Difference between revisions of "Magnetic methods"

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This page is currently being authored by a student at the University of Oklahoma. This page will be complete by December 1,2018.
 
This page is currently being authored by a student at the University of Oklahoma. This page will be complete by December 1,2018.
  
Magnetic surveys are a geophysical method to image anomalies in the earth’s magnetic field caused by source bodies within the sub-surface. Oil and gas exploration use magnetic anomalies to detect faults and igneous intrusions. Magnetics are commonly used with gravity ([[Gravity methods]]) as a low cost way to understand the structure of the subsurface during the beginning phases of exploration. Both gravity and magnetics are potential fields meaning that they are the spatial derivatives of their respected fields.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Gravity and magnetics are also low resolution and non -unique meaning that multiple geologic models can fit the data.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> The more that is known about the sub-surface, the more the geologic model can be narrowed down. Other uses of magnetics include detecting pipes, buried objects, and archaeological sites.[[File:Earth's magnetic feild.gif|thumb|Figure 1: Diagram of the Earth’s magnetic field showing the field lines flowing from the South (positive) to the North pole (negative). <ref>“The Earth’s Magnetic Field.” UNC.edu [http://www.unc.edu/depts/oceanweb/turtles/juvenilemap/EarthMF.html]</ref>]]
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Magnetic surveys are a geophysical method to image anomalies in the earth’s magnetic field caused by source bodies within the sub-surface. Oil and gas exploration use magnetic anomalies to detect faults and igneous intrusions. Magnetics are commonly used with gravity ([[Gravity methods]]) as a low cost way to understand the structure of the subsurface during the beginning phases of exploration. Both gravity and magnetics are potential fields meaning that they are the spatial derivatives of their respected fields.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Gravity and magnetics are also low resolution and non -unique, meaning that multiple geologic models can fit the data.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> The more that is known about the sub-surface, the more the geologic model can be narrowed down. Other uses of magnetics include detecting pipes, buried objects, and archaeological sites.[[File:Earth's magnetic feild.gif|thumb|Figure 1: Diagram of the Earth’s magnetic field showing the field lines flowing from the South (positive) to the North pole (negative). <ref>“The Earth’s Magnetic Field.” UNC.edu [http://www.unc.edu/depts/oceanweb/turtles/juvenilemap/EarthMF.html]</ref>]]
 
== Earth's magnetic field ==
 
== Earth's magnetic field ==
The Earth’s magnetic field is a dipole, meaning it has a positive and negative end.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Magnetic field lines flow from positive (south pole) to negative (north pole) as seen in figure 1. The Earth's magnetic field induces magnetism within the surface of the Earth and accounts for the majority of a rock’s magnetic field<ref name=":0" />. Other forms of magnetic fields found in rock include ferromagnetism and remnant magnetism. Ferromagnetism is caused by ferromagnetic materials like the mineral magnetite found within the rocks<ref name=":0" />. Ferromagnetic materials create their one magnetic field that may not be in line with the Earth’s. Remnant magnetism is the remaining magnetization after a magnetic field has been removed<ref name=":0" />. This records the direction of the magnetic field lines when the rock was formed preserving the inclination. Inclination is the angle in which the magnetic field lines are pointing in relation to the Earth’s surface <ref>“Magnetic Declination-Magnetic Inclination (Dip).” Geokov. [http://geokov.com/education/magnetic-declination-inclination.aspx]</ref>. At the poles the inclination is 90 while at the equator it is 0. Magnetism is the sum of all magnetic fields, both induced and remnant, acting on the media. The magnetism of a rock is described by the magnetic susceptibility (k) which is dimensionless.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Magnetic susceptibility describes ability for a rock to be magnetized and changes depending on lithology. Magnetic susceptibility is what we are trying to obtain from magnetic data.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Ferromagnetic and remnant magnetization only account for less then 1% of the total field strength measured in the magnetic susceptibility.
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The Earth’s magnetic field is a dipole, meaning it has a positive and negative end.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Magnetic field lines flow from positive (south pole) to negative (north pole) as seen in figure 1. The Earth's magnetic field induces magnetism within the surface of the Earth and accounts for the majority of a rock’s magnetic field.<ref name=":0" /> Other forms of magnetic fields found in rock include ferromagnetism and remnant magnetism. Ferromagnetism is caused by ferromagnetic materials like the mineral magnetite found within the rocks.<ref name=":0" /> Ferromagnetic materials create their own magnetic field that may not be in line with the Earth’s. Remnant magnetism is the remaining magnetization after a magnetic field has been removed.<ref name=":0" /> This records the direction of the magnetic field lines when the rock was formed preserving the inclination. Inclination is the angle in which the magnetic field lines are pointing in relation to the Earth’s surface .<ref>“Magnetic Declination-Magnetic Inclination (Dip).” Geokov. [http://geokov.com/education/magnetic-declination-inclination.aspx]</ref> At the poles, the inclination is 90 while at the equator it is 0. Magnetism is the sum of all magnetic fields, both induced and remnant, acting on the media. The magnetism of a rock is described by the magnetic susceptibility (k) which is dimensionless.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Magnetic susceptibility describes the ability for a rock to be magnetized and changes depending on lithology. Magnetic susceptibility is what we are trying to obtain from magnetic data.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Ferromagnetic and remnant magnetization only account for less than 1% of the total field strength measured in the magnetic susceptibility.
  
