Wave Types and Processing Methods

针对目前项目工作调研的结果，以海上和复杂地表地震数据为研究对象，主要解决的问题有两类：

1. 压制海上的多次波(Multiples)和导波(Guided waves)，
2. 压制复杂地表中面波(Ground-roll)和由于散射导致的噪声。

我把关于波的类型区分总结如下，并阐述了目前常用的解决方法。这个研究方向主要的工作就是如何最好的分离噪声和信号，之后有效的压制噪声。从采集角度看，如果检波器具有多分量的功用，并且可以按照各个分量最好的提取采集的信号，这样可以起到现场压制噪声的效果；如果从处理角度看，我们需要的就是采用变换域的方法，把信号和噪声转换到F-K或者其他有效的区分噪声和信号的域里进行处理。大家如果有什么想法可以讨论一下。

To summarize, field records contain (a) reflections, (b) coherent noise, and (c) random ambient noise. One important aspect of data processing is to uncover genuine reflections by suppressing noise of various types. Processing, however, cannot yield signal from field data without signal. At best, it suppresses whatever noise is in the field data and enhances the reflection energy that is buried in the noise.
Reflections on shot records are recognized by their hyperbolic traveltimes. If the reflecting interface is horizontal, then the apex of the reflection hyperbola is situated at zero offset. On the other hand, if it is a dipping interface, then the reflection hyperbola is skewed in the updip direction.
There are several wave types under the coherent noise category:

1. Ground roll is recognized by low frequency, strong amplitude, and low group velocity. It is the vertical component of dispersive surface waves. In the field, receiver arrays are used to eliminate ground roll. Ground roll can have strong backscattered components because of lateral inhomogeneities in the near-surface layer.
2. Guided waves are persistent, especially in shallow marine records in areas with hard water bottom. The water layer makes a strong velocity contrast with the substratum, which causes most of the energy to be trapped within and guided laterally through the water layer. The dispersive nature of these waves makes them easy to recognize on shot records. Guided waves also make up the early arrivals. The stronger the velocity contrast between the water layer and the substratum, the smaller the critical angel; thus more guided-wave energy is trapped in the supercritical region. When there is a strong velocity contrast, refraction energy propagates in the form of a head wave. Guided waves also are found in land records. These waves are largely attenuated by CMP stacking. Because of their prominently linear moveout, in principle they also can be suppressed by dip filtering techniques. One such filtering technique is based on 2-D Fourier transformation of the shot record. Another approach is based on slant stacking.
3. Side-scattered noise commonly occurs at the water bottom, where there is no flat, smooth topography. Irregularities of varying size act as point scatters, which cause diffraction arrivals with table-top trajectories. They can be on or off the vertical plane of the recording cable. These arrivals typically exhibit a large range of moveouts, depending on the spatial position of the scatterers in the subsurface.
4. Cable noise is linear and low in amplitude and frequency. It primarily appears on shot records as late arrivals.
5. The air wave with a 300 m/s velocity can be a serious problem when shooting with surface charges such as Geoflex, Poulter, or land air gun. Perhaps the only effective way to remove air waves is to zero out the data on shot gathers along a narrow corridor containing this energy (notch muting). It often is impossible to recover any data arriving after the air wave on Poulter data.
6. Power lines also cause noisy traces in the form of a monofrequency wave A monofrequency wave may be 50 or 60 Hz, depending on where the field survey was conducted. Notch filters often are used in the field to suppress such energy.
7. Multiples are secondary reflections with interbed or intrabed raypaths. Guided waves include supercritical multiple energy. Multiples are attacked by methods, which are based on moveout discrimination, and prediction theory, which uses the periodic behavior of multiples. The most effective moveout-based suppression technique often is CMP stack with inside-trace mute. Prediction theory should be particularly effective, at least in theory, in the slant-stack domain.

Random noise has various sources. A poorly planted geophone, wind motion, transient movements in the vicinity of the recording cable, wave motion in the water that causes the cable to vibrate, and electrical noise from the recording instrument all can cause ambient noise. The net result of scattered noise from many scatterers in the subsurface also contributes to random noise.
It is noted that energy propagating within the earth is subject to a decay in amplitude because of wavefront divergence and frequency-dependent absorption from the intrinsic attenuation of rocks. Signal strength therefore decreases in time, while random noise persists and eventually dominates. Unfortunately, gain corrections to restore signal strength at later times boost random
noise in the process. Fortunately, CMP stack suppresses a significant part of the random noise uncorrelated from trace to trace.

Seismic Sources

A seismic source generates controlled seismic energy that is used in 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 specialized air gun. Seismic sources can provide single pulses or continuous pulses of energy that generates seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract to receivers, such as geophones or hydrophones.

Seismic exploration using sound sources may be used to investigate shallow subsoil structure, for engineering site work, or deeper structures, usually in the search for oil, or for scientific investigation. The returning signals from the sources are detected by geophones, laid in known locations relative to the position of the source. The recorded signals are then subjected to specialist analysis and processing to yield comprehensible data.

Source model

A seismic source signal has the following characteristics:

1. generated as an impulsive source

2. band-limited

3. the generated waves are time-varying

Types of sources

1. Explosives

Explosives, such as dynamite, can be used as crude but effective sources of seismic energy.

2. Air gun

An air gun is used for marine reflection and refraction surveys. It consists of one or more pneumatic chambers that are pressurized with air. The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a bolt is retracted, allowing the air to escape the chamber and to produce a pulse of acoustic energy.

3. Plasma sound source

A plasma sound source (PSS), otherwise called a spark gap sound source, is a means of making very low frequency sonar pulse underwater.

For each firing, it stores electric charge in a large high-voltage bank of capacitors, and then releases all the stored energy in an arc across electrodes in the water. The underwater spark discharge produces a high-pressure plasma and vapor bubble, which expands and collapses, making a loud sound. Most of the sound produced is between 20 and 200 Hz.

The PSS has also been used for sonar. There are also plans to use PSS as a non-lethal weapon against submerged divers.

4. Thumper truck

A Thumper truck (or weight-drop) truck is a vehicle mounted ground impact which can used to provide the seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, possibly about three metres, to impact (or "Thump") the ground. To augment the signal, the weight may be dropped more than once at the same spot, the signal may also be increased by thumping at several nearby places in an array whose dimensions may be chosen to enhance the seismic signal by spatial filtering.

Thumping might be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few metres might create long-lasting disturbance of the soil. An advantage of the Thumper (later shared with Vibroseis) especially in politically unstable areas, was that no explosives were required.

5. Vibroseis sources

Vibroseis is a method used to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed.

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.