Difference between revisions of "Sonic logs"

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== Overview ==
 
== Overview ==
A sonic log produces data which illustrates P-wave travel time versus depth <ref name="Sonic Log." /> and is recorded as microseconds per foot (ms/ft). This data provides information about how fast acoustic waves travel through rock.Wave propagation which produces the P-waves in sonic logs follow properties according to [[Snell's law|Snell’s Law]] <ref name="Crain's">“Crain's Petrophysical Handbook: Sonic Travel Time (Slowness) Logs.” Crain's Petrophysical Handbook | Sonic Travel Time (Slowness) Logs, www.spec2000.net/07-soniclog.htm.</ref> and demonstrates how waves travel through different interfaces or rock layers in the subsurface.<gallery widths="250" heights="250">
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A sonic log produces data which illustrates P-wave travel time versus depth <ref name="Sonic Log." /> and is recorded as microseconds per foot (ms/ft). This data provides information about how fast acoustic waves travel through rock. Wave propagation which produces the P-waves in sonic logs follow properties according to [[Snell's law|Snell’s Law]] <ref name="Crain's">“Crain's Petrophysical Handbook: Sonic Travel Time (Slowness) Logs.” Crain's Petrophysical Handbook | Sonic Travel Time (Slowness) Logs, www.spec2000.net/07-soniclog.htm.</ref> (Figure 1<ref name=":0">“Snell's Law.” ''Wikipedia'', Wikimedia Foundation, 21 Nov. 2019, en.wikipedia.org/wiki/Snell%27s_law.</ref> and 2<ref name=":0" />) and demonstrates how waves travel through different interfaces or rock layers in the subsurface.
File:Snells Law.png|Figure 1: Snell's Law
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[[File:Snells law formula.png|left|thumb|Figure 1: Snell's Law]]
File:Surface refraction.png|Figure 2: Snell's Law applied to surface interfaces
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</gallery>
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Waves will propagate until [[Dictionary:Attenuation|attenuation]], which can be a result of several situations. Some degree of absorption will affect waves, turning the mechanical energy into heat. Waves can also be attenuated by coming in contact with fracture or bedding planes and are internally reflected <ref name="Crain's" />. Another form of attenuation occurs when a foreign substance, usually gas, enters the mud column and decreases the sonic signal<ref>Burch, Don. “Sonic Logs Need Troubleshooting.” ''AAPG Explorer'', AAPG Explorer, Mar. 2002, explorer.aapg.org/story/articleid/46752/sonic-logs-need-troubleshooting.</ref>. This type of attenuation, referred to as cycle skipping, can produce low quality logs (Figure 3<ref name="Crain's" />). All of these occurrences must be accommodated for when relating to seismic. Another characteristic to accommodate and correct for when analyzing sonic log data is tool stretch. As the wireline is lowered into the borehole, the weight of the line will cause some stretch that grows larger with increasing depth. This must be accommodated for especially when comparing sonic log data to core data.
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[[File:Interface reflection.png|left|thumb|343x343px|Figure 2: Snell's Law at interface]]
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[[File:Attenuation of sonic logs.png|thumb|Figure 3: Cycle Skipping]]
  
Waves will propagate until [[Dictionary:Attenuation|attenuation]], which can be a result of several situations. Some degree of absorption will affect waves, turning the mechanical energy into heat. Waves can also be attenuated by coming in contact with fracture of bedding planes and are internally reflected <ref name="Crain's" />. Another form of attenuation occurs when a foreign substance, usually gas, enters the mud column and decreases the sonic signal. This type of attenuation, referred to as cycle skipping, can produce low quality logs. All of these occurrences must be accommodated for when relating to seismic. Another characteristic to accommodate and correct for when analyzing sonic log data is tool stretch. As the wireline is lowered into the borehole, the weight of the line will cause some stretch that increases with increasing depth. This must be accommodated for especially when comparing sonic log data to core data.<gallery widths="400" heights="400">
 
File:Screen Shot 2019-10-14 at 11.36.24 AM.png|Figure 3: Cycle Skipping<ref name="Crain's/>
 
</gallery>
 
 
== Dipole Shear Sonic Logs ==
 
== Dipole Shear Sonic Logs ==
The most modern type of sonic logs are the dipole shear sonic logs. These logs measure values for [[Compressional wave|compressional]], [[Dictionary:S-wave|shear]], and [[Dictionary:Stoneley wave|Stoneley]] slowness through both monopole and dipole sources. A monopole source emits radially while a dipole energy source emits energy in one direction. When determining whether to use a monopole or a dipole measurement, the type of formation that the acoustic waves will be traveling through is most important<ref name="Crain's" />.  
+
The most modern type of sonic logs are the dipole shear sonic logs. These logs measure values for [[Compressional wave|compressional]], [[Dictionary:S-wave|shear]], and [[Dictionary:Stoneley wave|Stoneley]] slowness through both monopole and dipole sources. A monopole source emits radially while a dipole energy source emits energy in one direction. When determining whether to use a monopole or a dipole measurement, the type of formation that the acoustic waves will be traveling through is most important<ref name="Crain's" />. Figure 4<ref name="Crain's" /> demonstrates the different monopole and dipole systems.
 