 
== Acquisition ==
 
== Acquisition ==
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[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Absolute magnetometers measure the total value of the earth’s magnetic field at a fixed point while relative magnetometers measure the change in the magnetic field at a location compared to a base station. Absolute magnetometers are used less often because they are expensive and have long reading times for each measurement.<ref name=":1">Nicolas Mariita (2009). “The magnetic method.”  
 
[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Absolute magnetometers measure the total value of the earth’s magnetic field at a fixed point while relative magnetometers measure the change in the magnetic field at a location compared to a base station. Absolute magnetometers are used less often because they are expensive and have long reading times for each measurement.<ref name=":1">Nicolas Mariita (2009). “The magnetic method.”  
 
[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Acquisition can take place on land, airborne, marine, and satellite. Land has the best resolution but is usually used for small study areas due to time constraints. Airborne and marine acquisition is most common in oil and gas exploration with the flight line spacing controlling and the flight altitude of the plane controlling the resolution of the data.<ref name=":1">Nicolas Mariita (2009). “The magnetic method.”  
 
[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Acquisition can take place on land, airborne, marine, and satellite. Land has the best resolution but is usually used for small study areas due to time constraints. Airborne and marine acquisition is most common in oil and gas exploration with the flight line spacing controlling and the flight altitude of the plane controlling the resolution of the data.<ref name=":1">Nicolas Mariita (2009). “The magnetic method.”  
[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Satellites have global coverage but lower resolution. In all surveys the line spacing controls the resolution of the survey.
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[https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-05-18b.pdf]</ref> Satellites have global coverage but lower resolution. In all surveys, the line spacing controls the resolution of the survey.
 
=== External variation corrections ===
 
=== External variation corrections ===
Magnetic measurements do not need to be corrected for drift like gravity measurements but they still need to be corrected for external variations.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> The sun can have large effect on magnetic surveys due to it’s own magnetic field. If there is strong solar activity happening then acquisition must be stopped as solar storms can create outliers in the data, which need to be corrected.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Daily changes with in the sun also need to be monitored to know the effect they are having on the acquisition. This is solved by having a base station at a fixed location that is measuring the entire time of the acquisition to see any changes within the sun. These changes are compared to the quiet nighttime value to determine the correction needed for each measurement. Quiet nighttime values are measurements taken from the same base station at night when there is no influence from the sun.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref>[[File:Magnetic anomaly.jpg|thumb|Figure 3: Guo et al. 2015 shows the magnetic anomaly that is the result of a buried pipe.<ref>Zhi-Yong Guo et al. (2014). “Forward modeling of total magnetic anomaly over a pseudo-2D underground ferromagnetic pipeline.” Journal of Applied Geophysics. 113, 14-30. [https://www.sciencedirect.com/science/article/pii/S0926985114003644]</ref>]]
+
Magnetic measurements do not need to be corrected for drift like gravity measurements but they still need to be corrected for external variations.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> The sun can have a large effect on magnetic surveys due to its own magnetic field. If there is strong solar activity happening then acquisition must be stopped as solar storms can create outliers in the data, which need to be corrected.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> Daily changes within the sun also need to be monitored to know the effect they are having on the acquisition. This is solved by having a base station at a fixed location that is measuring the entire time of the acquisition to see any changes within the sun. These changes are compared to the quiet nighttime value to determine the correction needed for each measurement. Quiet nighttime values are measurements taken from the same base station at night when there is no influence from the sun.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref>[[File:Magnetic anomaly.jpg|thumb|Figure 3: Guo et al. 2015 shows the magnetic anomaly that is the result of a buried pipe.<ref>Zhi-Yong Guo et al. (2014). “Forward modeling of total magnetic anomaly over a pseudo-2D underground ferromagnetic pipeline.” Journal of Applied Geophysics. 113, 14-30. [https://www.sciencedirect.com/science/article/pii/S0926985114003644]</ref>]]
 
