Magnetic survey methods rely on the ability of a variety of instruments to measure very small magnetic fields associated with buried archaeological remains. Features such as a ditch, pit or kiln can act like a small magnet, or series of magnets, that produce distortions (anomalies) in the Earth's magnetic field. In mapping these slight variations, detailed plans of archaeological sites can be obtained as buried features often producing reasonably characteristic anomaly shapes and strengths (Gaffney and Gater 2003). The survey was undertaken using four Bartington Grad601 sensors mounted at 1m intervals (1m traverse interval) onto a rigid carrying frame. The system is programmed to take readings at a frequency of 10Hz (allowing for a 10-15cm sample interval) on roaming traverses 4m apart. These readings are stored on an external weatherproof laptop and later downloaded for processing and interpretation. The system is linked to a Trimble R8s Real Time Kinetic (RTK) differential Global Positioning System (dGPS) outputting in NMEA mode to ensure a high positional accuracy for each data point. MLGrad601 and MultiGrad601 (Geomar Software Inc) software has been used to collect and export the data. Terrasurveyor V3.0.32.4 (DWConsulting) software has been used to process and present the data. The survey has clearly demonstrated that the prevailing geological and pedological conditions are favourable for the detection of sub-surface archaeological remains and consequently it is assessed that the results provide a reliable indication of the extent of all the significant areas of sub-surface archaeological remains within the ODA, subject to the limitations of the technique. Anomalies indicative of probable or possible archaeological features and activity have been identified throughout the ODA, the majority of which were previously unknown, thus adding significantly to the archaeological understanding of the landscape across which the cable corridor will traverse. Although the suspected archaeological remains extend throughout the ODA there are still large areas where no anomalies of archaeological potential have been identified from the geophysical survey. However, the low magnitude exhibited by some of the anomalies and the partial and discontinuous nature of others suggests that, in certain instances, the archaeological remains may be more extensive than revealed by the survey, either due to partial truncation by modern agricultural techniques and/or a lack of magnetic contrast on a variable geological substrate. Nevertheless, eleven broad areas comprising both concentrations of anomalies or single clearly defined features are identified as AAA's. Most of the linear anomalies are interpreted as being the result of soil-filled ditches forming an extensive and complex network of field systems and enclosures, most likely for animals, which extends across the full length and width of the ODA. These field systems and potential stock enclosures are of uncertain date but probably date to the later prehistoric or early Roman periods and possibly post-medieval. Smaller, sub-divided, enclosures with numerous discrete anomalies are interpreted as more likely to have been the sites of human occupation. Several of these settlement sites are identified, particularly in the western half of the ODA, again varying dates are likely including medieval. As well as the enclosures and settlement sites two circular anomalies, interpreted as locating a round barrow of possible Bronze Age date and a windmill of likely post-medieval date, are also highlighted.
Alistair Webb (Wed,) studied this question.
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