Typical 3-D processing sequence

Excerpted from "3-D Seismic Interpretation" by M. Bacon, R. Simm and T. Redshaw
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1. Reformat
2. Designature
3. Resample from 2 to 4 ms
4. Low cut filter (5/12 minimum phase filter)
5. Remove bad traces
6. Merge navigation with seismic headers
7. Spatial resampling from 12.5 to 25 m groups. Normal moveout (NMO) correction, K-filter, trace drop, inverse NMO
8. Spherical divergence gain corrections
9. Deconvolution before stack
10. Shot interpolation to double fold in CMP gathers
11. Radon demultiple on interpolated gathers
12. High-frequency noise removal
13. Drop of interpolated traces
14. Flex binning increasing from 37.5 m on near to 50 m on far offsets
15. Sort to common offset
16. Dip moveout (including approximated NMO) halving the number of offset planes on output
17. Pre-stack time migration using constant velocity 1600 m/s
18. Inverse NMO
19. Re-pick velocities (0.5 km grid)
20. NMO

Processing hereafter continuing on three volumes:
21. Stack to generate three volumes: near offset stack, far offset stack and full offset stack
22. 3-D constant velocity inverse time migration
23. Bulk static (gun and cable correction)
24. K-notch filter to remove pattern caused by the acquisition
25. FXY deconvolution
26. FXY interpolation to 12.5 m X 12.5 m bin grid
27. Pre-migration data conditioning (e.g. amplitude balance, edge tapers, etc.)
28. One pass steep dip 3-D time migration (using time and spatially varying velocity field)
29. Zero-phase conversion by matching to wells
30. Spectral equalisation
31. Bandpass filter
32. Residual scaling

Seismic Noise Suppression

Suppression

In seismic acquisition and processing, the attenuation of amplitudes to reduce the effects of noise or to prevent overload from the high energy of first breaks.

1. Alias filter: A filter, or a set of limits used to eliminate unwanted portions of the spectra of the seismic data, to remove frequencies that might cause aliasing during the process of sampling an analog signal during acquisition or when the sample rate of digital data is being decreased during seismic processing. (Synonyms: antialias filter)
2. Coherence filtering: A technique for removing noise and emphasizing coherent events from multiple channels of seismic data.
3. Mute: To remove the contribution of selected seismic traces in a stack to minimize air waves, ground roll and other early-arriving noise. Low-frequency traces and long-offset traces are typical targets for muting.
4. Tail mute: A cutoff in time, offset or both that has the effect of eliminating some types of noise from seismic data. A tail mute can be used to exclude slow surface waves such as ground roll.

Seismic Noise and Distortion

Noise
Anything 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.

1. Aliasing: The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. Aliasing can be avoided by sampling at least twice the highest frequency of the waveform or by filtering frequencies above the Nyquist frequency, the highest frequency that can be defined accurately by that sampling interval.
2. Bubble effect: Bubble pulses or bubble noise that affect data quality. In marine seismic acquisition, the gas bubble produced by an air gun oscillates and generates subsequent pulses that cause source-generated noise. Careful use of multiple air guns can cause destructive interference of bubble pulses and alleviate the bubble effect. A cage, or a steel enclosure surrounding a seismic source, can be used to dissipate energy and reduce the bubble effect.
3. Coherent noise: Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples.
4. Cultural noise: Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods.
5. Ground roll: A type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.
6. Surface wave: A wave that propagates at the interface between two media as opposed to through a medium. A surface wave can travel at the interface between the Earth and air, or the Earth and water. Love waves and Rayleigh waves are surface waves.
7. 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. (Synonyms: secondary reflection)

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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.

Diagram of ghosts and multiple reflections

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.

Diagram of multiple reflections

Multiple can be divided into three major types:

1. 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.
2. 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. Long-path multiples originate deep in the subsurface and generate distinct events because their travel path is longer than primary reflections from the same depth.
   
   
3. Peg-leg multiple: A 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.

Comparison of forward modeling and inversion

Seismic inversion begins with a seismic trace and outputs an acoustic impedance model.

A mathematical process by which data are used to generate a model that is consistent with the data, the process of solving the inverse problem. In seismology, surface seismic data, vertical seismic profiles and well log data can be used to perform inversion, the result of which is a model of Earth layers and their thickness, density and P- and S-wave velocities. Successful seismic inversion usually requires a high signal-to-noise ratio and a large bandwidth.

Wave

Wave:
A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation.

Body wave:
A wave that propagates through a medium rather than along an interface. P-waves and S-waves are examples of body waves.

