Difference between revisions of "Acquisition"

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Acquisition of seismic data can be done using two main sources: earthquakes and controlled sources. For exploration purposes, controlled sources are used both for land and marine acquisition.An ideal seismic source is a source that is capable of generating a repeatable pulse of known frequency and other properties.
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==Land Acquisition==
  
{{geophysics-stub}}
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=== Sources ===
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Land acquisition for [[Reflection and refraction|reflection seismology]] uses an array of sources and receivers. The choices of which sources and receivers to use depend on the goals of the survey along with cost and environmental conditions.
  
There are three major phases of seismic data:
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Dynamite is a commonly used impulse source for exploration. Dynamite is preferred  when the survey area is in harsh terrain that Vibroseis cannot traverse such as marshes, mountains, or environmentally sensitive areas. The dynamite must be buried prior to detonation to increase the amount of energy transmitted into the subsurface and for safety. Since the energy is produced instantly from the detonation, dynamite sources produce a wavelet that is roughly minimum phase. However, dynamite does have its drawbacks. Inconsistencies in the blasts along with variations in the burial depth and the local ground conditions will cause variations in the produced signal. Another impulse source used is modified shotguns called Betsy Guns. Betsy Guns are used for shallower and smaller surveys.
1) Acquisition
 
2) Processing
 
3) Interpretation
 
  
Data acquisition involves sending energy into the ground and recording the energy as it returns to the surface after bouncing (or echoing) off of the underground rock type boundaries.
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Another commonly used source type for petroleum exploration are vibratory sources. Vibroseis trucks are used to transmit energy into the earth using a specified range of frequencies over a specified time. The trucks feature a heavy mass that vibrates vertically on a base plate to transfer energy into the subsurface. The range of frequencies (i.e. how fast the mass vibrates) and the length of time that the vibration occurs are unique for each survey. Since the signal inputted into the subsurface is known, it can be mathematically removed in processing to help remove noise and create a trace that resembles the true reflectivity of the survey area. In an effort to improve the post-correlation signal to noise ratio, an array of vibroseis trucks may be used, as the post-correlation signal to noise ratio is proportional to:<math>S:R ~ F\bulletsqrt{LW}</math>, where F = the weight of the truck(force applied), L = the length of the sweep and N = the number of sweeps<ref>Dean, Timothy & Tulett, J. (2014). The Relationship between the Signal-to-Noise Ratio of Downhole Data and Vibroseis Source Parameters. 10.3997/2214-4609.20140916. </ref>. Using an array of trucks will increase the force applied, therefore enhancing the SNR. Generally, Vibroseis trucks are generally only produce P-waves, as they are designed to vibrate the mass vertically. There exists Vibroseis trucks designed to produce S-waves, but they are rare and infrequently used.  
The boundaries between rock types cause a change in what is called "acoustic impedance" which is a technical term that simply means that the energy changes when it hits that boundary. Some energy passes through the boundary and some energy bounces off of it and returns to the surface where it is recorded.
 
Acquisition has two main components
 
  
1) sources
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Weight drops are another type of source. These are impulse sources which are generally used for shallow subsurface due to being much lower energy than dynamite or vibroseis. Examples of weight drops are sledgehammers hitting a metal plate on the ground and weights dropped heights of at least two meters. Accelerated weight drops (AWD) also fall in this category. AWD work by using a hydraulic system to lift a heavy steel hammer up, and a gas-charged piston forces the piston down. These have been proven as viable sources for [[Vertical seismic profiling (VSP)|VSP's]] and tool-orientation for micro-seismic surveys. <ref>Botelho, Marco & Schinelli, Marco & Guerra, Rafael. (2015). Successful Application of Accelerated Weight Drop on VSP Acquisition. 10.1190/sbgf2015-019. </ref>
2) receivers
 
  
Sources and recievers are different for land and marine cases.
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=== Receivers ===
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For land surveys, the receiver used is a geophone. The instrument must have a solid connection to the ground, so it is usually buried. Inside the geophone, a magnet is a attached to the sides with a coil of wire suspended inside the magnet. As reflected waves return, the body of the geophone vibrates with the ground caused by the up-going energy. The coil vibrates at a different rate than the body, so the coil moves in and out of the magnetic field which induces an electrical current. The produced electrical current is recorded and is called the seismic trace, which is a representation of the subsurface's response to the inputted energy from the source. 3C geophones are designed to record 3 components of the wavefield: the P, SH, and SV waves. Another option to use for land receivers are land streamers that are towed behind a vibroseis truck <ref>van der Veen, M & Spitzer, Roman & Green, AG & Wild, P. (2001). Design and application of a towed land-streamer for cost-effective 2D and pseudo-3D shallow seismic data acquisition. Geophysics. 66. 10.1190/1.1444939. </ref>.
  
