JNCC) is "...the public body that advises the UK Government and devolved administrations on UK-wide and international nature conservation." has had a long term vested interest in the impact of geophysical or seismic surveys on the marine environment for many years. Even back in the 1990s, it was understood at a government level that the impact of the sound energy produced by seismic surveys needed to be investigated and monitored. JNCC guidelines have been and continue to be one of the references used internationally as a possible baseline standard for surveys in seismic contracts world-wide, such as the 'JNCC guidelines for minimising the risk of injury to marine mammals from geophysical surveys (seismic survey guidelines)', 2017. A complicating factor in the discussion of seismic sound energy as a disruptive factor to marine mammals is that of the size and scale of seismic surveys as they are conducted into the 21st century. Historically, seismic surveys tended to have a duration of weeks or months and to be localised, but with OBN technology, surveys can cover thousands of square kilometres of ocean and can continue for years, all of the time putting sound energy into the ocean 24 hours a day from multiple energy sources. One current example of this is the 85,000 square kilometre mega seismic survey contract signed by the
Abu Dhabi Abu Dhabi (, ; ar, أَبُو ظَبْيٍ ' ) is the capital and second-most populous city (after Dubai) of the United Arab Emirates. It is also the capital of the Emirate of Abu Dhabi and the centre of the Abu Dhabi Metropolitan Area. ...
national oil company
ADNOC The Abu Dhabi National Oil Company ( ar, شركة بترول أبوظبي الوطنية) or ADNOC is the State-owned oil company of the United Arab Emirates (UAE). It is the world's 12th largest oil company by production. As of 2021, the compan ...
in 2018 with an estimated duration into 2024 across a range of deep-water areas, coastal areas, islands and shallow water locations. It may be very difficult to assess the long-term impact of these huge operations on marine life. In 2017, IOGP recommended that, to avoid disturbance whilst surveying: * Protective measures are employed to address site-specific environmental conditions of each operation to ensure that sound exposure and vessel traffic do not harm marine mammals. * Surveys planned to avoid known sensitive areas and time periods, such as breeding and feeding areas. * Exclusion zones are typically established around the seismic source to further protect marine fauna from any potentially detrimental effects of sound. The exclusion zone is typically a circle with a radius of at least 500 meters around the sound source. * Trained observers and listening devices are used to visually and acoustically monitor that zone for marine mammals and other protected species before any sound-producing operations begin. These observers help ensure adherence to the protective practices during operations and their detailed reports provide information on the biodiversity of the survey area to the local governments. * Sound production typically begins with a “soft-start” or “ramp-up” that involves a gradual increase of the sound level from the air gun source from a very low level to full operational levels at the beginning of the seismic lines – usually over 20 to 40 minutes. This soft-start procedure is intended to allow time for any animal that may be close to the sound source to move away as the sound grows louder. As second factor is the regulatory environment where the seismic survey is taking place. In highly regulated locations such as the
North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
or the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
, the legal requirements will be clearly stated at the contract level and both contractor and client will comply with the regulations as the consequences of non-compliance can be severe such as substantial fines or withdrawal of permits for exploration blocks. However, there are some countries which have a varied and rich marine
biome A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
but where the environmental laws are weak and where a regulator is ineffective or even non-existent. This situation, where the regulatory framework is not robust can severely compromise any attempts at protecting marine environments: this is frequently found where state-owned oil and gas companies are dominant in a country and where the regulator is also a state-owned and operated entity and therefore it is not considered to be truly independent.


See also

*
Deconvolution In mathematics, deconvolution is the operation inverse to convolution. Both operations are used in signal processing and image processing. For example, it may be possible to recover the original signal after a filter (convolution) by using a deco ...
* Depth conversion, the conversion of acoustic waves two-way travel time to actual depth *
Exploration geophysics Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of ...
*
LIGO The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
*
One-way wave equation A one-way wave equation is a first-order partial differential equation describing one wave traveling in a direction defined by the vector wave velocity. It contrasts with the second-order two-way wave equation describing a standing wavefield resu ...
*
Passive seismic Passive seismic is the detection of natural low frequency earth movements, usually with the purpose of discerning geological structure and locate underground oil, gas, or other resources. Usually the data listening is done in multiple measurement p ...
* SEG-Y, a popular file format for seismic reflection data *
Seismic migration 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 ...
*
Seismic refraction Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic st ...
* Seismic Unix, open source software for processing of seismic reflection data *
Seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy ...
* Swell filter * Synthetic seismogram


References


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 Society of Exploration Geophysicists (SEG) is a learned society dedicated to promoting the science and education of exploration geophysics in particular and geophysics in general. The Society fosters the expert and ethical practice of geophys ...
, the
American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, atmospheric, ocean, hydrologic, space, and planetary scientists and enthusiasts that according to their website includes 130,000 people (not members). AGU's a ...
, and the
European Association of Geoscientists and Engineers The European Association of Geoscientists and Engineers (EAGE) is a professional organization for geoscientists and engineers, established in 1951 with a worldwide membership. The association provides a platform for professionals in geophysics, pe ...
.


External links


Biography of Henry Salvatori

Proving That The Seismic Reflection Method Really Works
– Geophysical Society of Tulsa
Reflection Seismology Literature at Stanford Exploration Project

Website of the International Association of Geophysical Contractors

IAGC/IOGP position paper on seismic surveys and marine mammals
(PDF)
Tutorial on seismic reflection data processing

Information about using Seismic Survey in oil and gas exploration in Australia
{{DEFAULTSORT:Reflection Seismology Seismology measurement Petroleum geology