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Supplementary material to “Magnetic Anomaly Map of the World”

Michael E. Purucker, NASA Goddard Space Flight Center, Greenbelt, Maryland

Citation:
Purucker, M. E. (2007), Magnetic Anomaly Map of the World, Eos Trans. AGU, 88(25), 263, [Full Article (pdf)]

Least-squares collocation (LSC), a technique commonly used in geodesy (Moritz, 1980) was the primary method used for gridding and estimating the anomalies at 3 minutes of arc spacing. The model correlation functions were tuned to the observed correlations from the data over Australia, Russia and North America. The details of the LSC model functions and the process itself are described in Maus et al. (2007b). Other methods of interpolation and gridding, embedded in Oasis Montaj (Geosoft®) or GMT (Wessel and Smith, 1998), were also used.

Different pre-existing data compilations were merged by initially removing linear trends and then using the LSC techniques, with weights proportional to distance from the margins of the grids. Long-wavelengths (> 400 km, or spherical harmonic degrees ≤ 100) were removed from the individual compilations, and replaced with the CHAMP magnetic anomaly field downward continued to 5 km altitude (Maus et al., 2007a; Hemant et al., 2007). The marine data available from the National Geophysical Data Center (NGDC) was reduced using the Comprehensive Model (Sabaka et al., 2004). The marine data were interpolated whenever the data density was sufficiently high (Hamoudi et al., 2007).

In marine areas where no near-surface data exist, the digital age map of the oceans (Müller et al., 1997) was combined with the magnetic time scales of Gee and Kent (2007) and Kent and Gradstein (1986) in order to estimate the near-surface fields. The assumptions utilized in estimating these fields do not work well over the Cretaceous and Jurassic quiet zones, and hence the fields over these features are not shown in this map. Magnetic fields calculated from the digital age map of the oceans should only be used to indicate the general character of the magnetic field pattern in a region, and may prove unreliable indicators of actual individual magnetic field amplitudes or polarities.

Two versions (A and B) of the map are available in digital form. The B version is shown in the accompanying map. The A version differs in its handling of areas without near-surface data, which are filled with the downward-continued CHAMP magnetic field model. In contrast, the B version contains both model data derived from CHAMP, and marine ages, with a priority given to the marine age data. Both versions, when upward-continued to satellite altitude, reproduce the magnetic anomaly field derived from the CHAMP satellite.

References

• Gee, J.S. and Kent, D.V., 2007, Source of oceanic magnetic anomalies and the geomagnetic polarity time scale, Chapter 12, in Kono, M., ed., Volume 5. Geomagnetism: Treatise on Geophysics: Amsterdam, Elsevier, in press.
• Hamoudi, M., Thebault, E., Lesur, V., and Mandea, M., 2007. GeoForschungsZentrum Anomaly Magnetic Map (GAMMA): A candidate model for the World Digital Magnetic Anomaly Map, Geochem. Geophys. Geosyst., in press
• Hemant, K., Thebault, E., Mandea, M., Ravat, D., and Maus, S., 2007, Magnetic anomaly map of the world: merging airborne, marine and ground-based magnetic data sets, Earth Planet. Sci. Lett., in press
• Kent, D.V. and Gradstein, F.M., 1986, A Jurassic to Recent chronology, in P.R. Vogt and B.E.Tucholke (editors), The Western North Atlantic Region, Geology of North America Volume M (Geological Society America, Boulder), 45–50.
• Maus, S., Lühr, H., Rother, M., Hemant, K. Balasis, G. Ritter, P., and Stolle, C., 2007a. Fifth generation lithospheric magnetic field model from CHAMP satellite measurements, Geochem. Geophys. Geosyst., in press.
• Maus, S., Sazonova, T., Hemant, K., Fairhead, D. and Ravat, D, 2007b, The NGDC candidate for the World Digital Magnetic Anomaly Map, Geochem.Geophys. Geosyst., in press
• Moritz, H., 1980, Advanced Physical Geodesy, Herbert Wichmann Verlag, Karlsruhe.
• Müller, R.D., Roest, W.R., Royer, J.-Y., Gahagan, L.M., and Sclater, J.G.,1997, Digital isochrons of the world’s ocean floor, J. Geophys. Res, 102, 3211-3214.
• Sabaka, T.J., Olsen, N. and Purucker, M.E., 2004. Extending comprehensive models of the Earth's magnetic field with Oersted and CHAMP data, Geophys. J. Int., 159, 521-547, doi:10.1111/j.1365-246X.2004.02421.x
• Wessel, P., and Smith, W, 1998. New, improved version of Generic Mapping Tools released, EOS transactions of the American Geophysical Union, 79, 579.

Figure 1. Magnetic Anomaly Map of the World with Mercator and polar stereographic projections. The anomaly field is shown at an altitude of 5 kilometers above the WGS84 ellipsoid. The near-surface compilations are distinguished from the satellite-based and oceanic model data by way of shading, and their distribution can be seen in the index map included within the map. Finally, the entire data set is displayed using the natural color scale (red = high, blue = low) with a shaded relief effect using artificial illumination. The white lines on the map locate undifferentiated tectonic elements and include ridges, fracture zones, and trenches. The original map is at a scale of 1:50 million.

Major data sets, their WDMAM codes, spatial resolution, and links

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