The earth's magnetic field at any point on or near the earth's surface is the vector sum of the contributions from two main sources; the primary field due to the dynamo in the earth's liquid core and the crustal field from the magnetic mineral content of local rocks. This vector has both an orientation and an amplitude. Move slightly in any direction and this orientation and amplitude will vary. Introduce a highly magnetic rock formation into an otherwise homogenous host, and the local magnetic vector will change. In a three-dimensional world, there are nine (3×3) spatial gradients forming a tensor which defines the anomalous field.

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