1. P-wave: An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. (Synonyms: acoustic wave, compressional wave, dilatational wave)
2. S-wave: An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson's ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. (Synonyms: shear wave, tangential wave)
3. SH-wave: A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane.
4. SV-wave: A shear wave that is polarized so that its particle motion and direction of propagation occur in a vertical plane.
5. Converted wave: A seismic wave that changes from a P-wave to an S-wave, or vice versa, when it encounters an interface.

Surface wave:
A wave that propagates at the interface between two media as opposed to through a medium. A surface wave can travel at the interface between the Earth and air, or the Earth and water. Love waves and Rayleigh waves are surface waves.

1. Love wave: A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation.
2. Rayleigh wave: A type of surface wave in which particles move in an elliptical path within the vertical plane containing the direction of wave propagation. At the top of the elliptical path, particles travel opposite to the direction of propagation, and at the bottom of the path they travel in the direction of propagation. Because Rayleigh waves are dispersive, with different wavelengths traveling at different velocities, they are useful in evaluation of velocity variation with depth. Rayleigh waves make up most of the energy recorded as ground roll.
   
3. Stoneley wave: A type of large-amplitude interface, or surface, wave generated by a sonic tool in a borehole. Stoneley waves can propagate along a solid-fluid interface, such as along the walls of a fluid-filled borehole and are the main low-frequency component of signal generated by sonic sources in boreholes. Analysis of Stoneley waves can allow estimation of the locations of fractures and permeability of the formation. Stoneley waves are a major source of noise in vertical seismic profiles.
4. Tube wave: (1) A Stoneley wave that occurs at the low frequencies of seismic data. (2) An interface wave that occurs in cased wellbores when a Rayleigh wave encounters a wellbore and perturbs the fluid in the wellbore. The tube wave travels down the wellbore along the interface between the fluid in the wellbore and the wall of the wellbore. A tube wave suffers little energy loss and typically retains a very high amplitude which interferes with reflected arrivals occurring later in time on vertical seismic profile (VSP) data. Because the tube wave is coupled to the formation through which it is traveling, it can perturb the formation across open fractures intersecting the borehole. This squeezing effect can generate secondary tube waves which travel both up and down from the fracture location. Such events can be diagnostic of the presence of open fractures and their amplitude related qualitatively to the length and width, e.g., volume of the fluid-filled fracture space. This effect is generally seen only in shallow formations where the overburden pressure is lower.
5. Dispersion: A type of distortion of a wave train in which the velocity of the wave varies with frequency. Surface waves and electromagnetic body waves typically exhibit dispersion, whereas P-waves in most rocks show little change in velocity with frequency.
6. Ground roll: A type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.

Sources of free exploration-geophysical software

• Madagascar (A GPL software package with a separated header / data format very similar to classic SEPlib, but rewritten from scratch. Madagascar includes an environment for creating and testing reproducible software experiments.)
• CWP/SU Seismic Un*x Home Page (A free seismic processing system that uses an expanded SEGY-like data format.)
• qiWorkbench (A processing environment originally from BHP Billiton that includes the bhpViewer as one component.)
• Software from the Stanford Exploration Project (SEPlib, a data-cube-based seismic processing system, along with other miscellaneous software.)
• The FreeUSP home page (An Amoco/BP-heritage processing system that uses an expanded SEGY-like data format.)
• The Data Dictionary System (DDS) home page (An Amoco/BP-heritage processing system supporting high-dimensional flexible-format data files.)
• ARCO heritage source code (The Arco SPARC processing system, and some reflectivity modeling codes.)
• Computers and Geosciences journal software archive

干扰波的类型和特点

根据干扰波的出现规律，可以分为规则干扰和无规则干扰（随机干扰）两大类。

无规则干扰主要指没有一定频率，也没有一定的传播方向的波。在记录上形成杂乱无章的干扰背景。

规则干扰波是指一定的主频和一定视速度的干扰波。例如面波、声波、浅层折射波、侧面波等。它们的主要特点如下:

