= Solution
At low northern latitude, a simple induced <total-field magnetic anomaly> is generally \b[asymmetric and bipolar], not a symmetric central positive peak. A compact body more magnetic than its surroundings, magnetized parallel to a northward, downward ambient field, gives its principal positive lobe on the southern side and a negative lobe toward the north. At exactly zero <magnetic inclination> its central anomaly reverses relative to the pole; latitude and survey orientation must therefore be included.<Magnetic line profiles by UBC Geophysical Inversion Facility>[Line-profile examples 1–5: latitude, profile orientation and remanence effects]{ref}
For an original explicit sketch, put a dipole at depth $d$ and let $x$ increase northward. In a north–south vertical section with inducing inclination $I$, an <induced magnetization> parallel to the ambient field gives
$$
\Delta B(x)\ \propto\ \Delta M\,
\frac{3(x\cos I-d\sin I)^2-(x^2+d^2)}{(x^2+d^2)^{5/2}}.
$$
A geocentric-dipole approximation at latitude $20^\circ$ gives $I\simeq\arctan(2\tan20^\circ)=36.1^\circ$, only an illustrative field direction, not a measured local value.
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2022/paper-1-magnetic-contrast.png]
{title=Original induced-dipole profiles: a positive magnetic body and an excavated or weakly magnetized replacement give opposite signs at the same field inclination}
For the actual crater, breccia, water and <sediment> may replace more magnetic <basalt>, giving \b[negative <magnetization contrast>]. In that case reverse the simple positive-body sketch: a dominant southern low and a northern high are possible even with purely induced contrasts. The magnetic sign cannot be inferred from density or from the word “crater” alone.
Back to article page
