some basic concepts

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Offset Offset In surface seismic acquisition, the In surface seismic acquisition, the horizontal distance from source to horizontal distance from source to receiver. Offset between seismic receiver. Offset between seismic source and receiver creates a source and receiver creates a delay, or moveout, in the arrival delay, or moveout, in the arrival time of a reflection that can be time of a reflection that can be corrected before stacking and can corrected before stacking and can be used to determine velocity. be used to determine velocity.

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Page 1: Some basic concepts

OffsetOffset

In surface seismic acquisition, the In surface seismic acquisition, the horizontal distance from source to horizontal distance from source to receiver. Offset between seismic source receiver. Offset between seismic source and receiver creates a delay, or and receiver creates a delay, or moveout, in the arrival time of a moveout, in the arrival time of a reflection that can be corrected before reflection that can be corrected before stacking and can be used to determine stacking and can be used to determine velocity. velocity.

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Acoustic ImpedanceAcoustic Impedance

The product of density and seismic The product of density and seismic velocity, which varies among different velocity, which varies among different rock layers, commonly symbolized by Z. rock layers, commonly symbolized by Z. The difference in acoustic impedance The difference in acoustic impedance between rock layers affects the reflection between rock layers affects the reflection coefficient. coefficient.

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SeismogramSeismogram

Traces recorded from a single Traces recorded from a single shotpoint. Numerous seismograms shotpoint. Numerous seismograms are displayed together in a single are displayed together in a single seismic section.seismic section.

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SeismographSeismograph

A device or system that records the A device or system that records the ground oscillations that make up ground oscillations that make up exploration seismic data or earthquakes, exploration seismic data or earthquakes, sometimes used incorrectly as a sometimes used incorrectly as a synonym for geophone. A seismograph synonym for geophone. A seismograph can include amplifiers, receivers and a can include amplifiers, receivers and a recording device (such as a computer recording device (such as a computer disk or magnetic tape) to record disk or magnetic tape) to record seismograms. seismograms.

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GeophoneGeophone

A device used in surface seismic A device used in surface seismic acquisition, both onshore and offshore, acquisition, both onshore and offshore, that detects ground velocity produced by that detects ground velocity produced by seismic waves and transforms the motion seismic waves and transforms the motion into electrical impulses. Geophones detect into electrical impulses. Geophones detect motion in only one direction. Conventional motion in only one direction. Conventional seismic surveys on land use one geophone seismic surveys on land use one geophone per receiver location to detect motion in per receiver location to detect motion in the vertical direction. the vertical direction.

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Seabed GeophoneA type of receiver that can be positioned on the seafloor to acquire seismic data. Marine Seismic acquisition can be performed by a source vessel and a recording vessel with streamers or, as shown here, seabed geophones. Energy from the source vessel in the form of P-waves travels through the Earth as P-waves and S-waves and is recorded by the receiver groups and relayed to the recording vessel.

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Hydrophone

A hydrophone (Greek "hydro" = "water" and "phone" = "sound") is a microphone designed to be used underwater for recording or listening to underwater sound. A device designed for use in detecting seismic energy in the form of pressure changes under water during marine seismic acquisition. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. Geophones, unlike hydrophones, detect motion rather than pressure.

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Takeout on Seismic Cable

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Streamer

A surface marine cable, usually a buoyant assembly of electrical wires that connects hydrophones and relays seismic data to the recording seismic vessel. Multistreamer vessels tow more than one streamer cable to increase the amount of data acquired in one pass.

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MultiplexA process that permits transmitting several channels of information over a single channel without crossfeed. Usually different input channels are sampled in sequence at regular intervals and the samples are fed into a single output channel; digital seismic tapes are multiplexed in this way. Multiplexing can also be done by using different carrier frequencies for different information channels and in other ways.

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Binary and Decimal Amplitude

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Air Gun

A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles.

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Marine seismic vessels typically tow arrays of air guns and streamers containing hydrophones a few meters below the surface of the water. The air guns are activated periodically, such as every 25 m (about 10 seconds), and the resulting sound wave travels into the Earth, is reflected back by the underlying rock layers to a hydrophone and then relayed to the recording vessel.

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Vibrator An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data.

Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data. The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear "sweep" of at least seven seconds, beginning with high frequencies and decreasing with time ("downsweeping") or going from low to high frequency ("upsweeping").

