Most major oil companies also have actively conducted research into seismic methods as well as collected and processed seismic data using their own personnel and technology. Reflection seismology has also found applications in non-commercial research by academic and government scientists around the world. Seismic waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are travelling.
The acoustic or seismic impedance, Z , is defined by the equation:. When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface. At its most basic, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers or geophones at the surface.
In common with other geophysical methods, reflection seismology may be seen as a type of inverse problem. That is, given a set of data collected by experimentation and the physical laws that apply to the experiment, the experimenter wishes to develop an abstract model of the physical system being studied. In the case of reflection seismology, the experimental data are recorded seismograms, and the desired result is a model of the structure and physical properties of the Earth's crust. In common with other types of inverse problems, the results obtained from reflection seismology are usually not unique more than one model adequately fits the data and may be sensitive to relatively small errors in data collection, processing, or analysis.
For these reasons, great care must be taken when interpreting the results of a reflection seismic survey. The general principle of seismic reflection is to send elastic waves using an energy source such as dynamite explosion or Vibroseis into the Earth, where each layer within the Earth reflects a portion of the wave's energy back and allows the rest to refract through.
These reflected energy waves are recorded over a predetermined time period called the record length by receivers that detect the motion of the ground in which they are placed.
On land, the typical receiver used is a small, portable instrument known as a geophone , which converts ground motion into an analogue electrical signal. In water, hydrophones are used, which convert pressure changes into electrical signals. Typically, the recorded signals are subjected to significant amounts of signal processing before they are ready to be interpreted and this is an area of significant active research within industry and academia. In general, the more complex the geology of the area under study, the more sophisticated are the techniques required to remove noise and increase resolution.
Modern seismic reflection surveys contain large amount of data and so require large amounts of computer processing, often performed on supercomputers or computer clusters. When a seismic wave encounters a boundary between two materials with different acoustic impedances, some of the energy in the wave will be reflected at the boundary, while some of the energy will be transmitted through the boundary.
For a wave that hits a boundary at normal incidence head-on , the expression for the reflection coefficient is simply. Similarly, the amplitude of the incident wave is multiplied by the transmission coefficient to predict the amplitude of the wave transmitted through the boundary. The formula for the normal-incidence transmission coefficient is. As the sum of the squares of amplitudes of the reflected and transmitted wave has to be equal to the square of amplitude of the incident wave, it is easy to show that.
By observing changes in the strength of reflectors, seismologists can infer changes in the seismic impedances. In turn, they use this information to infer changes in the properties of the rocks at the interface, such as density and elastic modulus. The situation becomes much more complicated in the case of non-normal incidence, due to mode conversion between P-waves and S-waves , and is described by the Zoeppritz equations.http://police-risk-management.com/order/online/pymyn-nascondere-numero.php
In , Karl Zoeppritz derived 4 equations that determine the amplitudes of reflected and refracted waves at a planar interface for an incident P-wave as a function of the angle of incidence and six independent elastic parameters. The reflection and transmission coefficients, which govern the amplitude of each reflection, vary with angle of incidence and can be used to obtain information about among many other things the fluid content of the rock.
Practical use of non-normal incidence phenomena, known as AVO see amplitude versus offset has been facilitated by theoretical work to derive workable approximations to the Zoeppritz equations and by advances in computer processing capacity. AVO studies attempt with some success to predict the fluid content oil, gas, or water of potential reservoirs, to lower the risk of drilling unproductive wells and to identify new petroleum reservoirs.
The 3-term simplification of the Zoeppritz equations that is most commonly used was developed in and is known as the "Shuey equation". A further 2-term simplification is known as the "Shuey approximation", is valid for angles of incidence less than 30 degrees usually the case in seismic surveys and is given below: . The time it takes for a reflection from a particular boundary to arrive at the geophone is called the travel time.
If the seismic wave velocity in the rock is known, then the travel time may be used to estimate the depth to the reflector. A series of apparently related reflections on several seismograms is often referred to as a reflection event. By correlating reflection events, a seismologist can create an estimated cross-section of the geologic structure that generated the reflections. Interpretation of large surveys is usually performed with programs using high-end three-dimensional computer graphics.
In addition to reflections off interfaces within the subsurface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded:. The airwave travels directly from the source to the receiver and is an example of coherent noise.
Seismic Data Processing - MATLAB & Simulink
A Rayleigh wave typically propagates along a free surface of a solid, but the elastic constants and density of air are very low compared to those of rocks so the surface of the Earth is approximately a free surface. Low velocity, low frequency and high amplitude Rayleigh waves are frequently present on a seismic record and can obscure signal, degrading overall data quality.
A head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface. This motion causes a disturbance in the upper medium that is detected on the surface. An event on the seismic record that has incurred more than one reflection is called a multiple. Multiples can be either short-path peg-leg or long-path, depending upon whether they interfere with primary reflections or not.
Multiples from the bottom of a body of water 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. Cultural noise includes noise from weather effects, planes, helicopters, electrical pylons, and ships in the case of marine surveys , all of which can be detected by the receivers.
Reflection seismology is used extensively in a number of fields and its applications can be categorised into three groups,  each defined by their depth of investigation:. A method similar to reflection seismology which uses electromagnetic instead of elastic waves, and has a smaller depth of penetration, is known as Ground-penetrating radar or GPR. Reflection seismology, more commonly referred to as "seismic reflection" or abbreviated to "seismic" within the hydrocarbon industry, is used by petroleum geologists and geophysicists to map and interpret potential petroleum reservoirs.
