The effectiveness of geophysical methods in detecting underground tunnels − the case study from the Ponikowska adit in Olkusz, Poland

Abstract

During the Medieval period, Olkusz (located approximately 50 km northwest of Kraków, southern Poland) was one of the most important European cities. Its development was closely related to silver extracted from lead ores. When shallow deposits were depleted at the end of the 15th century, the construction of drainage adits became necessary to access deeper resources. The Ponikowska Adit was constructed from 1563 to 1621. It was the longest (7860 m) and most utilised. It drained rocks to the west and north of the town until 1712, when it became clogged. In the late 19th century, it was refurbished and partially reactivated. However, insufficient maintenance caused it to close again in the early 20th century.

The locations of the tunnels are not known accurately. Geophysical surveys have provided insights into the underground areas. Electromagnetic profiling in frequency domain (FDEM) and electrical resistivity tomography (ERT) were used to locate these tunnels. The FDEM surveys indicated variations in the subsurface conductivity. However, it did not precisely locate the adit. A sequence of low-resistivity anomalies was observed in the ERT cross-sections, rather than the anticipated high-resistivity anomalies originating from voids. The 2D modelling validated this result as an effect of filling the tunnels with deposited clay material. The ERT anomalies are consistent with the likely paths of the adit. Moreover, numerous anomalies originating from higher mining excavations were detected.

The effectiveness of noninvasive geoelectrical techniques for relatively deep archaeological identification in significantly altered post-industrial areas is an innovative aspect of this study. The results indicate that combining geophysical identification and modelling with historical information is an essential and efficient tool to support the preservation of unique underground mining heritage sites. It would allow for better understanding and securing of effective historical resources for future generations.