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Vibrators (right) are commonly used for acquisition of onshore seismic data and are mounted on large trucks (left).

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Frequency

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Wavelength

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Amplitude

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The term phase in seismic signals is often referred to as minimum phase, maximum phase, mixed phase, or zero phase.

Minimum phase: The minimum-phase signal, shown in (a), is described as a front-loaded signal. This means that the energy in the signal is concentrated in the front of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. Mixed phase: The mixed-phase signal, shown in (b), is described as a signal with its energy concentrated in the center of the pulse. It can be divided into minimum-phase and maximum-phase signals. The signal is usually not symmetrical. The phase of this signal will vary for each frequency component of the signal. Maximum phase: The maximum-phase signal, shown in Figure (c), is described as an end-loaded signal. This means that the energy in the signal is concentrated toward the end of the pulse. The signal is not symmetrical. The phase of this signal will vary for each frequency component of the signal. The characteristics of the maximum-phase signal are the opposite of the minimum-phase signal.Zero phase: The zero-phase signal, shown in (d), is symmetrical and centered on zero time. The zero-phase signal has the shortest duration and largest peak amplitude of any signal with the same amplitude spectrum. These characteristics make it the most desirable of all the signals because of its resolution capability. The phase of the zero-phase signal is zero for all frequency components contained within the signal.

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Correlation

The comparison of seismic waveforms in the time domain, similar to coherence in the frequency domain.

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Signal

The portion of the seismic wave that contains desirable information. Noise is the undesirable information that typically accompanies the signal and can, to some extent, be filtered out of the data.

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NoiseAnything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy, such as shot generation ground roll, surface waves, multiples, effects of weather and human activity, or random occurrences in the Earth. Noise can be minimized by using source and receiver arrays, generating minimal noise during acquisition and by filtering and stacking data during processing.

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Coherent Noise

Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples.

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Random Noise

Disturbances in seismic data that are not coherent (they lack a phase relationship between adjacent traces, unlike air waves and ground roll) and cannot be correlated to the seismic energy source. Random noise can be reduced or removed from data by stacking traces, filtering during processing or using arrays of geophones during acquisition.

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Ground RollGround Roll

A type of coherent noise generated by A type of coherent noise generated by a surface wave, typically a low-a surface wave, typically a low-velocity, low-frequency, high-amplitude velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure Rayleigh wave. Ground roll can obscure signal and degrade overall data signal and degrade overall data quality, but can be alleviated through quality, but can be alleviated through careful selection of source and careful selection of source and geophone arrays, filters and stacking geophone arrays, filters and stacking parameters.parameters.

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Signal-to-Noise ratioSignal-to-Noise ratio

The ratio of desirable to undesirable The ratio of desirable to undesirable (or total) energy. The signal-to-noise (or total) energy. The signal-to-noise ratio can be expressed mathematically ratio can be expressed mathematically as S/N. The signal-to-noise ratio is as S/N. The signal-to-noise ratio is difficult to quantify accurately because difficult to quantify accurately because it is difficult to completely separate it is difficult to completely separate signal from noise. It also depends on signal from noise. It also depends on how noise is defined. how noise is defined.

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Multifold Fold CMP Coverage

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If there are N geophones and the array moves forward a distance of nΔx between shots, it can be shown that the number of rays that share the same common midpoint is N/2n. This is the fold of the data. Fold is sometimes reported as a percentage of coverage (100×N/2n %)

In the example below, there are 6 geophones and the array moves forward a distance of1Δx between shots. This gives 3-fold CMP coverage or 300% coverage.

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Marine Seismic AcquisitionMarine seismic vessels are typically about 75 m [246 ft] long and travel about 5 knots [9.3 km/hr or 5.75 miles/hr] while towing arrays of air guns and streamers containing hydrophones a few meters below the surface of the water. The tail buoy helps the crew locate the end of the streamers. The air guns are activated periodically, such as every 25 m (about 10 seconds), and the resulting sound wave travels into the Earth, is reflected back by the underlying rock layers to hydrophones on the streamer and then relayed to the recording vessel.

