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The angle of the geomagnetic field to the horizontal, conventionally positive downwards. It controls the skew and polarity pattern of an induced total-field magnetic anomaly.

Teaching examples show how total-field magnetic anomalies depend on magnetic inclination, survey direction, geometry and remanent magnetization.

The difference $\Delta \mathbf{M}={\mathbf{M}}_{\mathrm{target}}-{\mathbf{M}}_{\mathrm{reference}}$ between a target region and the reference material it replaces. A weakly magnetic crater infill inside magnetic basalt can have a negative induced contrast, even though both materials have positive magnetic susceptibility. The sign and direction of this contrast, including remanent magnetization, determine a total-field magnetic anomaly together with geometry and the ambient field. Geophysical density contrast does not determine magnetic contrast.

At low northern latitude, a simple induced total-field magnetic anomaly is generally 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.^[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 givesA geocentric-dipole approximation at latitude $20^{\circ }$ gives $I\simeq \arctan (2\tan 20^{\circ })=36.1^{\circ }$, only an illustrative field direction, not a measured local value.

For the actual crater, breccia, water and sediment may replace more magnetic basalt, giving 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.

Measure total magnetic field along the same south–north line with a calibrated magnetometer, recording position, time and sensor height. A simultaneous base-station magnetometer measures temporal variation so that diurnal changes can be removed from the moving survey. Avoid disturbed periods and magnetic contamination from vehicles, fences, power lines and equipment. Choose station spacing much smaller than the expected anomaly width, for example tens to hundreds of metres for a kilometre-scale target.

Subtract the epoch-appropriate International Geomagnetic Reference Field or another stated regional main-field model to obtain a total-field magnetic anomaly. Level overlapping traverses and remove offsets or drift where appropriate. Accurate terrain/sensor geometry is needed for comparison with a geological model, but there is no universal magnetic analogue of a constant-density Bouguer slab correction: the effect depends on the distribution and direction of magnetization. Measure magnetic susceptibility and remanent magnetization on representative greenstones, granite and host rock to test the assumed magnetic sources.

Assume the greenstone belt is more magnetically susceptible than the host rocks and granite, its magnetization is mainly induced magnetization, and the body is roughly east–west elongated so that the south–north line crosses it. At southern magnetic latitudes the main field has negative magnetic inclination in a north-and-down coordinate convention. The total-field magnetic anomaly is then generally asymmetric: the principal negative lobe is on the southern side and the positive lobe is displaced northward for the simple induced source illustrated here. The exact lobe sizes and peak positions depend on body geometry and the actual magnetic inclination.^[ref]

An explicit sketching model is an induced magnetic dipole at depth $d$. With northward coordinate $x$, $\mathbf{r}=(x,-d)$ and unit field direction $\hat{\mathbf{F}}=(\cos I,\sin I)$, its normalized total-field magnetic anomaly isThe plot uses $I=-36^{\circ }$ only as an illustrative dipole-field inclination for a latitude near $20^{\circ }$ south, not a measured modern field at the site. A wide greenstone belt would broaden the response and can produce prominent edge anomalies. Remanent magnetization or a magnetic granite could change the sketch substantially; gravity alone cannot identify the magnetic source.

Assume that the intrusion has a positive magnetic susceptibility contrast, that induced magnetization dominates its remanent magnetization, and that the local present-day geomagnetic field points roughly northward and downward, as in this northern mid-latitude setting. Writewhere $\mathbf{b}$ is the anomalous magnetic field and the last expression assumes $|\mathbf{b}|\ll |{\mathbf{B}}_0|$. A total-field magnetic anomaly measures a field projection, so it need not have the same sign everywhere above a high-susceptibility body.^[ref]

The 100 km east–west extent makes an approximately two-dimensional, east–west-striking body a useful model away from its ends. On a profile drawn north on the left, south on the right, the expected normally induced response has a subsidiary negative lobe on the northern side and a stronger positive lobe over the body, displaced towards the south. The northward horizontal field component makes the anomaly asymmetric; its downward component produces the dominant positive response. The field falls back towards the regional value away from the intrusion. Broad bodies can have edge shoulders or a broad central high, so the sketch represents a qualitative shape rather than one uniquely specified curve.

