Magnetometry Survey

Because human activity changes the magnetic properties of the ground. Burning is especially powerful: heating soil and clay enhances their magnetic susceptibility and, as fired material cools in the Earth's field, it acquires a permanent remanent magnetization, so hearths and kilns stand out strongly. Digging also matters: pits and ditches that fill with darker, more organic, more magnetically susceptible topsoil contrast with the less magnetic subsoil into which they were cut. A magnetometer detects the small local field variations these contrasts create. Where no such contrast exists — uniform soils, non-fired stone — the features may be magnetically invisible, which is why understanding the physics is essential to predicting success.

The Earth's magnetic field is large and varies both regionally and through the day, while archaeological anomalies are minute by comparison. A gradiometer uses two sensors a fixed distance apart and records the difference between them, which cancels out the broad regional field and the diurnal drift that affect both sensors almost equally, leaving only the steep, local gradients produced by shallow buried features. This makes the instrument far more sensitive to archaeology and removes the need for a separate base-station correction. Aspinall and colleagues explain that gradiometry is the standard configuration for archaeological prospection precisely because of this strong suppression of unwanted variation.

Only indirectly. Magnetometry maps the strength and shape of anomalies at the surface, and the form and amplitude of an anomaly carry some information about the size, depth, and magnetic nature of its source — broader, weaker anomalies suggest deeper or larger features, sharp dipoles suggest near-surface fired material or iron. But the method does not measure depth directly, and an anomaly's archaeological meaning (ditch, pit, kiln, modern pipe) must be inferred from its pattern and context. Reliable identification of depth, date, and function usually requires complementary techniques such as ground-penetrating radar, modeling of the anomaly, or targeted excavation to ground-truth the interpretation.