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VSPVSPA class of A class of borehole seismic measurements used for borehole seismic measurements used for correlation with surface seismic datacorrelation with surface seismic data, for obtaining , for obtaining images of higher resolution than surface seismic images of higher resolution than surface seismic images and for looking ahead of the drill bit; also images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using measurements made in a vertical wellbore using geophones inside the wellbore and a source at the geophones inside the wellbore and a source at the surface near the well. Most VSPs use a surface seismic surface near the well. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an source, which is commonly a vibrator on land and an air gun in offshore or marine environments. air gun in offshore or marine environments. A VSP is a A VSP is a much more detailed survey than a check-shot survey much more detailed survey than a check-shot survey because the geophones are more closely spaced, because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals shot survey might include measurements of intervals hundreds of meters apart.hundreds of meters apart. Also, a VSP uses the Also, a VSP uses the reflected energy contained in the recorded trace at reflected energy contained in the recorded trace at each receiver position as well as the first direct path each receiver position as well as the first direct path from source to receiver. The check-shot survey uses from source to receiver. The check-shot survey uses only the direct path traveltime. only the direct path traveltime.

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Zero-offset VSPsZero-offset VSPs (A) have sources close to the wellbore (A) have sources close to the wellbore. .

Offset VSPsOffset VSPs (B) have sources some distance from the (B) have sources some distance from the receivers in the wellbore. receivers in the wellbore.

Walkaway VSPsWalkaway VSPs (C) feature a source that is moved to (C) feature a source that is moved to progressively farther offset and receivers held in a fixed progressively farther offset and receivers held in a fixed location. location.

Walk-above VSPsWalk-above VSPs (D) accommodate the recording (D) accommodate the recording geometry of a deviated well, having each receiver in a geometry of a deviated well, having each receiver in a different lateral position and the source directly above the different lateral position and the source directly above the receiver. receiver.

Salt-proximity VSPsSalt-proximity VSPs (E) are reflection surveys to help (E) are reflection surveys to help define a salt-sediment interface near a wellbore by using a define a salt-sediment interface near a wellbore by using a source on top of a salt dome away from the drilling rig. source on top of a salt dome away from the drilling rig.

Drill-noise VSPsDrill-noise VSPs (F), also known as seismic-while-drilling (F), also known as seismic-while-drilling (SWD) VSPs, use the noise of the drill bit as the source and (SWD) VSPs, use the noise of the drill bit as the source and receivers laid out along the ground. receivers laid out along the ground.

Multi-offset VSPsMulti-offset VSPs (G) involve a source some distance from (G) involve a source some distance from numerous receivers in the wellbore.numerous receivers in the wellbore.

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check-shot surveycheck-shot surveyA type of borehole seismic data designed to A type of borehole seismic data designed to measure the measure the seismic traveltime from the surface to a known depthseismic traveltime from the surface to a known depth. P-. P-wave velocity of the formations encountered in a wellbore wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant the surface of the Earth, and recording the resultant signal. signal. The data can then be correlated to surface seismic The data can then be correlated to surface seismic data by correcting the sonic log and generating a data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic synthetic seismogram to confirm or modify seismic interpretationsinterpretations. It differs from a vertical seismic profile in . It differs from a vertical seismic profile in the number and density of receiver depths recorded; the number and density of receiver depths recorded; geophone positions may be widely and irregularly located geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. regularly spaced intervals in the wellbore.

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Synthetic SeismogramSynthetic SeismogramThe result of one of many forms of forward The result of one of many forms of forward modeling to predict the seismic response of the modeling to predict the seismic response of the Earth. A more narrow definition used by seismic Earth. A more narrow definition used by seismic interpreters is that a synthetic seismogram, interpreters is that a synthetic seismogram, commonly called a synthetic, is a direct one-commonly called a synthetic, is a direct one-dimensional model of acoustic energy traveling dimensional model of acoustic energy traveling through the layers of the Earth. through the layers of the Earth. The synthetic The synthetic seismogram is generated by convolving the seismogram is generated by convolving the reflectivity derived from digitized acoustic and reflectivity derived from digitized acoustic and density logs with the wavelet derived from density logs with the wavelet derived from seismic data.seismic data. By comparing marker beds or other By comparing marker beds or other correlation points picked on well logs with major correlation points picked on well logs with major reflections on the seismic section, interpretations reflections on the seismic section, interpretations of the data can be improved. The quality of the of the data can be improved. The quality of the match between a synthetic seismogram depends match between a synthetic seismogram depends on well log quality, seismic data processing on well log quality, seismic data processing quality, and the ability to extract a representative quality, and the ability to extract a representative wavelet from seismic data, among other factors.wavelet from seismic data, among other factors.

