Complex prospection of medieval underground salt chambers in the village of Wislica, Poland

In the period of its great boom, i.e. between the fourteenth and the sixteenth centuries, Wilica (a village in the southeast part of Poland) was one of the administrative capitals of Poland, an important centre of commerce and trade, as well as one of the most important centres of salt trade in the country. There is a hypothesis that underground chambers previously existed in Wilica where salt was stored. This paper delineates archaeological, geological, and hydrogeological studies conducted in Wilica and presents historical background with a particular focus on the salt trade. The main part of the paper consists of the results of geophysical studies carried out in the Solny Square, and the objective of the investigations was verification of the hypothesis of the existence of underground salt chambers. Three geophysical techniques were employed, i.e. ground penetrating radar (GPR), electrical resistivity tomography (ERT), and a micro-gravimetric method. The GPR method was applied for examination of the near-surface zone to find the remains of medieval shafts located over the underground chambers facilities. ERT and micro-gravimetric methods were applied for detection of chambers or their remains located at depths from a few to a dozen or so metres. The results of geophysical surveys were correlated with information from boreholes. ERT and micro-gravimetric data were correlated, which allowed the determination of three main anomalous regions located several metres under Solny Square. A control borehole was drilled in the central part of the largest geophysical anomaly and samples were collected to establish pH; the borehole data confirmed the existence of a depression in the rock mass filled with Quaternary material with alkaline pH. It is likely that two of three anomalies indicated the locations of remains of medieval underground salt chambers.