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The development of electronic magnetometers, i.e., the proton-procession and fluxgate instruments, for use in ground magnetic surveys has permitted the measurement of the first-vertical derivative of the total, field, or of the vertical component of that field, with negligible addition to the total cost of the survey. The gain in information is, however, significant. Curves for the vertical gradient over a vertical contact, point pole, and finite dipole are presented. The vertical contact is outlined by the zero contour for the vertical gradient of the vertical component, and the depth of burial is half the horizontal distance between the positive and negative maxima. The depth of burial of the point pole and finite dipole is approximately equal to the horizontal distance between the negative half-maximum points on the vertical-gradient curves.
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