Results of Geophysical Investigations Related to the Excavated Remains of the Late Antique and Early Mediaeval Iron Production Sites in the Podravina Region, Croatia

Several sites containing relatively high quantities of waste products from the bloomery iron production collected during archaeological field surveys in the Podravina region as part of the TransFER project were subsequently investigated by magnetic prospecting and topsoil apparent magnetic susceptibility mapping. The magnetic method proved to be very effective in identifying various archaeological remains of iron production activity in situ due to the high magnetic susceptibility of materials present in iron production workshops, which was confirmed in relation to the results of archaeological excavations at Sušine near Virje as well as Velike Hlebine and Dedanovice near the Hlebine sites. Reasons for the higher magnetic susceptibility include, in addition to the strongly magnetic minerals in iron production waste, fragments of fired clay from furnace construction and features such as shallow pits with burnt bottoms, as well as any other materials that have been exposed to high temperatures (burnt house plaster, etc.). The sites with remains of bloomery iron production were therefore reliably identified on the basis of their magnetic properties and the results were evaluated in relation to the excavated features. This study has also demonstrated that the sites with iron production activity present can also be reliably identified on the basis of changes in the apparent magnetic susceptibility of the topsoil to a depth of only 5 cm, after partial destruction of the archaeological layers by deep ploughing. These anomalies generally have a wider spatial distribution than those detected by magnetometers, due to a wide dispersion of material by ploughing mechanisms. With the aim of identifying layers with major enrichments of bog iron ore, electrical resistivity tomography (ERT) measurements were carried out at the site Novigrad Podravski–Milakov Berek, where pieces of bog iron ore appeared on the surface. Based on these results, we have not been able to reliably identify ore deposits, but it has been shown that it is possible to identify layers of relatively low resistivity on ERT profiles that have increased iron mineral content, as confirmed by X-ray analysis of core samples from shallow drillings at several other locations in the Podravina region.