Magnetic field fluctuations at the earth's surface of at <1 Hz are shown to result from motion with a peak velocity of ∼4 cm/s of ground water (∼4 S/m) at ∼5 km depth. Surface field changes can occur due to either divergence free fluid motion with transverse spatial wavelengths of a few tens of km, and/or homogeneous flow which displaces local inhomogeneities in the earth magnetic field.
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Draganov et al. (1991) studied this question.
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