An exactly vertical inducing field would remove this north–south skew for a symmetric induced body. Real magnetic inclination and magnetic declination should be taken from a suitable geomagnetic field model for the survey date and location; no exact local value is supplied here. Strong oblique or reversed remanent magnetization can shift or reverse the lobes, and heterogeneous magnetic susceptibility, body dip and three-dimensional end effects can alter the profile. Higher magnetic susceptibility alone does not determine a unique amplitude or polarity pattern without these qualifications.

Lay out a surveyed grid extending beyond the possible burial area into unaffected ground. Use closely spaced parallel traverses with measurements along each line; spacing should resolve a car-scale magnetic anomaly, with a finer grid over any candidate. For a shallow car, a pilot survey with roughly metre-scale spacing is a sensible starting point, refined according to anomaly width rather than treated as a universal prescription. Record position, time, sensor height, repeat readings and any nearby interference. Cross-lines and repeat stations check reproducibility. Maintain a constant sensor height on a nonmagnetic staff and remove magnetic items from the operator; map fences, gates, pipes, buildings and vehicles before interpreting an anomaly.

Operate a synchronized stationary base-station magnetometer in quiet ground while the rover surveys, or make frequent base returns if a second instrument is unavailable. A total-field magnetic anomaly is the corrected field magnitude minus a specified background:For a field-sized target, a local background estimated away from the car is often sufficient. A larger regional survey may subtract the International Geomagnetic Reference Field and an independently justified regional trend. Do not remove the car's broad anomaly by overfitting that trend.

Correct rover observations for time-varying geomagnetic field using the base record, for example $F_c(\mathbf{x},t)=F_r(\mathbf{x},t)-[F_b(t)-F_b(t_0)]$. Check instrument offsets and tie-line differences, reject disturbed or poor-quality readings, and allow for differing sensor heights. Preserve localized target signals when interpolating a contour map or comparing with a magnetic dipole model. There are no compulsory gravity-style free-air or Bouguer gravity anomaly reductions for this magnetic search.

Postpone surveying during a geomagnetic storm or rapid irregular magnetic variation, when base correction may be inadequate. Avoid periods of moving farm machinery or electrical interference, and recognize that strong permanent cultural interference or highly magnetic host rock may make this particular method unsuitable. Ordinary cloud cover or darkness does not prevent a magnetic survey.^[ref]

The sketches assume the car can be approximated by a compact magnetic dipole whose induced magnetic moment is parallel to the local geomagnetic field. The host contributes negligible local contrast. Real steel also has remanent magnetization and shape-dependent magnetization, so the car's actual profile need not follow this idealization. A proton-precession magnetometer observeswhen $|\mathbf{b}|\ll |{\mathbf{B}}_0|$. This scalar total-field magnetic anomaly must not be confused with a fixed vertical field component. For induced $\mathbf{m}=m{\hat{\mathbf{B}}}_0$, the magnetic dipole field givesReversing both the inducing field and induced moment leaves this total-field magnetic anomaly unchanged, exactly for the magnitude definition as well as in the weak-anomaly approximation.^[ref]

Near the south magnetic pole, the inducing magnetic field and induced magnetic moment are vertically upward. Directly over the car its dipole field is also upward, again reinforcing the main field. The total-field magnetic anomaly is therefore the same positive central high and weak negative side lobes as at the north magnetic pole under the same induced-dipole assumptions. A fixed signed vertical field component would reverse, but the scalar field strength measured by the proton-precession magnetometer does not. If the car's remanent magnetization dominates, neither this symmetry nor these predicted signs is assured.

At the magnetic equator, the inducing magnetic field and induced magnetic moment are horizontal. Let $x$ point along this field, approximately magnetic north, and $y$ across it. Directly over the car, the dipole field opposes the main field, producing a negative central total-field magnetic anomaly. A north–south traverse through the car givesa symmetric central low with positive shoulders. An east–west traverse through the car instead has $\Delta T(y)\propto -(y^2+h^2{)}^{-3/2}$, a single negative trough. The equatorial anomaly therefore needs a stated traverse orientation; there is no universal single equatorial line profile.

Near the north magnetic pole, take the inducing magnetic field and induced magnetic moment as vertically downward. Directly above the car, its anomalous field reinforces the main field: a positive central total-field magnetic anomaly results. Along a horizontal traverse, the vertical-dipole approximation gives a symmetric central high flanked by weaker negative side lobes:where $h$ is the depth from the sensor to the effective dipole centre. The anomaly width scales with $h$; no absolute amplitude can be calculated without the car's magnetic moment.