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[[File:Monopole-dipole.png|thumb|Figure 4: Monopole versus Dipole system]]
<nowiki>***</nowiki>IMAGE HERE of how dipole shear sonic logs work
 
  
 
=== Slow Formations ===
 
=== Slow Formations ===
A slow formation refers to a formation in which the compressional wave velocity measured in the borehole fluid exceeds encompassing shear wave velocities <ref name="Sonic Log." />. This monopole measurement results in compressional and Stoneley arrivals but no detected shear waves. Therefore, a dipole measurement is better suited for a slow formation because it can generate flexural or bender waves which generate measureable shear waves. An example of a slow formation is a high porosity gas sand layer.
+
A slow formation refers to a formation in which the compressional wave velocity measured in the borehole fluid exceeds encompassing shear wave velocities <ref name="Sonic Log." />. This monopole measurement results in compressional and Stoneley arrivals but no detected shear waves. Therefore, a dipole measurement is better suited for a slow formation because it can generate flexural or bender waves which produce measurable shear waves. An example of a slow formation is a high porosity gas sand layer.
  
 
=== Fast Formations ===
 
=== Fast Formations ===
A fast formation has higher shear wave velocities than compressional wave velocities, and shear waves as well as compressional and Stoneley waves can all be observed with a monopole measurement. An example of a fast formation would a low porosity carbonate layer<ref name="Crain's" />. Although both monopole and dipole measurements have advantages for different types of rock layers, modern sonic logs contain both types of measurements as to most accurately measure the acoustic properties of the subsurface<ref>“Cross-Dipole Acoustic Log.” Cross-Dipole Acoustic Log | Open Energy Information, openei.org/wiki/Cross-Dipole_Acoustic_Log.</ref>.
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A fast formation has higher shear wave velocities than compressional wave velocities. Shear waves as well as compressional and Stoneley waves can all be observed with a monopole measurement. An example of a fast formation would be a low porosity carbonate layer<ref name="Crain's" />. Although both monopole and dipole measurements have advantages for different types of rock layers, modern sonic logs contain both types of measurements as to most accurately measure the acoustic properties of the subsurface<ref>“Cross-Dipole Acoustic Log.” Cross-Dipole Acoustic Log | Open Energy Information, openei.org/wiki/Cross-Dipole_Acoustic_Log.</ref>.
  
 
== Single Receiver Sonic Logs ==
 
== Single Receiver Sonic Logs ==
Single receiver sonic logs were the first type of sonic logs which contained only one transmitter and one receiver. These logs are no longer used unless additional data analysis methods are present; however, old data still remains in less updated files<ref name="Crain's" />.
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[[File:Timeline of sonic logs.png|thumb|Figure 5: Timeline of Sonic Logs]]
 
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Single receiver sonic logs were the first type of sonic logs and contained only one transmitter and one receiver. These logs are no longer used unless additional data analysis methods are present; however, old data still remains in less updated files<ref name="Crain's" />. Figure 5<ref name="Crain's" /> illustrates a time line of previous sonic logs that have since been outdated.
<nowiki>***</nowiki>INSERT image here of single receiver sonic logs
 
  
 
== Two Receiver Sonic Logs ==
 
== Two Receiver Sonic Logs ==
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== Borehole Compensated Sonic Logs ==
 
== Borehole Compensated Sonic Logs ==
The borehole compensated sonic logs <ref name="Borehole">“Borehole Compensated Sonic Tool (BHC*).” Division of Marine and Large Programs, mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/.</ref> has two sets of transmitters and receivers, which measure compressional waves and average the travel time <ref name="Borehole" />. This type of log automatically compensates for variations in the borehole diameter or size and sonde tilt, as well as, increases resolution from 2 feet to 9 inches.
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Borehole compensated sonic logs <ref name="Borehole">“Borehole Compensated Sonic Tool (BHC*).” Division of Marine and Large Programs, mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/.</ref> have two sets of transmitters and receivers, which measure compressional waves and average the travel time <ref name="Borehole" />. This type of log automatically compensates for variations in the borehole diameter or size and sonde tilt, as well as, increases resolution from 2 feet to 9 inches<ref name="Crain's" />.
  
