Compared to gravity, a magnetic anomaly has an added complexity: It depends on the direction (m) of magnetization and on the direction in which the field is measured. Modern magnetometers routinely measure the total field—i.e., the component parallel to the Earth's main magnetic field f (Figure 1). Unless m and f are both vertical, there is a phase contribution to the total magnetic intensity (TMI) anomaly, which can shift the anomaly laterally, distort the shape, and even change its sign. Reduction to the pole (RTP) transforms an observed TMI anomaly into an anomaly that would be measured at the north magnetic pole. This relocates extreme magnetic anomalies to be over their sources, thus making magnetic interpretation easier.

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