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Multiple apparent electrical conductivity (EC a ) measurements with an electromagnetic induction (EMI) sensor frequently reveal analogue patterns caused by conductive features in the soil. A procedure was proposed to highlight different archaeological anomalies based on combinations of the simultaneous EC a measurements with the DUALEM-21S instrument. After selection of a 3.5 ha study site, 0.79 ha has been recorded by archaeological excavation. Since the majority of the archaeological features were found between the plough layer and 1.0 m below the soil surface, a set of four equations were developed to model the EC within that predefined depth interval. This set of four equations employed the four depth response curves specific to the four DUALEM-21S coil configurations. The modelled conductivity between 0.5 and 1.0 m (EC Ã 2 ) showed a larger variability across the archaeological features than the raw EC data. To quantify the added value of this modelled conductivity, EC Ã 2 and measured EC a were compared with the rasterized map of the archaeological traces. Finally, the EC Ã 2 map proved to be better able to distinguish between the archaeological features and the 'empty' background. This technique allowed the highlighting of vague anomalies in the simultaneous DUALEM-21S EC a measurements. m, mean in mS m À1 after conversion to a reference temperature of 25 C; CV, coefficient of variation in %; RD, relative difference in %.

Buried unexploded ammunition is a major problem on arable land in former battle areas. Many battlefields of the First World War (WWI) still contain a lot of unexploded shells just below the plough layer, posing serious threats to soil editors and trenchers. Electromagnetic induction (EMI) sensors have been used for a variety of agricultural and archaeological purposes to map the natural soil variability and to locate buried archaeological remains. Besides its sensitivity to variations in soil texture and anthropogenic disturbances, EMI proves to respond strongly to metal objects in the soil. Most EMI sensors rely on a single signal, with magnitude and sign of the metal anomalies differing according to the instruments coil distance and separation. The multi-coil EMI sensor, the DUALEM-21S, provides four simultaneous apparent electrical conductivity (ECa) signals enhancing significantly the possibilities for signal processing. To calibrate our instrument, we buried different masses of metal at different depths. The four ECa measurements showed a response to the metal objects down to 1.2 m. The measurements were subtracted by their gradual trend to obtain the local anomalies (ΔECa). A combination of these four ΔECa's was used to amplify the signal response to metal, influenced by both depth and mass of the buried objects. At an intensively shelled former WWI battle field near Ypres (Belgium), a detailed prospection was conducted with the DUALEM-21S. Based on our multi-signal procedure, we located 40 positions, 20 where we predicted buried metal and 20 where we expected that no metal was present within 1.2 m depth. There were no false negative predictions and at the 20 locations where we expected metal, shells up to 90 kg were excavated. As a final outcome we produced a map with predictions of the mass of metal objects in the soil assuming a fixed depth and alternatively a map with predictions of the depth of metal objects assuming a given mass.Apart from their potential for agricultural and archaeological investigations, multi-ECa signals were shown to be useful for locating metal objects, like unexploded WWI shells, in the top 1.2 m of soil. description of the archaeological landscape. Indeed, all the buried structures that are potentially detectable will be detected, whatever their nature and age (Bossuet et al. 2001). Despite the fact that EMI is less frequently used in archaeology, measurements of the apparent magnetic susceptibility (MSa) with EMI exhibit similar anomalies due to magnetic susceptible materials such as gradiometers (Simpson et al. 2009). The great advantage of using EMI is the low cost of sampling data at sufficient intensities to provide accurate mapped information. It is a rapid, non-invasive method for collecting soil ECa and MSa information (Saey et al. 2009b). Moreover, EMI proves useful for the detection and location of buried metal objects (Casey and Baertlein 1999).

Previous research showed a regional Cu enrichment of 6 mg kg -1 in the top soil of the Ypres war zone (Belgium), caused by corrosion of WWI shell fragments. Further research was required since in addition to Cu, also As, Pb, and Zn were used during the manufacturing of ammunition. Therefore, an additional data collection was conducted in which the initial Cu data set was tripled to 731 data points and extended to eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) which permitted (1) to evaluate the environmental impact of the heavy metals at a regional scale and (2) to assess their regional spatial occurrence by performing an optimized geostatistical modeling. The results showed no pollution at a regional scale, but sometimes locally concentrations exceeded the soil sanitation threshold, especially for Cu, Pb, and Zn. The spatial patterns of Ni and Cr were related to variations in soil texture whereas the occurrences of Cu and Pb were clearly linked to WWI activities. This difference in spatial behavior was confirmed by an analysis of coregionalization.