1. 面波(surface wave)：地震勘探中遇到的面波，它的特点是频率低，一般为几Hz到30Hz；速度低，一般为100米/秒~1000米/秒，以200米/秒～500米/秒最为常见。面波时距曲线是直线，因此在小排列(100~150米)的波形记录上面波同相轴是直的。面波随着传播距离的增大，振动延续时间也越长，形成“扫帚状”，即发生频散。随着远离爆炸点，面波的强相位逐渐向后或向前转换。强相位追踪不远，一般只有几十米。面波能量的强弱与激发岩性、激发深度以及表层地震地质条件有关。这是因为在淤泥、厚黄土及沙漠地区，由于对有效波的能量强烈吸收，有效波能量减弱，面波能量相对增强；在疏松的低速岩层中激发或所用炸药量过大造成激发频率降低，使面波能量相对加强；爆炸井较深时面波减弱，井较浅时面波增强。妥善选择激发条件和组合是克服面波的主要办法。

2. 声波(acoustic wave)：在坑中，浅水池中、河中和干井中爆炸，都会出现强烈声波。声波是空气中传播的弹性波，速度为340米/秒左右，比较稳定，频率较高，延续时间较短，呈窄带出现。为了避免声波干扰，应尽量不在浅水及浅井中放炮，尽量采用井中爆炸，并用埋井的方法以增强有效波的能量和防止声波干扰。在山区工作时，有时还会碰到多次声波的干扰。

3. 浅层折射波：当表层存在高速层，或第四系下面的老地层埋藏浅，可能观测到同相轴为直线的浅层折射波。

4. 侧面波：在地表条件比较复杂的地区进行地震勘探工作时，还会出现一种被称为侧面波的干扰波。例如在黄土高原地区，由于水系切割，形成谷沟交错的复杂地形。黄土高原的侧面是沟，原和沟的相对高差达几百米，在原与沟的交界为陡峻的黄土与空气的接触面，形成一个强波阻抗分界面，因而地震波激发后，传播到黄土边沿，被反射回来，记录上可能出现来自不同方向的具有不同视速度的干扰波。这种干扰波是一种侧面波。

还有一类是次生的低速干扰和次生的高速干扰。它和随机干扰不同，但与上面谈的规则干扰相比又显得更复杂一些，所以单独列为一类。这类干扰在频率域中与有效波是不能分离的，在视速度和视波长域中次生高速干扰又和反射有效波部分重合。它们在记录上出现的位置可以占全部记录的任意一个角落。它们是地表附近各种地物障碍物（沟、坝、公路、树木、电杆、房屋建筑物以及小山包等等）以及地表岩性不均匀性所造成的，是由反射波到达地面后，使地面产生振动，地面上任何不均匀性和地物障碍物受激发而等于地面做“敲击”动作。于是在近处产生次生的直达波和面波，在远处产生次生的折射波。这些波种类繁多，个数太多且来自于不同方向，因此就造成了极大的复杂性。

为了识别和区分这些干扰波与有效波，可以抓住它们两者之间的一个主要差异，即干扰波的最大真速度和有效波的视速度范围不同。干扰波是沿着地表附近传播的；有效波是从地下垂直来到地面的，这是它们的区别。当然，当次生干扰源相对于测线的位置不同时，次生干扰波到达测线上的视速度可以等于或大于其真速度以至无穷大，但它总会在地面的某个方向上暴露出它传播的真速度。

最后，对于干扰波的类型作一个小结：干扰波可以分为规则干扰和随机干扰两大类，规则干扰又可分为（1）沿着水平方向传播的，如面波和车辆等引起的干扰；沿着垂直方向传播的，如多次波；（2）具有重复性的，如面波；不具有重复性的，如一些人为因素产生的干扰。随机干扰也可以分为能重复出现的，如地表不均匀性引起的散射；不重复出现的，如风吹草动等自然因素引起的随机干扰。

地震波频谱的特征及其应用

地震波的频谱是地震波的动力学特点之一。研究地震波的频谱特征有着多方面的意义。分析有效波和干扰波的频谱上的差异对我们设计地震仪器，选择处理参数和野外工作方法都有一定的指导意义。此外，地震波的频谱中包含着关于地下地层的岩性、构造方面的信息，可用于地震资料的地层岩性解释。

各种地震波的频谱的特征：

1. 与地震勘探有关的一些波的频谱特点：面波的频率较低在10~30Hz；反射波的主频一般在30～50Hz，近年来采用野外数字仪（它的记录频率范围向低频可扩展到5Hz）和低频检波器，能记录到6～7秒的反射波，它们的主频可以低到10Hz；风吹草动等微震的频谱比较宽；声波的频谱在100Hz以上是比较高频的范围；工业交流电干扰主频是50Hz，并有一个很窄的频带。