 
== Array Sonic Logs ==
 
== Array Sonic Logs ==
Array sonic logs were developed to provide information about a variety of acoustic properties such as porosity, permeability, gas zones, acoustic impedance, and elastic properties of a formation. This type of sonic log can have both a monopole or dipole source. Similar to the dipole shear sonic log, the array sonic log with a monopole source measures compressional, Stoneley, and wave velocities which are used to estimated shear wave velocities. The monopole source is not able to measure shear wave velocities in slow formations. In fast formations, the monopole source is able to measure compressional, shear and Stoneley waves. The dipole source produces a more consistent and accurate shear wave velocity for both slow and fast formations<ref name="Crain's" />.
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[[File:Sonic to seismic.png|thumb|Figure 6: Sonic Logs and Seismic Ties]]
 +
Array sonic logs were developed to provide information about a variety of acoustic properties such as porosity, permeability, gas zones, acoustic impedance, and elastic properties of a formation. This type of sonic log can have both a monopole or dipole source. Similar to the dipole shear sonic log, the array sonic log with a monopole source measures compressional, Stoneley, and wave velocities which are used to estimated shear wave velocities. The monopole source is not able to measure shear wave velocities in slow formations. In fast formations, the monopole source is able to measure compressional, shear, and Stoneley waves. The dipole source produces a more consistent and accurate shear wave velocity for both slow and fast formations<ref name="Crain's" />.
  
 
== Sonic Logs and Seismic Ties ==
 
== Sonic Logs and Seismic Ties ==
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# Both data sets must be converted to the same units. Seismic is collected in time (ms) whereas sonic logs are collected in depth (m of ft.). Using check shot data, either the seismic or sonic logs can be converted.
 
# Both data sets must be converted to the same units. Seismic is collected in time (ms) whereas sonic logs are collected in depth (m of ft.). Using check shot data, either the seismic or sonic logs can be converted.
 
# Once the units are converted, a wavelet extraction can take place in order to produce the synthetic seismogram.  
 
# Once the units are converted, a wavelet extraction can take place in order to produce the synthetic seismogram.  
In the seismic, reflections are indicative of changes in acoustic impedance which can indicate a change in lithology. Figure demonstrates the steps and information which can be obtained from the sonic log in order to produce the synthetic seismogram.  
+
In the seismic, reflections are indicative of changes in acoustic impedance which can indicate a change in lithology. Figure 6<ref name="Crain's" /> demonstrates the steps and information which can be obtained from the sonic log in order to produce the synthetic seismogram.
 
 
<nowiki>***</nowiki>Picture will be inserted here depicting sonic logs to wavelet to synthetic seismic to seismic
 
  
 
== External Links ==
 
== External Links ==

Latest revision as of 19:12, 4 December 2019

A sonic log is an acoustic log that emits sound waves which start at the source, travel through the formation, and return back to the receiver [1]. The travel time from the source to the receiver is called slowness and as a result sonic logs are sometimes referred to as sonic slowness logs.

Overview

A sonic log produces data which illustrates P-wave travel time versus depth [1] and is recorded as microseconds per foot (ms/ft). This data provides information about how fast acoustic waves travel through rock. Wave propagation which produces the P-waves in sonic logs follow properties according to Snell’s Law [2] (Figure 1[3] and 2[3]) and demonstrates how waves travel through different interfaces or rock layers in the subsurface.

Figure 1: Snell's Law

Waves will propagate until attenuation, which can be a result of several situations. Some degree of absorption will affect waves, turning the mechanical energy into heat. Waves can also be attenuated by coming in contact with fracture or bedding planes and are internally reflected [2]. Another form of attenuation occurs when a foreign substance, usually gas, enters the mud column and decreases the sonic signal[4]. This type of attenuation, referred to as cycle skipping, can produce low quality logs (Figure 3[2]). All of these occurrences must be accommodated for when relating to seismic. Another characteristic to accommodate and correct for when analyzing sonic log data is tool stretch. As the wireline is lowered into the borehole, the weight of the line will cause some stretch that grows larger with increasing depth. This must be accommodated for especially when comparing sonic log data to core data.

Figure 2: Snell's Law at interface
Figure 3: Cycle Skipping

Dipole Shear Sonic Logs

The most modern type of sonic logs are the dipole shear sonic logs. These logs measure values for compressional, shear, and Stoneley slowness through both monopole and dipole sources. A monopole source emits radially while a dipole energy source emits energy in one direction. When determining whether to use a monopole or a dipole measurement, the type of formation that the acoustic waves will be traveling through is most important[2]. Figure 4[2] demonstrates the different monopole and dipole systems.

