Magnetotellurics

Surveys are made of the magnetic and electric pulsation fields whose penetration into the conducting Earth depends on the frequency of the source fields and the conducting properties of the interior region. Geophysicists use these surveys to delineate the resistivity of the Earth's crust to depths of about 100 km (63 miles). Using special mathematical modeling of the measure-

Longitude

FIGURE 5.6 ► Top figure shows the surface field obtained from an Earth-centered, tilted dipole. The bottom figure shows the surface field obtained from the eccentric axis dipole. Both were computed from the 1995 IGRF model. The total field contours are in gammas (nanoteslas). Note the better representation of the South Atlantic-South American anomaly by the eccentric dipole. Figure from J. R. Heirtzler of GSFS/NASA.

Longitude

FIGURE 5.6 ► Top figure shows the surface field obtained from an Earth-centered, tilted dipole. The bottom figure shows the surface field obtained from the eccentric axis dipole. Both were computed from the 1995 IGRF model. The total field contours are in gammas (nanoteslas). Note the better representation of the South Atlantic-South American anomaly by the eccentric dipole. Figure from J. R. Heirtzler of GSFS/NASA.

ments taken along a fixed line on the surface, the scientists can produce a two-dimensional (2D) profile of the substructure. Figure 5.8 illustrates the results of one recent survey that was made for the purpose of understanding the source of local earthquakes.

WOO 00

FIGURE 5.7 ► This Earth-interior horizontal fluid velocity at the core-mantle boundary (CMB) was computed for the year 2000 from the nondipole components of the most recent IGRE Shading (scale to the right in 10-3 cm/sec) represents the motion speed and the direction is given by the arrows. Continental outlines indicate the compared Earth crust locations. Figure from J. Quinn of USGS.

FIGURE 5.7 ► This Earth-interior horizontal fluid velocity at the core-mantle boundary (CMB) was computed for the year 2000 from the nondipole components of the most recent IGRE Shading (scale to the right in 10-3 cm/sec) represents the motion speed and the direction is given by the arrows. Continental outlines indicate the compared Earth crust locations. Figure from J. Quinn of USGS.

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