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 Incoming links: Proton-precession magnetometer

F = ∣ B ∣

of the local magnetic field. Its sensor contains a proton-rich liquid, such as a hydrocarbon or suitable water-based fluid. A strong current through a coil briefly polarizes the nuclear magnetic moments, producing a small net magnetization. The polarizing field is directed sufficiently across the local magnetic field to leave a transverse component when the current is switched off.

The protons then undergo Larmor precession about the ambient magnetic field. Their rotating net magnetization induces a decaying alternating voltage in a receiving coil. Measuring the frequency determines the field:

ω = γ_{p} F, f = \frac{γ _{p}}{2 π} F, \frac{γ _{p}}{2 π} ≃ 42.58 MHz T^{- 1} .

For example,

50 μ T

gives about

2.13 kHz

. The known proton gyromagnetic ratio provides an absolute frequency-to-field scale; this instrument does not measure the field's direction. Strong spatial gradients cause different parts of the liquid to precess at different rates and shorten the usable signal, which matters very close to a buried steel object.^[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 observes

Δ T = ∣ B_{0} + b ∣ - ∣ B_{0} ∣ ≃ B_{0} \cdot b

when

∣ b ∣ ≪ ∣ B_{0} ∣

. This scalar total-field magnetic anomaly must not be confused with a fixed vertical field component. For induced

m = m B_{0}

Δ T ≃ \frac{μ _{0} m}{4 π r ^{3}} [3 (B_{0} \cdot r)^{2} - 1] .

Reversing 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.