2. 激发条件对地震波频谱有一定的影响。在用炸药激发，药量增大时，地震波的频谱移向低频。因此在高分辨率地震勘探中，常采用小药量多井组合激发方式。

3. 不同类型的地震波的频谱也有差别。例如，同一界面的反射纵波比反射横波具有较高的频谱分量，横波比纵波具有较低频谱分量的原因，初步认为是因为在沉积岩中，至少在频率大于40～50Hz时，横波的吸收系数比纵波的大。

4. 同一类型的地震波（例如反射纵波），随着传播距离的增加，因为高频成分被介质吸收，频谱中低频成分相对增强。因而会造成同一界面的反射波在不同炮检距的道上，它的频谱是不同的，在炮检距大的道上频率较低。另一方面，在同一道里，由浅到深各界面的反射波的主频一般来说也是逐渐降低的。多次波由于在低速层中经过一些路程，频率一般也偏低一些。

野外地震仪记录频率范围的选择：

数字记录频率范围扩展到3～250Hz。即在野外尽可能把地震波的全部频率成分记录下来。在石油勘探中，这样可以照顾到把浅，中，深层的反射都记录下来，当然也不可避免记录了一些干扰波。

Key #5: Data Interpretation

We must interpret the seismic data to understand the geology and assess the likelihood of finding oil and gas accumulations.

Geophysicists interpret the processed seismic data and integrate other geoscientific information to make assessments of where oil and gas reservoirs may be accumulated.

Data Visualization

Powered by advanced supercomputer power, rapid data loading, high-speed networking and high-resolution graphics, visualization centers provide the ability to display and manipulate complex volumes of 3D data in a collaborative, team environment.

The result is . . . better interpretation . . . of more data . . . in less time.

Data Integration

A broad range of advanced interpretation services including PSDM, seismic attribute analysis, amplitude variation with offset (AVO) analysis, and reservoir characterization should be offered.

Our visualization centers enhance our ability to integrate additional geophysical and geologic data such as well logs, and to visualize and rapidly mature prospects for testing.

The End Product

The end product of all this work and technology is a graphic 3D representation of the Earth's sub-surface geologic structure. Based largely on this information, exploration companies will decide where (or if!) to drill for oil and gas. This example (top) represents over 600 square kilometers of complex geology down to a depth of more than 6,000 meters!

Key #4: Data Processing

We must make sense of the recorded seismic 'squiggles' to produce the truest possible image of the Earth's sub-surface geologic structure. Reflected seismic response is a mixture of our output pulse, the effect of the Earth upon that pulse, and background noise, all convolved together.

We must remove the output pulse and the noise to leave just the 'Earth model'. This is the role of seismic data processing, which requires accuracy, reliability, speed and substantial computing power. The advanced mathematical algorithms and complex geophysical processes applied to 3D seismic data require enormous computing resources.

Not to mention the massive volumes of data involved.

For example, the amount of seismic data recorded by CGGVeritas during just ONE medium-sized marine 3D survey would fill more than 20,000 compact disks, forming a stack over 650 feet high!

Processing: Deconvolution

Ideal seismic response would be a single sharp reflection for each sub-surface rock layer boundary. Actual seismic response is less than ideal because our output pulse is not perfectly sharp and changes its shape while passing through the Earth.

Deconvolution 'deconvolves' our output pulse from the seismic response and converts it into a cleaner, sharper, less confusing pulse.

Can you determine the number of rock layers here by examining the actual seismic response (before deconvolution)?

Data Processing: Stacking

Seismic traces from the same reflecting point are gathered together (CRP gather) and summed, or 'stacked'.

The six seismic traces on the left are from the same reflecting point. As the traces are merged into one (right), background noise cancels itself out while the seismic signals add together, producing a stronger signal-to-noise ratio. (The output trace on the right is shown here six times only to provide a better comparison.)

The more of these seismic traces we can stack together into one output trace, the clearer the seismic image.

This first image shows a seismic section produced after the seismic traces have been sorted, adjusted for varying path lengths and signal strength, and stacked.

Here, each trace is the summation of 48 individual 'shot' traces.

Note the water bottom 'multiple' reflection (arrowed) -- a seismic 'echo' of the seafloor caused by energy bouncing back-and-forth within the water layer to produce a 'false' reflection obscuring the real data.

This second image shows the result of suppressing the water bottom multiple.

The seismic image is enhanced by a process that suppresses the multiple without harming real reflections.

This third image is further enhanced by 'focusing' energy for both flat and steep reflectors.

Any missing traces are 'filled in' by interpolation.

This fourth image most closely resembles the true sub-surface geology.

