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Earth and Space Sciences · 术语表

什么是 Total-field anomaly; diurnal correction; reduction to the pole？

又称： total-field anomaly · diurnal correction · reduction to the pole

Definition 31.7 University Earth and Space Sciences — Year 3 · 第 31 章 — Field and Lab Methods III

Let $\mathbf{F}$ be the main field at the survey and $\Delta\mathbf{B}$ the field of the magnetised rocks.

The total-field anomaly is the measured intensity minus that of the main field, $\Delta T = |\mathbf{F} + \Delta\mathbf{B}| - |\mathbf{F}|$ ; when $|\Delta\mathbf{B}| \ll |\mathbf{F}|$ it is the projection $\Delta\mathbf{B}\cdot\mathbf{F}/|\mathbf{F}|$ .
The diurnal correction removes the variations of the field with time: a magnetometer at a fixed base station records the field throughout the survey, and each reading $T(t)$ of the moving instrument is replaced by $T(t) - T_\mathrm{base}(t)$ .
The reduction to the pole computes, from the measured anomaly, the anomaly the same rocks would give if the main field were vertical, as at a magnetic pole; the anomaly then sits above its source.

Left: the total-field anomaly of a vertical dyke 20\, m thick, its top 100\, m deep, susceptibility 0.01, in a field of 50\,000\, nT; on profiles across an east–west strike at inclinations of 90 and 60, and across a north–south strike at 60. Right: the readings along the Varden magnetic line, one station every 50\, m and every two minutes from west to east, and the base station at the same times (plotted at the position the moving instrument then had): the daytime rise of the field is half the size of the anomaly. — Left: the total-field anomaly of a vertical dyke $20\,\mathrm{m}$ thick, its top $100\,\mathrm{m}$ deep, susceptibility $0.01$ , in a field of $50\,000\,\mathrm{nT}$ ; on profiles across an east–west strike at inclinations of $90{}^{\circ}$ and $60{}^{\circ}$ , and across a north–south strike at $60{}^{\circ}$ . Right: the readings along the Varden magnetic line, one station every $50\,\mathrm{m}$ and every two minutes from west to east, and the base station at the same times (plotted at the position the moving instrument then had): the daytime rise of the field is half the size of the anomaly.

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