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.