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A proton-precession magnetometer measures the scalar magnitude $F=|\mathbf{B}|$ 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:For example, $50\mu \mathrm{T}$ gives about $2.13\mathrm{kHz}$. 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]

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]