Difference between revisions of "Magnetic methods"

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 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 magnetic pole.[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.[1]

Acquisition

Magnetic data can be acquired by using both absolute and relative magnetometers.[4] 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 read times per measurement.[4] 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.[4] 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.[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 to measure 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]

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.[5] A reduction-to-pole (RTP) correction shifts anomalies so that they are directly above the source bodies like if the measurements took place at a pole as seen in figure 2. RTP corrections can be used for the correct location of source bodies but the amplitude of the anomaly is unreliable.

Figure 2: Ravat (2007) image shoes the effects of using the reduction-to-pole correction. On the left shows the magnetic anomaly imaged in the mid-latitudes, the anomaly appears to be off centered from the source from the latitude. The right shows the anomaly directly above the source after the reduced-to-pole correction is applied.[5]

Anomalies

Figure 4:  The magnetic anomalies associated with various archaeological sites.[6]

Magnetic anomalies are the result of small deviations within the Earth’s magnetic field.[7] 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.[7] Lower wavelengths are used for deeper or broader structures.

Figure 3: Guo et al. 2015 shows the magnetic anomaly that is the result of a buried pipe.[8]

Magnetic surveys and interpretation 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.[7]

Figure 5: A depression seen in a seismic cross section correlates to a positive magnetic anomaly.[7]
Figure 6: Example and data from Geophysics for practicing geoscientists. Magnetic data in map view on the right showing a linear magnetic anomaly that is inferred to be an intrusive dyke. A cross like taken across the anomaly shows a large positive magnetic response. The left image is a modeled response from an intrusive dyke showing a positive magnetic anomaly.[9]

An example on how to interpret magnetic data can be seen in figure 6.[9] The map view image on the upper right on figure 6 shows the magnetic anomalies of a section of the sub-surface.[9] The linear magnetic anomaly in the image is inferred as an intrusive dyke. [9] To test this theory, the modeled magnetic response of an intrusive dyke is on the left side of the image.[9] This shows a similar response to the acquired data.[9] Model magnetic data is often used to determine what is causing the anomalies seen in the acquired magnetic data.

More advance magnetic interpretation can be employed to extract as much geologic information from the magnetic anomalies. One way is using the half-width method which allows for the determination of the depth of the anomaly. [10] Another technique is pseudo-gravity transformation. Magnetic anomalies are a combination of the response of all magnetic bodies within the sub-surface and the amplitude is controlled mainly by the sources in the near surface. Pseudo-gravity transformations can be used to simplify the anomaly response to different depth ranges.[11] Inversions of magnetic data can also be calculated to get the magnetic susceptibility of the sub-surface.[12]