For land - the source is predominantly an array of vibratory source trucks that "excite" the ground and cause the energy to propagate through the earth.
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=== Survey Design ===
For marine - the source is an array of "air guns" The airguns build a tuned bubble of air in the water that causes a pressure wave to travel through the water which continues to propagate in the earth at the sea floor.
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The design of land surveys need to account for several factors:
  
For land - the receiver is an array of geophones.  At it's core, the geophone is a magnet and a coil. As the energy returns from the rock boundary interfaces it causes the earth to move at the surface and the geophone records that movement. The voltage values caused by the coil moving over the magnet is digitized into a digitally sampled seismic representation of how the surface of the earth moves and responds to the input stimulus from the vibrator trucks.
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1) The target horizon's approximate depth needs to be known along with the regional geological structure. Steeply dipping strata can be difficult to image with seismic, and there needs to be enough offset to image the target depth.
  
For marine - the receiver is a pressure sensor commonly referred to as a hydrophone. As the energy travels through the water the pressure changes are recorded and digitized into a digitally sampled seismic representation of how the water responds to the input stimulus from the air gun bubble pulse.
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2) Logistics: Permitting(getting permission from land owners), weather, and equipment availability will determine where and when a survey can be done.
  
In both cases the source is generally referred to as a "point source" which means that all of the energy emanates from a single location, but the receivers are laid out in a big areal array. The array can be a single long line of receivers for the 2D seismic or in a variety of different 3D surface arrays for 3D seismic.
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3)Trace/Bin Spacing: The trace spacing in 2-D seismic data helps determine lateral resolution of the data. The trace spacing must be close enough to identify true reflection dip, or else spatial aliasing will occur. For 3-D data, the bin size helps determines lateral resolution. However, the Fresnel zone also must be considered, and whichever is larger will determine the lateral resolution of the data.
  
Other terms you can look up for the 3D cases might include Full Azimuth and Narrow Azimuth.
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4) Survey size: Ideally, the area of interest should be covered by full-fold data.<ref>Chaouch, A., and J. L. Mari, 2006, 3-D Land Seismic Surveys: Definition of Geophysical Parameter: Oil & Gas Science and Technology - Revue de lIFP, v. 61, no. 5, p. 611–630, doi:10.2516/ogst:2006002.</ref>
  
Recording at multiple locations generates seismic "traces" or digital representation of the energy returning to the surface at different locations at different directions and distances from the source.  The traces are combined in different ways in processing to enhance the replication of reflected signal and remove the energy that is noise or unwanted energy.
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For further information, see the [[Land acquisition geometry]] page.
  
There are many articles and web sites that you can look at for more information, for example [https://en.wikipedia.org/wiki/Reflection_seismology WikipediA] has a good write up
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==Marine Acquisition==
 +
Marine Acquisition is accomplished by using large vessels outfitted with sources and streamers that are towed behind the ship.
 +
 
 +
=== Sources ===
 +
Airguns are the most popular source used for offshore seismic acquisition. These are metal cylinders through which high pressure air is forced through and into the water column. The injection of air into the water creates a pressure pulse that travels through the water and into the subsurface. It is common to have multiple airguns firing at once to create an array.
 +
 
 +
Sparkers are another source used for marine acquisition. These generate a pressure pulse in the form of a bubble by discharging an electrical current into the water.
 +
 
 +
Boomers are sources used for relatively shallow surveys and generate the pressure differential mechanically.
 +
 
 +
Chirp systems, like boomers, are used for shallow surveys. Chirp systems are vibratory sources. The chirp systems, sparkers, and boomers are high-frequency sources. This results in providing high resolution shallow data but lacking the energy to clearly image deeper deposits due to attenuation of their signal<ref>https://archive.epa.gov/esd/archive-geophysics/web/html/marine_seismic_methods.html</ref>.
 +
 