A process called 'migration' moves reflected energy to its true sub-surface position of origin.

Advanced Data Processing

More advanced processing techniques, such as Prestack Depth Migration (PSDM), can significantly improve seismic imaging, especially in areas of complex geology.

In this example from the Gulf of Mexico, see how PSDM has improved the imaging of a) a massive salt body, and b) sedimentary layers beneath the salt.

Processes such as PSDM take more time, expertise and resources to apply, but accurate 3D seismic images can mean the difference between success or an expensive dry hole.

In-Field Data Processing

Our customers usually need the data delivered as fast as possible!

In fact, today's industry demands for ever-faster turnaround of seismic projects necessitates that data now be processed, at least to a preliminary stage, in the field immediately after recording.

This requires equipment and personnel in the field to be almost as sophisticated as those onshore.

Wave velocities

The earth's crust is approximately a Poisson solid, with elastic constant 0.3 Tdyn/square(cm). Thus, for a density of 3 g/cub(cm), the P-wave velocity is 5.5 km/s. Similarly, The S-wave velocity is 3.2 km/s. Hence a P wave propagating with a velocity of 5.5 km/s and a period of 2 shas a wavelength of 11 km and the wavenumber is 0.57 per km. On the other hand, a wave with a period of 10 s and the same velocity has a wvelength of 55 km and a frequency of 0.1 Hz. The longer-period wave has a longer wavelength and a lower frequency.

Key #3: Data Recording (Acquisition)

Some of the energy we send into the ground, or water, is reflected back from geologic boundaries in the sub-surface.

This reflected energy is detected by a connected network of geophones (left) planted in the ground, or by groups of hydrophones contained inside the neutrally buoyant seismic 'streamer(s)' towed behind the vessel at sea (main picture).

Similar to microphones, these devices convert the reflected energy into electrical energy which is transmitted to a central recording system, usually housed in the instrument room (or 'doghouse') for recording as raw seismic data, and for quality control checks.

Quality control is vital, not just during data recording, but at every stage of a seismic project.

Multiple Lines of Data at Once

At sea, several lines of seismic data can be recorded simultaneously by towing multiple source arrays and streamers.

Here, two source arrays and four streamers allow eight lines of seismic data (shown in yellow) to be recorded at once.

It is generally much faster to acquire seismic data at sea than on land.

Seismic spectrum

Studies of earth-quakes typically use the period range from approximately 0.1 s to more than 3000 s, or frequencies from 10 Hz to 0.3 mHz. Higher-frequency waves of 20 - 80 Hz generated by explosions or other artificial sources are used in reflection seismology to explore the earth's crust. Still higher frequencies, 3 - 12 kHz, propagating primarily in the ocean, are used by marine geophysicists to map the sea floor. At the other end of the spectrum, ground motions with periods longer than 10 ksare due to slow crustal motions rather than propagating seismic waves.

Seismic Overview

The words seismic and geophysics are often associated with earthquakes. But seismic data are also a valuable technology used extensively by the oil and gas industry in its exploration, development and reservoir management operations.

The Purpose of Seismic

The main purpose of seismic exploration is to render the most accurate possible graphic representation of specific portions of the Earth's subsurface geologic structure.

The images produced allow exploration companies to accurately and cost-effectively evaluate a promising target (prospect) for its oil and gas yielding potential.

Seismic Fundamentals

Seismic imaging is simple. But it takes knowledge, experience and advanced technology to do it right.

Acquisition of seismic data involves the transmission of controlled acoustic energy into the Earth, and recording the energy that is reflected back from geologic boundaries in the subsurface.

Information regarding the structure and nature of the reflecting strata can be derived from the two-way travel time, and other attributes, of the returning energy. Processing these reflections produces a synthetic image of the Earth's subsurface geologic structure.

Acquiring Seismic Data at Sea

3D seismic data are displayed as a three-dimensional cube that may be sliced into numerous planes or cross-sections.

At sea, the procedure is essentially the same except that our instruments are continuously moving!

The seismic (energy) source is usually an array of airguns towed behind the survey vessel and just below the sea surface. The airguns are fired at regular intervals as the vessel moves along pre-determined survey lines.

Energy reflected from beneath the seafloor is detected by numerous 'hydrophones' contained inside long, neutrally buoyant 'streamers' - often almost 5 miles long - also towed behind the vessel.

2D Seismic Data

Two types of seismic surveys are available to the geophysicist: two-dimensional (2D) surveys, or three-dimensional (3D) surveys.

