Fluxgate, Caesium Vapour and Excavation: Establishing the Validity of High Sensitivity and High Sample Density Magnetic Measurements
An extensive fluxgate gradiometer survey was undertaken on a large housing development at Northampton. England. The results, based on a 1.0 x 0.5 sample density, were unambiguous, with evidence for clear settlement foci. However, it was felt that some of the detail, especially for individual dwellings was missing. A research design was formulated to see if the missing elements could be located. In particular the strategy was aimed at establishing the differences, if any, between two magnetic
... een two magnetic instruments (fluxgate gradiometer and caesium vapour gradiometer) of different sensitivity on "typical" British soils and to establish what variation an increased sample density achieves. Firstly, two enclosures and a "ring" ditch were re-surveyed using the Geoscan FM 36 and data collected on a 0.5 x 0.125 m grid. The area was then surveyed using a Scintrex Smartmag SM-4G on the same nominal sample intensity. The results were analysed to highlight: 1) The differences between the two fluxgate gradiometer surveys. 2) The interpretable differences between the fluxgate and caesium vapour data. This study benefits from two further factors. Firstly, a "blank" area was surveyed to analyse soil noise in an effort to understand true levels of identifiable anomalies. Secondly, all areas were then stripped and archaeologically excavated. As a result, direct comparison has been made between the instruments and the physical reality of the buried evidence and not simply a qualitative "analysis" between greyscales. This work has far reaching implications about how we go about survey, both in terms of strategy and instrumentation.