 +
=== Receivers ===
 +
Hydrophones are the typical receiver for marine seismic data, and they measure pressure. Energy in the subsurface is reflected through the water column where it creates a pressure pulse, which is measured by the hydrophone. Factors to consider when recording marine seismic are: how deep to tow the hydrophones, the length of the streamer, along with the number of hydrophone groups. Difficulties of using streamers include location monitoring and [[Cable feathering|feathering]]. Additionally, wildlife must be considered as surveys cannot operate nearby dolphins, whales and other wildlife. Furthermore, only P waves can be recorded by hydrophones as shear waves cannot travel through fluids
 +
 
 +
Ocean bottom cables are receivers used when recording shear wave data is desired. Similarly, ocean bottom seismometers can be used. These are deployed and recovered via remote vehicles and are very costly to use.
 +
 
 +
=== Survey Design ===
 +
For information about marine survey design and geometries, see the [[Wide azimuth (WAZ)|Wide azimuth]] page.
 +
 
 +
== References ==
 +
{{reflist}}
 +
 
 +
==See also==
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*[[Processing of 3-D seismic data]]
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*[[Marine acquisition geometry]]
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*[[Vibroseis deconvolution]]
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 +
==External links==
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http://wiki.aapg.org/Seismic_data_acquisition_on_land
 +
https://en.wikipedia.org/wiki/Reflection_seismology
 +
http://wiki.aapg.org/Marine_seismic_data_acquisition

Revision as of 20:19, 23 October 2018

Acquisition of seismic data can be done using two main sources: earthquakes and controlled sources. For exploration purposes, controlled sources are used both for land and marine acquisition.An ideal seismic source is a source that is capable of generating a repeatable pulse of known frequency and other properties.

Land Acquisition

Sources

Land acquisition for reflection seismology uses an array of sources and receivers. The choices of which sources and receivers to use depend on the goals of the survey along with cost and environmental conditions.

Dynamite is a commonly used impulse source for exploration. Dynamite is preferred when the survey area is in harsh terrain that Vibroseis cannot traverse such as marshes, mountains, or environmentally sensitive areas. The dynamite must be buried prior to detonation to increase the amount of energy transmitted into the subsurface and for safety. Since the energy is produced instantly from the detonation, dynamite sources produce a wavelet that is roughly minimum phase. However, dynamite does have its drawbacks. Inconsistencies in the blasts along with variations in the burial depth and the local ground conditions will cause variations in the produced signal. Another impulse source used is modified shotguns called Betsy Guns. Betsy Guns are used for shallower and smaller surveys.

Another commonly used source type for petroleum exploration are vibratory sources. Vibroseis trucks are used to transmit energy into the earth using a specified range of frequencies over a specified time. The trucks feature a heavy mass that vibrates vertically on a base plate to transfer energy into the subsurface. The range of frequencies (i.e. how fast the mass vibrates) and the length of time that the vibration occurs are unique for each survey. Since the signal inputted into the subsurface is known, it can be mathematically removed in processing to help remove noise and create a trace that resembles the true reflectivity of the survey area. In an effort to improve the post-correlation signal to noise ratio, an array of vibroseis trucks may be used, as the post-correlation signal to noise ratio is proportional to:Failed to parse (unknown function "\bulletsqrt"): {\displaystyle S:R ~ F\bulletsqrt{LW}} , where F = the weight of the truck(force applied), L = the length of the sweep and N = the number of sweeps[1]. Using an array of trucks will increase the force applied, therefore enhancing the SNR. Generally, Vibroseis trucks are generally only produce P-waves, as they are designed to vibrate the mass vertically. There exists Vibroseis trucks designed to produce S-waves, but they are rare and infrequently used.

Weight drops are another type of source. These are impulse sources which are generally used for shallow subsurface due to being much lower energy than dynamite or vibroseis. Examples of weight drops are sledgehammers hitting a metal plate on the ground and weights dropped heights of at least two meters. Accelerated weight drops (AWD) also fall in this category. AWD work by using a hydraulic system to lift a heavy steel hammer up, and a gas-charged piston forces the piston down. These have been proven as viable sources for VSP's and tool-orientation for micro-seismic surveys. [2]

Receivers

For land surveys, the receiver used is a geophone. The instrument must have a solid connection to the ground, so it is usually buried. Inside the geophone, a magnet is a attached to the sides with a coil of wire suspended inside the magnet. As reflected waves return, the body of the geophone vibrates with the ground caused by the up-going energy. The coil vibrates at a different rate than the body, so the coil moves in and out of the magnetic field which induces an electrical current. The produced electrical current is recorded and is called the seismic trace, which is a representation of the subsurface's response to the inputted energy from the source. 3C geophones are designed to record 3 components of the wavefield: the P, SH, and SV waves. Another option to use for land receivers are land streamers that are towed behind a vibroseis truck [3].