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Synthetic SeismogramSynthetic Seismogram

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The synthetic seismogram in the fifth column was The synthetic seismogram in the fifth column was generated by convolving the digitized sonic and density generated by convolving the digitized sonic and density logs with a wavelet shown in the column to the right. By logs with a wavelet shown in the column to the right. By comparing geological markers picked on logs, such as the comparing geological markers picked on logs, such as the top of the chalk in this display, with major reflections on top of the chalk in this display, with major reflections on the seismic section, seismic data can be used to map the seismic section, seismic data can be used to map physical properties between wells.physical properties between wells.

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The property of the rocks that defines the acoustic contrast is the acoustic impedance, which is the product of density and seismic velocity. The appropriate measure of the acoustic contrast is the reflection coefficient, which is the difference of the two acoustic impedances divided by their sum.

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The preparation of the synthetic therefore has these basic stages:

The calculation of the velocity log as a function of two-way time;

The calculation of the edited density log as a function of two-way time;

The multiplication of the two to obtain the acoustic-impedance log as a function of two-way time;

The calculation of the reflection-coefficient log also as a function of two-way time;

The convolution of the reflectivity series with a waveform (Zerophase Wavelet) representing the seismic pulse;

The display of the resulting synthetic in juxtaposition with the seismic section through the well, so that the match between synthetic and surface seismic can be confirmed, and so that each reflection can be positively tied to an interface observed in the well.

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Zero-Phase WaveletZero-Phase Wavelet

Pertaining to seismic data whose wavelet is Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution symmetrical about zero time. Deconvolution during seismic processing can convert data during seismic processing can convert data of mixed phase to zero-phase data, but is of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend not always successful. Zero-phase data tend to provide sharper definition and less to provide sharper definition and less distortion between stratigraphic features in distortion between stratigraphic features in the subsurface, such as sand and shale the subsurface, such as sand and shale layers. layers.

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One-Dimensional Seismic DataOne-Dimensional Seismic Data

A check-shot survey of a well, which can A check-shot survey of a well, which can be used to correct the sonic log and be used to correct the sonic log and generate a synthetic seismogram that generate a synthetic seismogram that displays changes in amplitude versus displays changes in amplitude versus traveltime.traveltime.

OROR

A single seismic trace. A single seismic trace.

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Two-Dimensional Seismic DataTwo-Dimensional Seismic Data

A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially.

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Diagram of a Two-Dimensional seismic surveyDiagram of a Two-Dimensional seismic survey

A 2D survey commonly contains numerous lines A 2D survey commonly contains numerous lines acquired perpendicular to the strike of geological acquired perpendicular to the strike of geological structures (such as the folds shown here) with a structures (such as the folds shown here) with a minimum of lines acquired parallel to geological minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the structures to allow line-to-line correlation of the seismic data and mapping of structuresseismic data and mapping of structures

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Concept of 3DWe employ the same types of sources and receivers used in 2-D surveys to acquire three-dimensional seismic data. The energy sources include dynamite, vibrators and air guns, and the receivers are either geophones or hydrophones. The main differences between 2-D and 3-D surveys are in the layout of the survey and the quantity of data that is acquired. These differences in acquisition procedures affect the way we process and interpret our data.When acquiring 3-D data on land, multiple lines of geophones are active at the same time, and the geophone array and shots do not fall on the same line. As a result, reflection points are spread over an area rather than being focused along a single, two-dimensional line.Acquiring 3-D marine data is similar to acquiring 2-D marine data, in that a boat with an energy source (such as an air gun) and a trailing streamer with many hydrophones are used. The significant difference in 3-D acquisition is the fact that we use multiple sources, multiple streamers and even multiple boats.

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In-Lineseismic line within a 3D survey parallel to the direction in which the data were acquired. In marine seismic data, the in-line direction is that in which the recording vessel tows the streamers.

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Cross-Line

A seismic line within a 3D survey perpendicular to the direction in which the data were acquired.

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The crosslines of a 3D survey are perpendicular to the The crosslines of a 3D survey are perpendicular to the direction in which the data were acquired.direction in which the data were acquired.The in-lines of a 3D survey are parallel to the direction in The in-lines of a 3D survey are parallel to the direction in which the data were acquired, or in marine data, the which the data were acquired, or in marine data, the direction in which the boat moves.direction in which the boat moves.

