Comparison between electromagnetic induction and fluxgate gradiometer measurements on the buried remains of a 17th century castle
Introduction
Anomalies of magnetic properties are a useful indicator of human disturbance in soil. For this reason, gradiometers are routinely used in archaeological prospection (Gaffney and Gater, 2003). Gradiometers measure passively small deviations from the earth's magnetic field, as remanent magnetism from past magnetisations as well as present magnetic induction of magnetic susceptible materials.
Electromagnetic induction (EMI) is less frequently used, but also allows measuring the apparent magnetic susceptibility (MSa or χa), whereby a magnetic field is artificially generated. So, in theory, sensors based on EMI are able to measure similar anomalies due to magnetic susceptible materials as gradiometers. In practice however, EMI is far less often used to measure magnetic susceptibility than gradiometers (e.g. English Heritage, 1995). Tabbagh (1984) compared magnetic and electromagnetic prospection to detect magnetic anomalies with theoretical models. He found differences in the modelled spatial response to magnetic features due to the different nature of the magnetising field. Gradiometer measurements were more influenced by metal objects, creating a larger area so that subtle anomalies were masked by extreme values. On the other hand, Desvignes and Tabbagh (1995) tested their EMI prototype and a Caesium gradiometer on a brick workshop site and a neolithic ring ditch site, which resulted in very similar anomaly maps. Persson and Olofsson (2004) used both the electrical conductivity (ECa or σa) and the MSa, which corresponded in different degrees to both GPR (ground penetrating radar) and gradiometer maps.
However, other case studies do not always show a good resemblance between the gradiometer and EMI sensor data, because the evaluation of EMI instruments for archaeological prospection is often done without considering the MSa, as in Fröhlich et al. (1996) and Maillol et al. (2004). Linford (1998) compared both the ECa and MSa of an EM38 instrument with twin probe resistivity and fluxgate gradiometer data. The ECa showed more resemblance to the gradiometer than to the resistivity map but his test area was very low in ECa, so that the signal was probably masked by the small influence of the magnetic susceptibility in the ECa measurement (Tabbagh, 1990). One of the reasons of changing success with an EMI sensor is the diversity of measurement configurations that can be used. Both MSa and ECa can be measured in different coil orientations and coil distances, each having a distinct spatial sensitivity.
A disadvantage of an EMI sensor like the EM38 is that it can only measure one property (either ECa or MSa) at a time. Therefore at least two surveys are needed to measure both, duplicating the total survey time. And then the measurements are also not collocated and not taken under identical ambient conditions. So investigating their relationship involves interpolation uncertainty. Contrarily, the EM38DD sensor (Geonics Limited, Canada) is able to measure ECa in both the vertical and the horizontal coplanar orientations (Cockx et al., 2007). The EM38DD is also able to measure simultaneously ECa and MSa (Simpson et al., 2008). Such collocated observations have a much better potential to investigate their relationship and complementarity than a single EMI instrument. But so far there has not been a systematic evaluation of the different options the EM38DD offers.
The objective of this study was to evaluate the different EM38DD configurations, installed on a mobile platform guided by a dGPS, for archaeological prospection. The significant advantage of this system is the fast acquisition of simultaneous and colocated ECa and MSa measurements. These measurements were compared with a fluxgate gradiometer survey. An archaeological site containing the buried remains of a 17th century castle was selected as a test case.
Section snippets
Study site
The study site was located in Vinkem, a small village near the west coast of Belgium (51°00′50.2292″ N, 2°39′40.8037″ E) (Fig. 1). The site consisted of three pasture fields of 0.8, 0.8 and 0.5 ha. The central field was surrounded by a large ditch of approximately 5 m wide. The site was located on a plateau of Tertiary clay, close to the edge of the low coastal area, 8.5 m above the mean sea level (expressed in m TAW, “Tweede Algemene Waterpassing”, the reference level of the Belgian ordnance).
Total site survey with EM38DD (survey 1)
The elevation of the site was obtained from a LIDAR survey, conducted for the whole Flanders with an average point density of 1 point per 20 m2 (Fig. 6a). LIDAR or “light detection and ranging” measures the distance between an airplane and the ground by recording the travel time of a laser pulse. In general, the elevation increased gently from north to south. The ditch surrounding the central and southern field was clearly visible as a local minimum. The northern and central field had a similar
Conclusions
The mobile EM38DD unit was able to detect the remains of the 17th century castle in the pasture fields of Vinkem. The wall foundations were clearly visible as a strong anomaly in the in-phase response of the 1 m vertical coplanar orientation, which was due to the enhanced magnetic susceptibility of the bricks. In the horizontal coplanar orientation, the anomalies were less clear and showed both positive and negative responses, which was probably caused by the change in sign of the response at a
Acknowledgements
The authors wish to thank the reviewers for their comments that greatly improved this article and the Fund for Scientific Research-Flanders (FWO), for the financial support to the research projects G.0162.06 and G.0078.06. This study was conducted in cooperation with the Flemish Land Management Institute (VLM).
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