Survey Design

The design of land surveys need to account for several factors:

1) The target horizon's approximate depth needs to be known along with the regional geological structure. Steeply dipping strata can be difficult to image with seismic, and there needs to be enough offset to image the target depth.

2) Logistics: Permitting(getting permission from land owners), weather, and equipment availability will determine where and when a survey can be done.

3)Trace/Bin Spacing: The trace spacing in 2-D seismic data helps determine lateral resolution of the data. The trace spacing must be close enough to identify true reflection dip, or else spatial aliasing will occur. For 3-D data, the bin size helps determines lateral resolution. However, the Fresnel zone also must be considered, and whichever is larger will determine the lateral resolution of the data.

4) Survey size: Ideally, the area of interest should be covered by full-fold data.[4]

For further information, see the Land acquisition geometry page.

Marine Acquisition

Marine Acquisition is accomplished by using large vessels outfitted with sources and streamers that are towed behind the ship.

Sources

Airguns are the most popular source used for offshore seismic acquisition. These are metal cylinders through which high pressure air is forced through and into the water column. The injection of air into the water creates a pressure pulse that travels through the water and into the subsurface. It is common to have multiple airguns firing at once to create an array.

Sparkers are another source used for marine acquisition. These generate a pressure pulse in the form of a bubble by discharging an electrical current into the water.

Boomers are sources used for relatively shallow surveys and generate the pressure differential mechanically.

Chirp systems, like boomers, are used for shallow surveys. Chirp systems are vibratory sources. The chirp systems, sparkers, and boomers are high-frequency sources. This results in providing high resolution shallow data but lacking the energy to clearly image deeper deposits due to attenuation of their signal[5].

Receivers

Hydrophones are the typical receiver for marine seismic data, and they measure pressure. Energy in the subsurface is reflected through the water column where it creates a pressure pulse, which is measured by the hydrophone. Factors to consider when recording marine seismic are: how deep to tow the hydrophones, the length of the streamer, along with the number of hydrophone groups. Difficulties of using streamers include location monitoring and feathering. Additionally, wildlife must be considered as surveys cannot operate nearby dolphins, whales and other wildlife. Furthermore, only P waves can be recorded by hydrophones as shear waves cannot travel through fluids

Ocean bottom cables are receivers used when recording shear wave data is desired. Similarly, ocean bottom seismometers can be used. These are deployed and recovered via remote vehicles and are very costly to use.

Survey Design

For information about marine survey design and geometries, see the Wide azimuth page.

References

  1. Dean, Timothy & Tulett, J. (2014). The Relationship between the Signal-to-Noise Ratio of Downhole Data and Vibroseis Source Parameters. 10.3997/2214-4609.20140916.
  2. Botelho, Marco & Schinelli, Marco & Guerra, Rafael. (2015). Successful Application of Accelerated Weight Drop on VSP Acquisition. 10.1190/sbgf2015-019.
  3. van der Veen, M & Spitzer, Roman & Green, AG & Wild, P. (2001). Design and application of a towed land-streamer for cost-effective 2D and pseudo-3D shallow seismic data acquisition. Geophysics. 66. 10.1190/1.1444939.
  4. Chaouch, A., and J. L. Mari, 2006, 3-D Land Seismic Surveys: Definition of Geophysical Parameter: Oil & Gas Science and Technology - Revue de lIFP, v. 61, no. 5, p. 611–630, doi:10.2516/ogst:2006002.
  5. https://archive.epa.gov/esd/archive-geophysics/web/html/marine_seismic_methods.html

See also

External links

http://wiki.aapg.org/Seismic_data_acquisition_on_land https://en.wikipedia.org/wiki/Reflection_seismology http://wiki.aapg.org/Marine_seismic_data_acquisition