The size and scale of seismic surveys has increased alongside the significant concurrent increases in computer power during the last 25 years. This has led the seismic industry from laboriously — and therefore rarely — acquiring small 3D surveys in the s to now routinely acquiring large-scale high resolution 3D surveys. The goals and basic principles have remained the same, but the methods have slightly changed over the years.
Land — The land environment covers almost every type of terrain that exists on Earth, each bringing its own logistical problems. Examples of this environment are jungle, desert, arctic tundra, forest, urban settings, mountain regions and savannah. Transition Zone TZ — The transition zone is considered to be the area where the land meets the sea, presenting unique challenges because the water is too shallow for large seismic vessels but too deep for the use of traditional methods of acquisition on land.
Examples of this environment are river deltas, swamps and marshes,  coral reefs, beach tidal areas and the surf zone. Transition zone seismic crews will often work on land, in the transition zone and in the shallow water marine environment on a single project in order to obtain a complete map of the subsurface. Marine — The marine zone is either in shallow water areas water depths of less than 30 to 40 metres would normally be considered shallow water areas for 3D marine seismic operations or in the deep water areas normally associated with the seas and oceans such as the Gulf of Mexico.
Seismic surveys are typically designed by National oil companies and International oil companies who hire service companies such as CGG , Petroleum Geo-Services and WesternGeco to acquire them. Another company is then hired to process the data, although this can often be the same company that acquired the survey. Finally the finished seismic volume is delivered to the oil company so that it can be geologically interpreted. Land seismic surveys tend to be large entities, requiring hundreds of tons of equipment and employing anywhere from a few hundred to a few thousand people, deployed over vast areas for many months.
Vibroseis is a non-impulsive source that is cheap and efficient but requires flat ground to operate on, making its use more difficult in undeveloped areas. The method comprises one or more heavy, all-terrain vehicles lowering a steel plate onto the ground, which is then vibrated with a specific frequency distribution and amplitude.
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For a long time, it was the only seismic source available until weight dropping was introduced around ,  allowing geophysicists to make a trade-off between image quality and environmental damage. Compared to Vibroseis, dynamite is also operationally inefficient because each source point needs to be drilled and the dynamite placed in the hole. A land seismic survey requires substantial logistical support.
In addition to the day-to-day seismic operation itself, there must also be support for the main camp for catering, waste management and laundry etc. Unlike in marine seismic surveys, land geometries are not limited to narrow paths of acquisition, meaning that a wide range of offsets and azimuths is usually acquired and the largest challenge is increasing the rate of acquisition. The rate of production is obviously controlled by how fast the source Vibroseis in this case can be fired and then move on to the next source location.
The goal of this primer is to provide a basic near-surface seismic-reflection processing guide for workers who have not had industry- or academic-supported training or guidance but wish to maintain the integrity of seismic imaging as a tool for near-surface exploration. This primer will focus on processing two small data sets using standard common-midpoint CMP processing and will include many significant processing pitfalls encountered in previous work. As a PDF file, each item on the Contents page is linked to its corresponding location, and the user can return to Contents by clicking the Page Number at the bottom- center of every page.
Additionally, all references in this tutorial are linked to the appropriate location in Appendix A the Reference list. The main portion of this tutorial is Section 2, Processing Steps and Associated Pitfalls, which focuses on individual processing procedures, each of which is broken into three parts: i. Geological Survey Pakiser and Mabey, ; Pakiser et al. Digital seismographs were not yet available, and the method was not strongly pursued until the late s and early s.
A classic paper by Hunter et al. At that time, however, the high cost of seismographs, computers, and processing systems limited the seismic investigation of the shallow subsurface.
Aspects of Seismic Reflection Data Processing
Thus, in the past 17 years the cost of seismographs has come down about an order of magnitude, factoring in the importance of dynamic range. Shallow-reflection seismic surveying is now fairly commonplace in academia for example, see Special Section - Shallow Seismic Reflection Papers, , Geophysics; Near Surface Geophysics Special Issue, , The Leading Edge and in industry, as noted by the significant amount of use by the environmental industry.
Processing shallow-seismic-reflection data is different from the seismic data processing done in hydrocarbon exploration e. Processing shallow-seismic reflection data is most precarious when nonreflective coherent events are generated on final stacked sections and can be misinterpreted as reflections. The goal of this tutorial is to attempt to reduce the number of misinterpretations by avoiding pitfalls and to advocate the adoption of a conservative approach to processing.
Specifically, the processor should attempt to eliminate or aviod generating coherent noise events, even at the expense of a final image of poorer quality. It is important to examine the objectives early and keep them in mind throughout processing.
Typically, the clarity and resolution of the final product are of the highest importance. Processes that tend to generate artifacts while increasing coherency such as f-k filtering, trace mixing, or migration are appropriate for goal-oriented processing. However, these processes can generate significant coherent events that have no geologic basis in the geometry of the subsurface.
Therefore, before performing any of the aforementioned processes, the seismic- data processor must be absolutely certain that the resulting coherent events can be correlated to reflections on the original shot or CMP gathers. A coherent event in a final stacked section that cannot be corroborated by supporting evidence from minimally processed shot or CMP gathers must be assumed to be artificial. An example of how processing procedures can create coherent events with no substantiation in the subsurface is shown in Figures 1.
The seismic reflection section in Fig. Figure 1. Note that the section in Fig. No unconformity Subhorizontal Bedding bedding dips to left? Time s 0. No faulting 0.