Geoelectric characterisation of furnace and slag trench structures: Case study Obafemi Awolowo University ancient iron smelting site, Ile-Ife, southwest Nigeria

ABSTRACT We utilised resisto-thermal effect of iron smelting to precisely locate concealed furnaces and slag trenches at an ancient iron smelting site situated within the estate of Obafemi Awolowo University, Ile-Ife, Southwest Nigeria. The methodology involved closely spaced, small electrode spacing (micro) Vertical Electrical Soundings across diagnostic dipolar magnetic anomaly closures. Both the furnace and the slag trench sites were characterised by thermal induced enhanced layer resistivity values and anomalously high depth extent of an index (second) resisto-archaeological geoelectric layer with higher influence beneath the furnace than the slag trench. The results showed that the location of the furnace is shifted from the centre of the magnetic low (negative), sometimes overlapping the transition zone towards the magnetic high (positive) while the slag trench is generally situated within the magnetic high zone. Preliminary archaeological excavation at one of the investigated sites (site 1) identified a furnace and the adjoining slag trench structures at the precise locations predicted from the geoelectric section. The study concluded that thermal-induced resistivity attribute is effective in precisely locating concealed ancient iron smelting furnace and slag trench structures.