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Geomagnetism

The Magnetic Field
The geomagnetic field is at any location the four-dimensional (space & time) vector sum of several discrete components. The temporal component has categories separated by the time over which any variation in their intensity becomes noticeable. Archaeological surveys are concerned with the two most rapidly changing categories, micropulsations & the diurnal field. The former may only last a few seconds & have amplitudes comparable with anomalies from archaeological sources, e.g., 2-5nT. The second is the daily fluctuation in the regional field that is broadly predictable & varies by some 30-40nT per day. This can be complicated by magnetic storms which can contribute field variations of well over 100nT, frequently associated with intense bursts of magnetic noise within the spread of amplitudes associated with archaeological sources. A third temporal variation is due to variations in the distribution of magnetic sources within the Earth's core. Unlike the other two, these occur over years, influencing both the amplitude & direction of the regional field & for archaeological purposes can be safely ignored. The stationary (non-temporal) component of the magnetic field is the sum of the myriad of magnetic sources within the Earth's crust. These range from deeply buried magnetic minerals through to changes in soil structure & properties due to environmental, agricultural & of course archaeological sources. To provide a sense of scale, the deeply buried sources can contribute anomalies of a few thousand nT across many kilometres of landscape, though visible as changes of only a few nT across the sizes of areas associated with many archaeological projects. In contrast, the environmental & archaeological sources may contribute just 10nT or so, detectable at distances of no more than perhaps 3m for the larger anomalies. Where anomalies exist of a larger spatial extent than the survey area they form part of the regional field & are caused by the deepest magnetic components of the ground. The remaining field is called the residual & represents roughly the sum of the magnetic sources present within the survey area, whatever their depth of burial. In basic terms, the more sensitive the instrument used to generate this data & the less cluttered the soil, the deeper the source that can be imaged magnetically, perhaps ditch fills or settlement sites concealed beneath marginal peat for example. A branch of geophysical processing called potential field analysis allows the geophysicist to further subdivide these sources, allowing the very shallowest ones, indicative of archaeological sources, to dominate the deeper.
The burial environment
Topsoil is usually magnetic relative to other soils & hence is important for magnetic survey. If topsoil is exceptionally deep it can mask more weakly magnetic features beneath it. Alternatively, regions where the topsoil is locally deeper than elsewhere are usually associated with enhanced magnetic field strength. Archaeological features that incorporate relict topsoil tend to enhance the magnetic field around them. In some cases, features may exist magnetically that cannot be detected during excavation. This is normal, as some soils with enhanced magnetic properties do not exhibit any visible difference from their surroundings. In addition, some features survive as shadows in the topsoil after they have been physically removed by ploughing. The converse scenario is of course also true: there are many archaeological features that have no detectable magnetic component. Finally, sometimes it will be the case that the archaeological feature itself is not magnetic but some secondary characteristic still allows its detection by magnetic survey. An example is where a ditch has been filled, perhaps soon after excavation, with the same material as its surroundings & therefore lacks magnetic contrast with the surrounding material. As this fill settles, deeper topsoil (whether contemporary or modern) can accumulate in the resulting hollow, creating a local slightly positive magnetic anomaly. An example of this is a grave site where the grave itself is usually nonmagnetic but can occasionally be located by the disturbance of the contemporary surface. Of course if the top of the feature has been truncated by ploughing this effect will disappear. Hearths, burnt or fired soil & clay, & similar contexts involving the application of heat to soil, tend to become strongly magnetic due to chemical changes in the soil, in particular the conversion of iron oxides to maghaemite & magnetite. For more information on this see magnetic susceptibility. Assuming there is adequate iron in the soil initially, the process results in a particularly strong enhancement that is effectively permanent (the degradation that does occur can be regarded as negligible over usual archaeological time scales). This means that hearths can usually be detected with confidence. In addition, the presence of domestic fires at settlement sites tends to lead to an accumulation of magnetic soil throughout the settled area & for a distance beyond. It is possible therefore, that features that are undetectable away from a settlement will become more detectable the closer survey proceeds to the inhabited area, an effect that has been observed in large surveys. A secondary effect of the same process is that the presence of non-magnetic features may become detectable if magnetic material has accumulated in or around them. A common example is wall footings against which magnetic soil has accumulated, even in trace quantities.