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

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.