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As mentioned earlier, the source of magnetization of rocks is primarily from magnetic induction by the earth's field of particles of magnetite within the rocks. Usually, the lighter colored rocks such as granite and porphyries have much less magnetite than the dark basic rocks such as gabbro and diabase. if we consider the magnetizable material in the rock as disseminated sources, the nature of the magnetic anomaly will depend on the way the sources are distributed and the distance from the rock surface at which measurements are made. This is illustrated by Figure 61 which show magnetic curves calculated from point sources in a north-south vertical plane and with vertical magnetization.

The upper curve with high relief and much detail is calculated for an elevation of 0.1 unit corresponding to the depth to the first row of dots. The lower curve, which is much smoother and of lower relief, is calculated for an elevation of 0.5 unit. At very low flight levels, the influence of individual sources is distinguishable as small variations or waves in the total effect. At greater distances above the surface these effects are merged together and the magneric field is practically the same as it would be if the sources of the magnetic disturbances were simple block (or sheets) of magnetized material with boundaries corresponding to the divisions between areas with different degrees of concentration of sources. In a way this is analogous to the manner in which a picture made up of small dots (as in a coarse-grained newsprint half tone) changes when viewed from different distances.

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