2D seismic data are displayed as a single vertical plane or cross-section sliced into the Earth beneath the seismic line's location.

2D is generally used for regional reconnaissance, or for detailed exploration work where economics may not support the greater cost of 3D . . .

3D Seismic Data

More expensive than 2D data, 3D produces spatially continuous results which reduce uncertainty in areas of structurally complex geology and/or small stratigraphic targets.

4D Seismic Data

Two or more 3D seismic surveys acquired at different times can be compared in order to search for changes in the fluids within the rock formations.

This type of survey is known as 4D, where elapsed TIME is the fourth dimension of information.

The Five Key Ingredients

There are five key ingredients to acquiring useful seismic data:

1. Positioning / Surveying

2. Seismic Energy Source

3. Data Recording

4. Data Processing

5. Data Interpretation

Key #1: Positioning / Surveying

Accurate positioning is fundamental and vital to acquiring seismic data.

We must know PRECISELY where all our instruments are on the Earth's surface.

Otherwise, however good the quality of the recorded seismic data . . . the data are worthless if we don't know where they came from.

In both marine (left) and land (right) environments, energy source and receiver layout patterns are pre-planned, and their positions pre-determined, so that we can calculate precisely where our recorded seismic data originate.

Positioning Technology

Today we are in the 'space age' of GPS - the Global Positioning System - which offers unprecedented accuracy.

GPS is a constellation of 24 satellites in orbit about 20,200 kms above the Earth. The satellites act as precise reference points in space and transmit radio signals that allow a GPS receiver on Earth to triangulate its position to within about 10 meters.

While 10-meter accuracy is adequate for many purposes, for seismic we use Differential GPS (DGPS) correction techniques to bring our levels of accuracy to between 2 meters and 30 centimeters!

Positioning at Sea

At sea, positioning is more difficult than on land because our vessel - and all its towed equipment - is continuously in motion.

Nevertheless, the precise locations of the energy source(s) and the streamer(s) MUST be known at all times.

In such a dynamic environment, real-time positioning is extremely complex and highly computer-intensive.

We use an integrated combination of multiple reference site DGPS, Relative GPS, laser measurements of ranges and angles, underwater acoustic ranging and digital compasses along the streamer(s).

Literally hundreds of complex mathematical position calculations are carried out every few seconds, enabling the precise positions of the vessel, the seismic source(s) and the individual hydrophone groups in the streamer(s) to be calculated in real-time as the vessel continuously moves along.

Key #2: Energy Source

At Sea: AirgunsTo gather seismic data, we must first generate and transmit controlled acoustic energy into the ground.

In the past, dynamite was the preferred seismic energy source both on land and at sea. Dynamite is still used on land, particularly in areas of soft, unconsolidated or weathered surface layers. When buried and detonated in safely plugged shot holes below the surface layer, dynamite produces a sharp, acoustically clean energy pulse.

However, in urban and/or populous areas, dynamite is obviously not practical! There are several other energy source technologies used for acquiring seismic data, but the main one is 'vibroseis'.

On Land: Vibroseis

Large servo-hydraulic vibrators on vibroseis trucks are safer, faster, more adaptable and more environmentally friendly than dynamite, and can yield equal (or sometimes better) data quality.

How Vibroseis Works

A vibroseis truck generates a controlled vibratory force of up to 70,000 lbs through a baseplate that is placed in contact with the ground.

At Sea: Airguns

In the marine environment, and sometimes in swamp or marsh, dynamite has been almost completely replaced by airguns.

In an airgun，high pressure air is stored in a firing chamber and explosively released through portholes by the action of a sliding shuttle with pistons at each end.

Seismic energy is generated by the rapid, explosive release of compressed air through the airgun's ports

How Airguns Are Deployed

into the surrounding water. This produces a primary energy pulse and an oscillating bubble.

Typically, multiple airguns are towed behind the vessel, several meters below the sea surface in a pre-determined combination, or 'array' of different chamber volumes designed to generate an optimally tuned energy output of desirable sound frequencies.

Opening Frontiers

Passionate performance and powerful innovation now go by a group name: Dr. Cheng Linghao; Dr. Xu Pengfei, Dr. Shao Yu and Che Jing.

Dr. Cheng linghao
IEEE Member,
The Hong Kong Polytechnic University.

Dr. Xu pengfei
Professor
Beijing Normal University.

Shao Yu
IEEE Member
SEG Associate Member
China National Petroleum Corporation.

Che Jin
Intermec Technologies Pte Ltd.
Nanyang Technologies University.