A magnetometer doesn’t see objects — it feels the faint dents they leave in the Earth’s magnetic field. The result is a grey map dappled with light and dark, and every spot is a clue about iron, fire, or disturbed soil below. This room is where you bring that map and learn to read its dots and dipoles. Post your survey, bring your settings, and let experienced eyes help you sort the targets from the trash.
Magnetic survey is one of the core methods of archaeological geophysics, and it works on a beautifully simple principle: the Earth has a magnetic field, and buried things distort it ever so slightly. A magnetometer measures those tiny spatial variations — anomalies measured in nanoteslas — as you walk a grid, and software renders them as a plan-view map, a bird’s-eye greyscale image of the field where buried features show up as bright and dark marks. Unlike radar, it produces no depth slice; it gives you the footprint of what lies below, seen from above. Reading that footprint is the craft, and it’s what this room is for.
The two things that light up
A magnetometer responds strongly to two kinds of buried thing. The first is iron: ferrous objects are intensely magnetic and produce a sharp dipole — a paired bright-and-dark spot, a little magnetic plus-and-minus — that stands out hard against the background. The second is more subtle and more archaeological: disturbed and fired soil. Topsoil is naturally more magnetic than the subsoil beneath it, so pits, ditches, and post-holes that have filled with topsoil read as soft positive anomalies, tracing out the plan of a vanished structure. And anything burned — a kiln, a hearth, a fired-clay floor, a destroyed building — takes on a strong permanent magnetism as it cools, lighting up the map. Iron, fire, and disturbance: that is the magnetometer’s whole alphabet.
Gradiometers and clean data
How the survey was taken decides how readable it is. A single-sensor instrument measures the total field, which drifts through the day as the Earth’s field wanders — noise that smears archaeology. Most archaeological work therefore uses a gradiometer: two sensors stacked a fixed distance apart, reading the difference between them. That difference cancels the daily drift and the deep geology, leaving only the shallow, local anomalies — exactly the buried features you care about. If a map looks clean and crisp, a gradiometer usually made it; if it looks streaky or washed out, that’s worth knowing before you interpret a single spot.
What fools you
The magnetometer’s sensitivity to iron is also its curse. A single modern nail, fence wire, horseshoe, or buried pipe throws a dipole so strong it can swamp the faint archaeology around it, and a field littered with farm scrap can be nearly unreadable. Igneous and volcanic geology adds its own magnetic noise, and lightning strikes and brick rubble can mimic features. The reader’s discipline is to recognise the loud, tight dipoles of modern ferrous junk and mentally set them aside, hunting for the softer, more organised patterns — lines, rectangles, circles — that betray human design rather than a dropped tool.
The treasure-hunter’s angle
Here is the honest truth every hunter must hold onto: gold and silver are not magnetic, so a magnetometer does not detect them directly. What it detects is everything around the prize — the iron-bound chest, the iron nails of a buried strongbox, the disturbed soil of a refilled pit, the hearth of an old homestead, the fittings of a wreck. A discrete, strong dipole can mark a ferrous cache or relic; a soft, organised anomaly can mark a structure worth investigating. Magnetics narrows the search and reveals the human story written in iron and fire — but it reads the container and the disturbance, never the coin itself. Pair it with a detector or GPR before you dig.
How to post your survey here
For a useful read, include the instrument type (gradiometer or total-field, fluxgate or caesium), the grid and line spacing, the units and the greyscale range (the nanotesla window your image is stretched to), and the site’s geology and history. Note any known modern iron — fences, pipes, buried cables — so we can discount it. A clear export of the processed grid beats a phone photo of a laptop screen. With that, the room can help you separate the trash dipoles from the targets, and tell the buried wall from the buried tractor part.
Related rooms
GPR Results · Resistivity Results · Seismic Results · General Data Analysis
Sources & further reading
- Magnetic survey as a method of archaeological geophysics, recording spatial variation in the Earth’s magnetic field to detect and map buried features (terrestrial and marine)
- Single-sensor (total-field) instruments versus gradiometers, which use paired sensors to measure the field gradient and cancel diurnal drift
- The greater magnetic susceptibility of topsoil over subsoil, so topsoil-filled pits and ditches register as anomalies; and the strong magnetism of fired features (kilns, hearths)
- The strong dipolar response of ferrous (iron) objects, and the masking effect of modern iron debris and igneous geology
- The key limit for treasure work: gold and silver are non-magnetic, so magnetometry detects associated iron, fired material, and soil disturbance rather than precious metal itself
Post your magnetic map below, with your instrument and grid details. The room reads together — bring the dots and we’ll help you sort the targets from the trash.