Figure 4: Monopole versus Dipole system

Slow Formations

A slow formation refers to a formation in which the compressional wave velocity measured in the borehole fluid exceeds encompassing shear wave velocities [1]. This monopole measurement results in compressional and Stoneley arrivals but no detected shear waves. Therefore, a dipole measurement is better suited for a slow formation because it can generate flexural or bender waves which produce measurable shear waves. An example of a slow formation is a high porosity gas sand layer.

Fast Formations

A fast formation has higher shear wave velocities than compressional wave velocities. Shear waves as well as compressional and Stoneley waves can all be observed with a monopole measurement. An example of a fast formation would be a low porosity carbonate layer[2]. Although both monopole and dipole measurements have advantages for different types of rock layers, modern sonic logs contain both types of measurements as to most accurately measure the acoustic properties of the subsurface[5].

Single Receiver Sonic Logs

Figure 5: Timeline of Sonic Logs

Single receiver sonic logs were the first type of sonic logs and contained only one transmitter and one receiver. These logs are no longer used unless additional data analysis methods are present; however, old data still remains in less updated files[2]. Figure 5[2] illustrates a time line of previous sonic logs that have since been outdated.

Two Receiver Sonic Logs

Soon after single receiver sonic logs, two receiver sonic logs became more commonly used in order to compensate for borehole effects[2]. Two receiver sonic logs are still in use but require a significant amount of editing before an evaluation can be made from the logs.

Borehole Compensated Sonic Logs

Borehole compensated sonic logs [6] have two sets of transmitters and receivers, which measure compressional waves and average the travel time [6]. This type of log automatically compensates for variations in the borehole diameter or size and sonde tilt, as well as, increases resolution from 2 feet to 9 inches[2].

Array Sonic Logs

Figure 6: Sonic Logs and Seismic Ties

Array sonic logs were developed to provide information about a variety of acoustic properties such as porosity, permeability, gas zones, acoustic impedance, and elastic properties of a formation. This type of sonic log can have both a monopole or dipole source. Similar to the dipole shear sonic log, the array sonic log with a monopole source measures compressional, Stoneley, and wave velocities which are used to estimated shear wave velocities. The monopole source is not able to measure shear wave velocities in slow formations. In fast formations, the monopole source is able to measure compressional, shear, and Stoneley waves. The dipole source produces a more consistent and accurate shear wave velocity for both slow and fast formations[2].

Sonic Logs and Seismic Ties

Once seismic data is processed and the sonic log data is acquired, they can be correlated in order to obtain a better understanding of the subsurface and particularly the lithology. This can be completed by producing a synthetic seismogram in order to accurately tie seismic to well data. Several steps must be taken in order to ensure that both sets of data are being accurately compared.

  1. It is important to start with sonic log data that has gone through a quality control process.
  2. Both data sets must be converted to the same units. Seismic is collected in time (ms) whereas sonic logs are collected in depth (m of ft.). Using check shot data, either the seismic or sonic logs can be converted.
  3. Once the units are converted, a wavelet extraction can take place in order to produce the synthetic seismogram.

In the seismic, reflections are indicative of changes in acoustic impedance which can indicate a change in lithology. Figure 6[2] demonstrates the steps and information which can be obtained from the sonic log in order to produce the synthetic seismogram.

External Links

Crain's Petrophyscial Handbook - https://www.spec2000.net/07-soniclog.htm

Schlumberger Oilfield Glossary - https://www.glossary.oilfield.slb.com/Terms/s/sonic_log.aspx

Division of Marine and Large Programs - http://mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/

References

  1. 1.0 1.1 1.2 “Sonic Log.” Sonic Log - Schlumberger Oilfield Glossary, www.glossary.oilfield.slb.com/Terms/s/sonic_log.aspx.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 “Crain's Petrophysical Handbook: Sonic Travel Time (Slowness) Logs.” Crain's Petrophysical Handbook | Sonic Travel Time (Slowness) Logs, www.spec2000.net/07-soniclog.htm.
  3. 3.0 3.1 “Snell's Law.” Wikipedia, Wikimedia Foundation, 21 Nov. 2019, en.wikipedia.org/wiki/Snell%27s_law.
  4. Burch, Don. “Sonic Logs Need Troubleshooting.” AAPG Explorer, AAPG Explorer, Mar. 2002, explorer.aapg.org/story/articleid/46752/sonic-logs-need-troubleshooting.
  5. “Cross-Dipole Acoustic Log.” Cross-Dipole Acoustic Log | Open Energy Information, openei.org/wiki/Cross-Dipole_Acoustic_Log.
  6. 6.0 6.1 “Borehole Compensated Sonic Tool (BHC*).” Division of Marine and Large Programs, mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/.