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BinA subdivision of a 3D seismic survey. The area of a three-dimensional survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the source and the receiver, reflection point or conversion point. Bins are commonly assigned according to common midpoint (CMP). Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold.

OR

To sort seismic data into small areas according to the midpoint between the source and the receiver, reflection point or conversion point prior to stacking.

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Three-Dimensional Seismic DataThree-Dimensional Seismic Data

A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m to over 600 m, and typical distances between shotpoints and receiver groups is 25 m (offshore) and 34 to 67 m (onshore). The resultant data set can be "cut" in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crossline. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data.

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3D Seismic3D Seismic

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WHY WE NEED 3-D?We live in a three-dimensional world. Perhaps that is all the justification we need for conducting 3-D seismic surveys. The truth of the matter is that the complexity of the problems now being addressed by 3-D techniques require such sophistication. However, this was not always the case. When the seismic reflection technique was developed by John Clarence Karcher and others in the 1920s, exploration objectives were shallow and their stratigraphy was fairly flat, although faults, reefs, or shallow folds were often present to create hydrocarbon traps. At that time, a 2-D approach was a satisfactory approximation of the subsurface and a reliable aid in finding new discoveries.In the deeper, more complex reservoirs we investigate today, the 3-D method helps us to better define the reservoir geometry because of its three significant advantages over 2-D — namely:· focusing, · positioning and · resolution

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3-D Advantages

Data DensityData AccuracyFlexibility of data displaysField development and appraisal3-D seismic better approximate 3-D

earth

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Swath Method

3D land acquisition where lines of geophones are orthogonal to source lines.

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4D Seismic4D SeismicThree-dimensional (3D) seismic data Three-dimensional (3D) seismic data acquired at different times over the same acquired at different times over the same area to assess changes in a producing area to assess changes in a producing hydrocarbon reservoir with time. Changes hydrocarbon reservoir with time. Changes may be observed in fluid location and may be observed in fluid location and saturation, pressure and temperature. 4D saturation, pressure and temperature. 4D seismic data is one of several forms of time-seismic data is one of several forms of time-lapse seismic data. Such data can be lapse seismic data. Such data can be acquired on the surface or in a borehole. acquired on the surface or in a borehole.

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These maps of an oil reservoir were made from 3D seismic These maps of an oil reservoir were made from 3D seismic data acquired at different times over the same area to data acquired at different times over the same area to assess changes in fluid saturation with time. In 1985 (left), assess changes in fluid saturation with time. In 1985 (left), the areas within red outlines A and B were predominantly the areas within red outlines A and B were predominantly oil-saturated. After ten years of oil production (1995, right), oil-saturated. After ten years of oil production (1995, right), both areas show an increase in water saturation.both areas show an increase in water saturation.

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3C Seismic DataA type of multicomponent seismic data acquired in a land, marine, or borehole environment by using three orthogonally oriented geophones. 3C is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, as for example, on land. This technique allows determination of both the type of wave and its direction of propagation.

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4C SeismicFour-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves.

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Obtaining four-component seismic data using a multiwave array system. Each sensor station within the cable comprises one hydrophone and three orthogonally oriented geophones to record both pressure and particle velocity.

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Need of Multi-Component Data The goal of exploration seismic is to obtain information about the subsurface. This includes;(1): Structural information (2): Lithologic information (3): Fluid content

Traditional seismic, using compression waves (P-waves), especially 3-D seismic, has been highly successful at achieving the first of these goals. However, lithology and fluid content have not been well identified. For example, bright spots are often taken to be indicators of gas. However, these may occur through changes in fluid content or lithology, so that the interpretation of P-wave sections may be ambiguous. Multi-component seismic offers the potential to achieve information about all three desired information types.

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UnconformityUnconformity

A geological surface separating older A geological surface separating older from younger rocks and representing from younger rocks and representing a gap in the geologic record. Such a a gap in the geologic record. Such a surface might result from a hiatus in surface might result from a hiatus in deposition of sediments, possibly in deposition of sediments, possibly in combination with erosion, or combination with erosion, or deformation such as faulting. deformation such as faulting.

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Angular Unconformity:Angular Unconformity:A surface that separates younger strata from eroded, dipping, older A surface that separates younger strata from eroded, dipping, older strata and represents a gap in the geologic record.strata and represents a gap in the geologic record.