=== Reduction-to-pole corrections ===
 
=== Reduction-to-pole corrections ===
 
Magnetic measurements are expressed in total magnetic intensity but need to be corrected for latitude.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> At different latitudes, the earth has different inclinations due to the magnetic field line. This means that an anomaly at the poles will image directly above the source body but if the source body is at lower latitude then it will be shifted.<ref name=":2">Shanti Rajagopalan (2003). ”Analytic signal vs. reduction to pole: solutions for low magnetic latitudes.” Exploration Geophysics, 34(4), 257-262. [https://library.seg.org/doi/abs/10.1071/EG03257?journalCode=exgeef&]</ref> A reduction-to-pole (RTP) correction shifts anomalies so that they are directly above the source bodies like seen in the poles as seen in figure 2. RTP corrections can be used for location of source bodies but the amplitude of the anomaly is unreliable.[[File:Archeological profiles.jpg|thumb|Figure 4:  The magnetic anomalies associated with various archaeological sites.<ref>“Archeological Investigations using High Sensitivity Magnetometers.”GEM systems. [http://www.gemsys.ca/archaeological-applications/discovering-mankinds-past-secrets/]</ref>]]
 
Magnetic measurements are expressed in total magnetic intensity but need to be corrected for latitude.<ref name=":0">Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”</ref> At different latitudes, the earth has different inclinations due to the magnetic field line. This means that an anomaly at the poles will image directly above the source body but if the source body is at lower latitude then it will be shifted.<ref name=":2">Shanti Rajagopalan (2003). ”Analytic signal vs. reduction to pole: solutions for low magnetic latitudes.” Exploration Geophysics, 34(4), 257-262. [https://library.seg.org/doi/abs/10.1071/EG03257?journalCode=exgeef&]</ref> A reduction-to-pole (RTP) correction shifts anomalies so that they are directly above the source bodies like seen in the poles as seen in figure 2. RTP corrections can be used for location of source bodies but the amplitude of the anomaly is unreliable.[[File:Archeological profiles.jpg|thumb|Figure 4:  The magnetic anomalies associated with various archaeological sites.<ref>“Archeological Investigations using High Sensitivity Magnetometers.”GEM systems. [http://www.gemsys.ca/archaeological-applications/discovering-mankinds-past-secrets/]</ref>]]

Revision as of 10:49, 30 November 2018

This page is currently being authored by a student at the University of Oklahoma. This page will be complete by December 1,2018.

Magnetic surveys are a geophysical method to image anomalies in the earth’s magnetic field caused by source bodies within the sub-surface. Oil and gas exploration use magnetic anomalies to detect faults and igneous intrusions. Magnetics are commonly used with gravity (Gravity methods) as a low cost way to understand the structure of the subsurface during the beginning phases of exploration. Both gravity and magnetics are potential fields meaning that they are the spatial derivatives of their respected fields.[1] Gravity and magnetics are also low resolution and non -unique, meaning that multiple geologic models can fit the data.[1] The more that is known about the sub-surface, the more the geologic model can be narrowed down. Other uses of magnetics include detecting pipes, buried objects, and archaeological sites.

Figure 1: Diagram of the Earth’s magnetic field showing the field lines flowing from the South (positive) to the North pole (negative). [2]

Earth's magnetic field

The Earth’s magnetic field is a dipole, meaning it has a positive and negative end.[1] Magnetic field lines flow from positive (south pole) to negative (north pole) as seen in figure 1. The Earth's magnetic field induces magnetism within the surface of the Earth and accounts for the majority of a rock’s magnetic field.[1] Other forms of magnetic fields found in rock include ferromagnetism and remnant magnetism. Ferromagnetism is caused by ferromagnetic materials like the mineral magnetite found within the rocks.[1] Ferromagnetic materials create their own magnetic field that may not be in line with the Earth’s. Remnant magnetism is the remaining magnetization after a magnetic field has been removed.[1] This records the direction of the magnetic field lines when the rock was formed preserving the inclination. Inclination is the angle in which the magnetic field lines are pointing in relation to the Earth’s surface .[3] At the poles, the inclination is 90 while at the equator it is 0. Magnetism is the sum of all magnetic fields, both induced and remnant, acting on the media. The magnetism of a rock is described by the magnetic susceptibility (k) which is dimensionless.[1] Magnetic susceptibility describes the ability for a rock to be magnetized and changes depending on lithology. Magnetic susceptibility is what we are trying to obtain from magnetic data.[1] Ferromagnetic and remnant magnetization only account for less than 1% of the total field strength measured in the magnetic susceptibility.

