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Seismic survey

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is one of the first steps in
searching for oil and gas resources that directly
affects the land and the landowners Seismic surveys are like sonar on steroids They are based on recording the time it takes for sound waves generated by controlled energy sources .The survey usually requires people and machinery
to be on private property and may result in
disturbances of the land such as the clearing of
trees

Published in: Science
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Seismic survey

  1. 1. Sources of seismic survey By : Muhammed Essayed
  2. 2. is one of the first steps in searching for oil and gas resources that directly  affects the land and the landowners Seismic surveys are like sonar on steroids They are based on recording the time it takes for sound waves generated by controlled energy sources .The survey usually requires people and machinery  to be on private property and may result in  disturbances of the land such as the clearing of  trees
  3. 3.  Seismic survey is used to develop an image of the underground geology. Seismic surveys use reflected sound waves to produce a scanning of the Earth’s subsurface. Seismic surveys can help  locate ground water, are used to investigate locations for landfills, and characterize how an area will shake during an a  earthquake, but they are primarily used for oil and gas exploration (Seismic acquisition)  Before starting discussion about Seismic acquisition, we must know some concepts about the seismic theory
  4. 4. Seismic waves are sound waves that travel through the Earth or other elastic bodies, for example as a result of an: earthquake, explosion, or some other process that imparts forces Types of seismic waves:  1-body wave(p-wave/s-wave)  2-surface wave(rayleigh wave-love wave-stonely wave-tube wave)
  5. 5.  1-Determining location: Land - marine.  2-seismic sources: Land sources - marine sources.  3-seismic detector: Land detectors (Geophone) Marine detectors (Hydrophone)
  6. 6. A seismic source is a device that generates controlled seismic energy used to perform 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 sweeps of energy. Both types of seismic sources generate seismic waves, which travel through a medium such as water or layers of rocks
  7. 7.  Sources of seismic energy come in a variety of sizes and shapes. Virtually anything that impacts, or causes motion on, the surface of the earth will be a source of seismic energy. Unfortunately, most sources are uncontrollable, such as road traffic, wind (this causes noise by making bushes and trees move), aircraft, people walking, etc. For our experiments, we would like to control the source of the ground motion. In this discussion, we will restrict our examples to those sources most commonly used in near-surface (i.e., environmental and engineering) investigations
  8. 8.  Three types of sources are most commonly used for both refraction and reflection investigations of the near surface :  1 - Impact Sources .  2-Gun Sources .  3-Explosive Sources .
  9. 9. Sources that generate seismic energy by impacting the surface of the Earth are probably the most common type employed. Although impact sources can be rather sophisticated in their construction, the most commonly used type of impact source is a simple sledgehammer. In this case, an operator does nothing more than swing the sledgehammer downward onto the ground. Instead of striking the ground directly, it is most common to strike a metal plate lying on the ground. The sledgehammer is usually connected to the recording system by a wire. The moment the sledgehammer strikes the plate, the recording system begins recording ground motion from the geophones
  10. 10.  Low Cost and  Simple to operate and maintain.
  11. 11.  It can be difficult to assure that the source is operated in a repeatable fashion,  Operation is manually strenuous,  Source outputs relatively small amounts of seismic energy. Therefore, it can be difficult to record reliable observations at great distances, and  Source outputs seismic energy that tends to be low frequency in nature (i.e. this source generates a lot of surface waves).
  12. 12.  - Like impact sources, gun sources generate seismic energy by transferring the kinetic energy of a moving object into seismic energy. In this case, the moving object is a bullet or shot-gun slug. Some sources use blanks instead of bullets or slugs. In this case, energy is transferred from the column of air in the gun's barrel that is set in motion by the blank to the ground
  13. 13.  Highly repeatable source,  Energy imparted into the ground is larger than is possible from a sledgehammer, and  Gun sources generally output higher-frequency energy. This helps to minimize surface wave generation.
  14. 14.  Safety,  Equipment is more bulky and expensive than simple impact sources, and  Getting permission (permitting) to use this source may be more difficult
  15. 15.  The choice of energy source is critical in land data acquisition because resolution and signal-to-noise ratio quality are limited by the source characteristics. A geophysicist should select a source based on the following five criteria :1-Penetration to the required depth  2-Bandwidth for the require resolution  3-Signal-to-noise- characteristics  4-environment  5-availability and cost
  16. 16. Land source Impulsive source Non-impulsive source
  17. 17.  1- Explosive sources such as Dynamite can be used as crude but effective sources of seismic energy. Generally the explosive charges are placed between 6 and 76 meters (20 and 250 ft) below ground. The charges are placed in a hole that is drilled with dedicated drilling equipment for this purpose. This type of seismic drilling is often referred to as "Shot Hole Drilling
  18. 18.  Have been used principally : gelatin dynamite and ammonium nitrate . the former is amixture of nitroglycerin and nitro cotton .ammonium nitrate is cheaper and less dangerous because it is more difficult to detonate than gelatin dynamites . ammonium nitrate and nitro carbonitrite are the dominant explosive used today.
  19. 19. 2-Non Explosive  such as Weight drop &  Hammers (common in shallow seismi c investigation).
  20. 20.  Pound for pound, these types of sources impart the most amount of seismic energy into the ground of any of the sources described here,  The energy tends to be very high frequency, and because the explosives are usually placed in a shallow borehole, it tends not to be contaminated by surface waves, and  Explosive sources are very repeatable.
  21. 21.  Safety,  Permitting. Landowners tend to be nervous about allowing the use of explosives on their property,  Data acquisition using explosive sources is much slower than using impact or gun sources. This is primarily because boreholes must be drilled within which the explosives are to be placed, and  Explosives tend to be expensive to acquire and maintain
  22. 22.  B‐Non impulsive sources: the main common i s Vibroseis which is a designed  vehicle lift its weight on large plate in conta ct with ground surface in sweeps.  Up Sweep: Frequency begins low & increase with time.  Down Sweep: Frequency begins high & decr ease with time  Vibroseis vehicles are often used rather than explosives  in open, accessible areas
  23. 23. Air gun: the common in offshore survey (first prod uced in 1960). This gun  releases highly compressed air into wate r. It uses a compressed air at 2000‐5000 PSI to produce an explosive blast of air. S everal air guns with different sizes are fired to enhance their initial pulses & re duce their bubble effects.
  24. 24.  An air gun is used for marine reflection and refraction surveys .The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a solenoid is triggered, which releases air into a fire chamber which in turn causes a piston to move, thereby allowing the air to escape the main chamber and to produce a pulse of acoustic energy Air gun arrays are built up of up to 48 individual air guns with different size chambers
  25. 25.  the aim being to create the optimum initial shock wave with minimum reverberation of the bubble after the first shot Gun arrays can be fired in flip-flop mode; typically this would be 48 guns per source, which would be selected and fired alternately. Large chambers (i.e., greater than 1.15 L or 70 cu in) tend to give low frequency signals, and the small chambers (less than 70 cubic inches) give higher frequency signals. The air gun is made from the highest grades of corrosion resistant stainless steel

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