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Magnetic surveys are a geophysical method to image anomalies in the earth’s magnetic field 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 (Blakely, 1996). Gravity and magnetics are also low resolution and non -unique meaning that multiple geologic models can fit the data (Blakely, 1996). 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.
Earth's magnetic field
The Earth’s magnetic field is a dipole, meaning it has a positive and negative end. Magnetic field lines flow from positive to negative as seen in figure 1. Magnetism is the sum of all magnetic fields acting on the media. The Earth’s magnetic field induces magnetization within the Earth’s subsurface and is described by the magnetic susceptibility (k). Magnetic susceptibility is what we are trying to obtain from magnetic data. Magnetism can also be affected by ferromagnetic materials that include minerals like magnetite. These minerals create their own magnetic field that can either add or subtract to the induced field and create the anomalies that are the targets of oil and gas magnetic surveys. Remnant magnetism which is the remaining magnetization after the magnetic field has been removed also adds to the magnetism of the sub-surface. 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 . At the poles the inclination is 90 while at the equator it is 0. Ferromagnetic and remnant magnetization only account for less then 1% of the total field strength measured in the magnetic susceptibility.
Magnetic data can be acquired by using both absolute and relative magnetometers. 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. Acquisition can take place on land, airborne, marine, and satellite. Land has the best resolution but due 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 the resolution of the data. Satellites have global coverage but lower resolution. In all surveys the lie 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. 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 and solar storms can create outliers in the data, which need to be corrected. Daily changes with in the sun also need to be monitored to know the effect they are having on the acquisition. This is solved my 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.
Magnetic measurements are expressed in total magnetic intensity but need to be corrected for latitude. 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. 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 1. RTP corrections can be used for location of source bodies but the amplitude of the anomaly is unreliable.
Magnetic anomalies are the result of small deviations within the Earth’s magnetic field . Anomalies that are closer to the surface appear clearer with strong amplitude but the deeper they are the lower resolution they get. 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 . 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, further information about the sub-surface is needed to tell what is causing 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.
- Shanti Rajagopalan (2003). "Analytic signal vs. reduction to pole: solutions for low magnetic latitudes." Exploration Geophysics, 34(4), 257-262.