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Magnetic survey

The magnetic field has a direction & intensity & hence it is possible to measure either the intensity of a directional component or the total intensity. The total intensity is measured using a total field magnetometer, e.g., a caesium magnetometer, however, it is common in UK archaeological surveys to measure just the vertical component, using a fluxgate gradiometer.

In addition, magnetometers can be configured in different ways, usually as single sensor magnetometers or as gradiometers. For this discussion it is assumed that the gradiometer is vertical. A single magnetic sensor measures all components of the ambient field, including the temporal which is not desired & hence needs to be removed from the data during processing. This is usually achieved either through reduction using software or by using a base station magnetometer, one that does not move & simply records the temporal variations so that they can be subtracted from the field data later.

A gradiometer avoids this by having two sensors measuring simultaneously, one sensor being mounted higher than the other. By subtracting the data from the upper sensor from the lower, the temporal component, common to both sensors, is removed. This has a disadvantage in that unless the upper sensor is quite high above the ground, e.g., 3m, the data from it can contain a large component due to shallow & hence archaeological sources. When the data is subtracted this reduces the anomaly strength from shallow sources as well as deep. For gradiometers using widely spaced sensors, e.g., the Bartington Grad601-2 (1m) or the ArchaeoPhysica wheeled Geometrics G858 (1.2m), this is much less of a problem than for shorter ones, e.g., the Geoscan Research FM36 (0.5m).

One advantage of vertical gradiometers is that they provide slightly better defined edges of anomalies due to magnetic sources close to them, e.g., magnetic fills in the tops of pits & ditches. A magnetometer, however, will quite often provide slightly larger anomaly strength & the calculated vertical gradient is nearly always a good model of the measured gradient.

Conversely, magnetometers are better at imaging laminar structures & can hence differentiate between soils at the same depth but with different magnetic susceptibility. This is of particular benefit when imaging small areas or sites with complex magnetic properties, e.g., settlement remains.

Base Station Magnetometer	Caesium Vapour Vertical Gradiometer	Four Channel Caesium Magnetometer