Acquisition

Figure 2: Rajagopalan (2003) shows how RTP corrections shift the anomaly to directly over the source body.[4]

Magnetic data can be acquired by using both absolute and relative magnetometers.[5] Absolute magnetometers measure the total value of the earth’s magnetic field at a fixed point while relative magnetometers measure the change in the magnetic field at a location compared to a base station. Absolute magnetometers are used less often because they are expensive and have long reading times for each measurement.[5] Acquisition can take place on land, airborne, marine, and satellite. Land has the best resolution but is usually used for small study areas due to time constraints. Airborne and marine acquisition is most common in oil and gas exploration with the flight line spacing controlling and the flight altitude of the plane controlling the resolution of the data.[5] Satellites have global coverage but lower resolution. In all surveys, the line spacing controls the resolution of the survey.

External variation corrections

Magnetic measurements do not need to be corrected for drift like gravity measurements but they still need to be corrected for external variations.[1] The sun can have a large effect on magnetic surveys due to its own magnetic field. If there is strong solar activity happening then acquisition must be stopped as solar storms can create outliers in the data, which need to be corrected.[1] Daily changes within the sun also need to be monitored to know the effect they are having on the acquisition. This is solved by having a base station at a fixed location that is measuring the entire time of the acquisition to see any changes within the sun. These changes are compared to the quiet nighttime value to determine the correction needed for each measurement. Quiet nighttime values are measurements taken from the same base station at night when there is no influence from the sun.[1]

Figure 3: Guo et al. 2015 shows the magnetic anomaly that is the result of a buried pipe.[6]

Reduction-to-pole corrections

Magnetic measurements are expressed in total magnetic intensity but need to be corrected for latitude.[1] At different latitudes, the earth has different inclinations due to the magnetic field line. This means that an anomaly at the poles will image directly above the source body but if the source body is at lower latitude then it will be shifted.[4] A reduction-to-pole (RTP) correction shifts anomalies so that they are directly above the source bodies like seen in the poles as seen in figure 2. RTP corrections can be used for location of source bodies but the amplitude of the anomaly is unreliable.

Figure 4:  The magnetic anomalies associated with various archaeological sites.[7]

Anomalies

Magnetic anomalies are the result of small deviations within the Earth’s magnetic field.[8] Anomalies that are closer to the surface appear clearer with strong amplitude but the deeper the source body is the lower resolution of the anomaly. Low resolution anomalies make determining the size of the source body difficult. The anomalies seen are also representative of the entire subsurface so wavelength filtering can be used to narrow down interpretations to shallow or deep structures.[8] Lower wavelengths are used for deeper or broader structures.

Figure 5: A depression seen in a seismic cross section correlates to a positive magnetic anomaly.[8]

Magnetic surveys can be used in a wide variety of industries. Figure 3 shows what the anomaly caused by a buried metal pipe in the ground. Figure 4 shows the anomalies caused by different archaeological sites. The deeper the artifacts are buried the harder it is to resolve them until they become undetectable. It should also be noted that figure 3 and 4 show similar positive anomalies for the pipe and artifact which relates to the non-unique problem of magnetic data and further information about the sub-surface is needed to tell what is the source of the anomaly. Figure 5 shows that a depression seen on a seismic line correlates to a positive magnetic anomaly. The positive magnetic anomaly could represent a possible trap in hydrocarbon exploration.


External links

AAPG magnetics

Historical development of the magnetic method

Gravity and magnetic geophysical methods oil exploration

Magnetics for hydrocarbon exploration

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 Richard Blakely (1996). “Potential Theory in Gravity & Magnetic Applications.”
  2. “The Earth’s Magnetic Field.” UNC.edu [1]
  3. “Magnetic Declination-Magnetic Inclination (Dip).” Geokov. [2]
  4. 4.0 4.1 Shanti Rajagopalan (2003). ”Analytic signal vs. reduction to pole: solutions for low magnetic latitudes.” Exploration Geophysics, 34(4), 257-262. [3]
  5. 5.0 5.1 5.2 Nicolas Mariita (2009). “The magnetic method.” [4]
  6. Zhi-Yong Guo et al. (2014). “Forward modeling of total magnetic anomaly over a pseudo-2D underground ferromagnetic pipeline.” Journal of Applied Geophysics. 113, 14-30. [5]
  7. “Archeological Investigations using High Sensitivity Magnetometers.”GEM systems. [6]
  8. 8.0 8.1 8.2 Christine Fichler (2017). “Magnetics for Hydrocarbon Exploration.” GEOExPro, 13(6). [7]