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Fault plane is oriented between 30 Fault plane is oriented between 30 and 90 degrees (measured from and 90 degrees (measured from

horizontal)horizontal)

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Fault plane is at less than 30 degreesFault plane is at less than 30 degrees

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Fault plane is oriented between 30 and 90 Fault plane is oriented between 30 and 90 degrees (measured from horizontal) degrees (measured from horizontal)

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Fault plane is at less than 30 degrees Fault plane is at less than 30 degrees

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Pinch-outPinch-out

A A type of stratigraphic trap. The termination by type of stratigraphic trap. The termination by thinning or tapering out ("pinching out") of a thinning or tapering out ("pinching out") of a reservoir against a nonporous sealing rock creates a reservoir against a nonporous sealing rock creates a favorable geometry to trap hydrocarbons, favorable geometry to trap hydrocarbons, particularly if the adjacent sealing rock is a source particularly if the adjacent sealing rock is a source rock such as a shale. rock such as a shale.

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ReefReef

A mound, ridge, or buildup of sediment or A mound, ridge, or buildup of sediment or sedimentary rock, most commonly produced by sedimentary rock, most commonly produced by organisms that secrete shells such as corals. organisms that secrete shells such as corals. Reefs are typically taller than the sediment that Reefs are typically taller than the sediment that surrounds them, resistant to weathering and surrounds them, resistant to weathering and wave action, and preserved within sediment of a wave action, and preserved within sediment of a different composition. Carbonate reefs form in a different composition. Carbonate reefs form in a limited range of temperatures, water depths, limited range of temperatures, water depths, salinities and wave activities, so their occurrence salinities and wave activities, so their occurrence can be used to interpret past environmental can be used to interpret past environmental conditions. Because the rocks that surround reefs conditions. Because the rocks that surround reefs can differ in composition and permeability, can differ in composition and permeability, porous reefs can form stratigraphic traps for porous reefs can form stratigraphic traps for hydrocarbons. Porosity of reefal limestones hydrocarbons. Porosity of reefal limestones depends on post-depositional diagenetic changes. depends on post-depositional diagenetic changes.

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First BreakFirst Break

The earliest arrival of energy propagated The earliest arrival of energy propagated from the energy source at the surface to from the energy source at the surface to the geophone in the wellbore in vertical the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or seismic profiles and check-shot surveys, or the first indication of seismic energy on a the first indication of seismic energy on a trace. On land, first breaks commonly trace. On land, first breaks commonly represent the base of weathering and are represent the base of weathering and are useful in making static corrections. useful in making static corrections.

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First Break

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Common Depth Point

In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data.

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Common Midpoint In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms.

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The traces from different source-receiver pairs that share a common midpoint, such as receiver 6 (R6), can be corrected during seismic processing to remove the effects of different source-receiver offsets, or NMO. After NMO corrections, the traces can be stacked to improve the signal-to-noise ratio.

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Common Depth Point & Common Mid PointCommon Depth Point & Common Mid Point

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Normal Moveout

The procedure in seismic processing that compensates for the effects of the separation between seismic sources and receivers in the case of a horizontal reflector.

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The traces from different source-receiver pairs that share a midpoint, such as receiver 6 (R6), can be corrected during seismic processing to remove the effects of different source-receiver offsets, called normal moveout or NMO. After NMO corrections, the traces can be stacked to improve the signal-to-noise ratio.

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Two-Way TraveltimeTwo-Way Traveltime

The elapsed time for a seismic wave The elapsed time for a seismic wave to travel from its source to a given to travel from its source to a given reflector and return to a receiver at reflector and return to a receiver at the Earth's surface. Minimum two-the Earth's surface. Minimum two-way traveltime is that of a normal-way traveltime is that of a normal-incidence wave with zero offset. incidence wave with zero offset.

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CreepCreep

Relatively slow, quiet movement Relatively slow, quiet movement along a fault. It is sometimes called along a fault. It is sometimes called "seismic creep" to distinguish it from "seismic creep" to distinguish it from the slumping of rock or soil on slopes the slumping of rock or soil on slopes (which is also known as (which is also known as creepcreep), and ), and sometimes called "aseismic creep", sometimes called "aseismic creep", since it does not trigger events since it does not trigger events greater than greater than microearthquakesmicroearthquakes..

