Comparison of Rapid and Precise Archaeo-magnetic Survey Techniques Over the Wroxeter Roman City Site, Shropshire, UK.

Magnetometer surveys have been carried out over a representative section of the Wroxeter roman city site to demonstrate the capabilities and limitations of current survey techniques. The site was chosen for this test because it is has an extensive literature of archaeological investigation, and has published blanket archaeomagnetic data coverage recorded with fluxgate gradiometer equipment. Repeated surveys of the chosen sample area, in excess of 2 hectares, tested the effects of; different makes of caesium vapour magnetometers, measurement configurations (total-field and gradient), navigation systems, and data processing techniques. All surveys were compared to the existing full coverage of the site obtained with differential fluxgate equipment. Specific contrasts are drawn between data acquisition techniques. English Heritage have recorded data along pre-located, gridded lines positioned by RTK DGPS navigation. Leicester data are recorded with navigation from real-time DGPS with no prior positional surveying, and survey lines decided by the physical features of the survey area and the incoming data, monitored in real time. Both vertical gradient data and total field data were acquired to contrast the relative merits of these alternative data types. The Leicester system is much more rapid and adaptable, but presents some more difficult and new challenges in terms of data processing. The two new data sets have been subject to alternative processing methods using different approaches and software. The results demonstrate the repeatability and integrity of the survey systems, such that archaeological anomalies are detectable by virtually all combinations investigated. Detailed analysis of the data provides useful guidance concerning the trade-offs that can be made to tune the survey technique for rapid survey of large areas, or best possible detail on specific areas.