What is Geophysics? - Cambridge Archaeology Field Group

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from geology or archaeological features (the latter values are typically only 1 – 2 nT).These local features add to the earth’s field at that exact location and so the fieldgradient is no longer zero. To measure field differences as small as 0.1 nT requires avery sensitive magnetometer and this sensitivity requirement gives rise to certainproblems – for example, iron is very magnetic and so a wire fence (or buried ironpipes or cables) can obscure the presence of more subtle features for several metreseither side of the fence. Magnetometry does not work so well, therefore, in urbansituations where buried ironwork is common.How do archaeological features acquire a magnetic field? It relies on the presence ofiron or iron oxides in the soil or artefact. Pottery, bricks and tiles or fire-heated soilssuch as hearths which have been heated above 575 – 675 o C acquire a permanentmagnetic field on cooling. Igneous rock is also more magnetic than most soils soartefacts or building stone made of igneous rock will also have a high field - but thisalso means that magnetometry will not work well where the local geology ispredominantly igneous. Pits and ditches, where the infill soil is different to the naturalsurrounding soil, may be more or less magnetic than the bulk soil and producepositive or negative responses.There are three types of magnetometer detectors:proton, alkali vapour and fluxgate. Of these, thefluxgate detector is the cheapest, basicallyconsisting of a metal rod surrounded by a coppercoil. An external magnetic field magnetises the rod,causing a current to flow which is then measured.This type is most used in non-commercial oruniversity archaeology. However, they are directionand temperature sensitive so they are normallyused in the gradiometer configuration, as shown inFigure 4.In this Geoscan system one detector is placed0.5m (some systems can use a 1.0m separation)vertically above a lower detector and the differenceFigure 4. Geoscan FM256 gradiometersystemin response between the two is equal to the magnetic gradient – it does not equal the4
total field. As any changes in the earth’s field affect both sensors, those changes areeffectively eliminated by this technique. Drift can still occur (especially due totemperature changes) and drift correction must be carried out at a fixed point, usuallybetween grid squares. Assume the ambient magnetic field is 50,000 nT and that a pit1m below the bottom sensor causes a local increase of 1 nT. The top sensor recordsa value of 0.3 nT so that the gradient is:50,001 – 50,000.3 nT = 0,7 nTA magnetometer survey uses a survey grid like that employed for resistivitymeasurements or field walking. However, because there are no probes to insert in theground, the operator carries out the survey by walking without pause and it is muchquicker to carry out than a resistivity survey. Modern instruments can take readings attimed intervals, usually marked by a beeping sound, so the operator has to walk at arate sufficient to cover a traverse within the time set. The overall time depends on theground conditions; for example short grass is fine but grass over 100cm tall can createdifficulties. Similarly the presence of iron fences and trees may give rise to problems.As with resistivity measurements, the data must be downloaded and processed.Magnetometry results may require more processing than resistivity ones. A typicalresult indicating various buried features is shown below in Figure 5. This example isfrom the estate areaimmediately behind theformal gardens at WimpoleHall. The interpretation,confirmed by the subsequentexcavation is as follows: thecircular feature is the brickwall surrounding the fountain.The large signal in the centreis given by a metal grillsubsequently built into thedemolished fountain and thevarious parallel features aredue to a number of brick-builtFigure 5. Magnetometry results showing the 18 th Cfountain and surrounding features at Wimpole Hall(courtesy of Peter Morris)5
Page 6 and 7: culverts and brick-filled drains(c)
Page 8 and 9: Which geophysical technique is the
Page 10: Further readingThere are a number o

What is Geophysics? - Cambridge Archaeology Field Group

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