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wavelet A one-dimensional pulse, usually the basic response from a single reflector. Its key attributes are its amplitude, frequency and phase. The wavelet originates as a packet of energy from the source point, having a specific origin in time, and is returned to the receivers as a series of events distributed in time and energy. The distribution is a function of velocity and density changes in the subsurface and the relative position of the source and receiver. The energy that returns cannot exceed what was input, so the energy in any received wavelet decays with time as more partitioning takes place at interfaces. Wavelets also decay due to the loss of energy as heat during propagation. This is more extensive at high frequency, so wavelets tend to contain less high-frequency energy relative to low frequencies at longer traveltimes. Some wavelets are known by their shape and spectral content, such as the Ricker wavelet.

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A wavelet is a one-dimensional pulse, usually the basic response from a single reflector. The zero-

phase wavelet to the right is a synthetic wavelet

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spherical wave

A wave generated from a point source, such as that generated by an underground explosion. Typical seismic sources such as vibrators and air-gun arrays emit elastic waves that are assumed to be spherical waves.

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Blowout An uncontrolled flow of reservoir fluids into the wellbore, and sometimes catastrophically to the surface. A blowout may consist of salt water, oil, gas or a mixture of these. Blowouts occur in all types of exploration and production operations, not just during drilling operations. If reservoir fluids flow into another formation and do not flow to the surface, the result is called an underground blowout. If the well experiencing a blowout has significant openhole intervals, it is possible that the well will bridge over (or seal itself with rock fragments from collapsing formations) downhole and intervention efforts will be averted.

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shadow zone Generally, an area of the Earth from which waves do not emerge or cannot be recorded. In seismology, the term is used to more specifically describe regions of the subsurface where P-waves and S-waves are difficult to detect, such as regions of the core at certain distances from the epicenter of an earthquake, or the point on the Earth's surface directly above an earthquake. Such zones were first observed in 1914 by Beno Gutenberg (1889 to 1960), an American geologist born in Germany. Because of the molten nature of the outer core, S-waves are especially difficult to detect at 103 to 142 degrees from the epicenter of an earthquake and not observable from 142 to 180 degrees from the epicenter. Areas below salt features are also called shadow zones because the high velocity of salt bends and traps energy, so seismic data quality beneath salt is generally poor unless special seismic processing is performed.

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Epicenter, Hypocenter and Focucs

Focus is also known as Hypocenter

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Critical AngleThe angle of incidence according to Snell's law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows:

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Critical Reflection A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle.

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ChannelA device to carry data from a receiver to a recorder, such as from a group of geophones. Simultaneous recording of 500 to 2000 channels is common during 3D seismic acquisition, and 120 to 240 channels during onshore 2D seismic acquisition.

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Exploratory Well

An Exploratory Well is a well drilled for the purpose of discovering new reserves in unproven areas. They are used to extract geological or geophysical information about an area with a view to exploiting untapped reserves. Exploratory Wells are sometimes known as Wildcat Wells.

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Appraisal Well

A well drilled as part of an appraisal drilling programm which is carried out to determine the physical extent, reserves and likely production rate of a field;

ORA well drilled as part of a programme to determine the size and likely yield of an oil or gas field.

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Development Well

A Development Well is a well drilled in a proved production field or area to extract natural gas or crude oil.

ORWell drilled for the production of oil or gas from a field already proven by appraisal drilling to be suitable for exploitation.

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Production Well

A well used to retrieve petroleum or gas from an underground reservoir.

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Root-mean-square Velocity

The value of the square root of the sum of the squares of the velocity values divided by the number of values, symbolized by vrms. The root-mean-square velocity is that of a wave through subsurface layers of different interval velocity along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity approach equality when source-receiver offset approaches zero and layers are horizontal and isotropic.

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Average Velocity

The depth divided by the traveltime of a wave to that depth. Average velocity is commonly calculated by assuming a vertical path, parallel layers and straight raypaths, conditions that are quite idealized compared to those actually found in the Earth.

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Anisotropy - isotropy comparison

Anisotropy. Anisotropy and isotropy can depend on scale. While a single crystal can be anisotropic, many crystals together can form an isotropic or homogeneous layer within an otherwise anisotropic rock.

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Multiple Reflection Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (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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Multiply-reflected seismic energy occurs in several ways but is typically removed by seismic processing. Long-path multiples appear as distinct events and generally originate deep in the subsurface. Short-path multiples are added to primary reflections and tend to come from shallow subsurface phenomena.

