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Modeling Earth’s Geomagnetic Fields

Our High-Definition Geomagnetic Model tracks forces we cannot see

Multi-color map of Real-Time High Definition Geomagnetic Model field and dip disturbance
Courtesy of NOAA NCEI and CIRES

A modern electronic compass helps us understand fields not in our vision. Our High Definition Geomagnetic Model (HDGM) provides a clearer picture of Earth’s magnetic field.

What is the HDGM?

Understanding the HDGM starts with knowledge of Earth’s magnetic fields. Around Earth and through its water and crust, magnetic forces interact and change. Magnetic fields arise due to the movement of the liquid metal within Earth’s outer core. The molten metal generates electricity as it moves, and that in turn creates most of Earth’s magnetism. These fields act like the unseen forces of a magnet.

Scientists use many instruments to measure Earth’s magnetic fields. Satellite, airborne, marine, and land-based observations show the position and strength of Earth’s invisible magnetism. NOAA scientists generate the model from a combination of satellite and ground-based data, as well as airborne and marine magnetic data compiled into the Earth Magnetic Anomaly Grid (EMAG2). From these, scientists create the HDGM to show fields of magnetism and their annual fluctuations. Known for its accuracy, the HDGM is the most advanced model to track changes in the global magnetic field.

“NOAA NCEI, in its capacity as a data center for magnetic trackline data, is uniquely positioned to continually improve the resolution of the crustal magnetic field,” says Manoj Nair, the NCEI research scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder who leads the HDGM operations for NOAA.

Changes in the magnetic field typically go unnoticed by people because they occur quite slowly. However, small changes can become significant over time and affect navigational information, also called “orientation” data, used by specialized users in certain services and industries.

The HDGM provides a view of Earth’s main magnetic field and the field below the surface, providing magnetic field values (total field, dip, and declination) at any point near Earth's crust [between 100 km (~62 miles) altitude and ~10 km (~6 miles) below sea level]. Although these fields primarily arise from Earth’s hot circulating core, they also come about due to:

  • Electric currents in Earth’s ionosphere

  • Static fields, such as contributions from minerals at Earth’s surface and lithosphere

  • Space weather events, which develop beyond the magnetosphere, Earth’s protective magnetic field

    Graphic of Earth's magnetic structure
    Earth's magnetic structure. Courtesy of NOAA NCEI.

The magnetosphere is a protective bubble that buffers us from the sun’s constant stream of charged particles—or solar wind. The wind from the sun, a continuous flow of plasma comprised of mostly electrons and protons with an embedded magnetic field, interacts with Earth and other objects in the solar system. Therefore, when conditions in space change, the magnetosphere can also change. Solar events, such as solar flares and coronal mass ejections, can cause these changes.

Uses of the HDGM

The accuracy of the HDGM makes it ideal for specialized applications and navigating in environments where data are sparse or non-existent. Many types of commercial and public enterprises rely on information about geomagnetic conditions to conduct operations.

The HDGM has specific applications in resource exploration below Earth’s surface, both onshore and offshore, playing a particularly important role where GPS (global positioning systems) can’t operate. The HDGM is a critical and highly trusted magnetic reference model for the oil and gas industry. Since 2011, the oil and gas industry has used the HDGM to increase safety and efficiency of directional drilling and other resource exploration. The model helps operators better position wells, preventing and mitigating the danger of collision with existing wellbores, and enabling real-time steering to save rig-time and reduce drilling costs. Other interesting uses of HDGM include drilling for geothermal exploration, earthquake studies, and long-term storage of CO2.

“If the field changes a few degrees over the course of a year, and you are drilling a 10 kilometer-long horizontal bore hole for energy extraction, you could end up being hundreds of meters away from where you expect,” says Brian Meyer, NCEI and CIRES geophysicist, who works on the HDGM.

To more accurately determine the direction of underground drilling, companies use several sensors near the drill head to determine position. The primary tool is the magnetometer, which can give accurate pointing direction. The magnetic field strength is proportional to its proximity to the source. To drill near rock that may contain magnetic minerals, local fields can affect a compass.

“Think of bringing a strong magnet close to your compass,” Meyer says. “The closer you get, the stronger the pull, and these drills are effectively inside the magnet.”

To be able to account for how magnetic rocks may affect a driller’s position, in combination with other magnetic sources, operators need magnetic models. The smaller the error tolerance in positioning, the closer together bore holes can be drilled to extract material efficiently and safely. Because several factors can complicate resource exploration, many companies rely on NOAA to provide the subject matter expertise and data access to help them make better decisions.

A Real-Time Model

In 2015, NCEI and researchers at CIRES introduced an addition to the HDGM that takes into account temporal changes due to space weather. It is known as the HDGM-RT, which stands for “real time.” The model estimates solar-wind driven disturbances to the magnetic field from electric currents in the ionosphere and magnetosphere, in near-real time. Additionally, the modeling tracks the diurnal magnetic field variations originating from the electric currents in the upper atmosphere.

These real-time data support a growing demand for even more accurate geomagnetic referencing. Although space weather contributes to changes in the magnetic field to a lesser extent than Earth’s core, large geomagnetic storms cause significant disturbances to the ground magnetic field. HDGM-RT reduces the magnetic referencing errors by providing a real-time correction to those magnetic disturbances.

References:

Stefan Maus, Nair, M. C., Poedjono, B., Okewunmi, S., Fairhead, D., Barckhausen, U., Milligan P. R., and Matzka, J. (2012). High definition geomagnetic models: A new perspective for improved wellbore positioning. In IADC/SPE Drilling Conference and Exhibition (pp.151436-MS). IADC/SPE Drilling Conference and Exhibition. DOI: 10.2118/151436-MS (For HDGM)

Manoj Nair, Woods, A., Chulliat, A., Alken, P., and Boneh, N. A real-time magnetic disturbance model to improve drilling accuracy in low and mid latitudes of the Earth. Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA) 42nd meeting. October 1, 2015, Houston, Texas. (FOR HDGM-RT)

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