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Long-path multiple

A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing.

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Multiply-reflected seismic energy from the water bottom is common in marine seismic data, but, like many multiples, seismic processing attempts to minimize its presence.

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Short-path Multiple

Multiply-reflected seismic energy with a shorter travel path than long-path multiples. Short-path multiples tend to come from shallow subsurface phenomena or highly cyclical sedimentation and arrive soon after, and sometimes very near, the primary reflections. Short-path multiples are less obvious than most long-path multiples and are less easily removed by seismic processing.

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GhostA short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as at the base of weathering or between sources and receivers and the sea surface.

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A ghost is a short-path multiple, or a spurious reflection that occurs when seismic energy reverberates in the shallow subsurface, such as at the base of weathering.

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Primary Reflection

Seismic events whose energy has been reflected once. Multiples, in contrast, are events whose energy has been reflected more than once. A goal of seismic data processing is to enhance primary reflections, which are then interpreted as subsurface interfaces.

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A primary reflection has been reflected only once. Multiples, in contrast, are events whose energy has been reflected more than once.

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Peg Leg MultiplesA type of short-path multiple, or multiply-reflected seismic energy, having an asymmetric path. Short-path multiples are added to primary reflections, tend to come from shallow subsurface phenomena and highly cyclical deposition, and can be suppressed by seismic processing. In some cases, the period of the peg-leg multiple is so brief that it interferes with primary reflections, and its interference causes a loss of high frequencies in the wavelet.

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Up, Middle & Down Stream ActivitiesThe petroleum industry is usually divided into three major components:Upstream, midstream and downstream. Midstream operations are usually included in the downstream category.

The upstream oil sector is a term commonly used to refer to the searching for and the recovery and production of crude oil and natural gas. The upstream oil sector is also known as the exploration and production (E&P) sector.

The upstream sector includes the searching for potential underground or underwater oil and gas fields, drilling of exploratory wells, and subsequently operating the wells that recover and bring the crude oil and/or raw natural gas to the surface

The downstream oil sector is a term commonly used to refer to the refining of crude oil, and the selling and distribution of natural gas and products derived from crude oil. Such products include liquified petroleum gas (LPG), gasoline or petrol, jet fuel, diesel oil, other fuel oils, asphalt and petroleum coke.

The downstream sector includes oil refineries, petrochemical plants, petroleum product distribution, retail outlets and natural gas distribution companies.

The downstream industry touches consumers through thousands of products such as gasoline, diesel, jet fuel, heating oil, asphalt, lubricants, synthetic rubber, plastics, fertilizers, antifreeze, pesticides, pharmaceuticals, natural gas and propane.

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Christmas Tree

The set of valves, spools and fittings connected to the top of a well to direct and control the flow of formation fluids from the well.

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Seismometer

A device that records seismic energy in the form of ground motion and transforms it to an electrical impulse.

Commonly used for Earthquake.

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lithosphere The brittle outer layer of the Earth that includes the crust and uppermost mantle. It is made up of six major and several minor tectonic plates that move around on the softer asthenosphere. The lithosphere of the oceans tends to be thinner (in some oceanic areas, less than 50 km [30 miles] thick) and more dense than that of the continents (more than 120 km [70 miles] thick in places like the Himalayas) because of isostasy. The movement of the plates of the lithosphere results in convergence, or collisions, that can form mountain belts and subduction zones, and divergence of the plates and the creation of new crust as material wells up from below separating plates. The lithosphere and asthenosphere are distinguished from the crust, mantle and core of the Earth on the basis of their mechanical behavior and not their composition.

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Asthenosphere The relatively plastic layer of the upper mantle of the Earth on which the tectonic plates of the lithosphere move. The asthenosphere is approximately 200 km [124 miles] thick and, owing to its depth below the Earth's surface, warm (~ 1400 oC) [2640 oF] but not molten. Here the mantle deforms by plastic flow in response to applied pressures above 100 MPa [14,500 psi]. This zone is considered coincidental, at least below oceanic crust, with the low-velocity zone of the upper mantle.

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Automatic Gain Control

A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay.

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Uphole Survey

Technique in which seismic sources are energized within a borehole and arrival times recorded by surface geophones. It is used particularly to determine weathered-layer velocity.