Illuminating traces of an Achaemenid's monumental complex in the Southern Caucasus by electrical resistivity tomography

The Achaemenid Empire controlled the Western Asian landmass between ca. 550-300 BCE. By some estimates, its subject peoples made up almost 44% of the world population. In recent years, studies have moved from a consideration of its core region, located in the province of Fars, Iran, to the investigation of imperial rule in the dependent regions. Since Achaemenid centers were often built of mudbrick and thus not easy to recognize by traditional archaeological methods, geophysical methods play an important role in this research. This paper reports the results of an innovative Electrical Resistivity Tomography (ERT) study undertaken at one such center, Saaklemo, in the Kakheti Province, Republic of Georgia and shows the wide impact of the geophysical methods in the detection of the archaeological sites.In three areas of this ancient site between 2020 and 2021, following archaeological survey and magnetometer prospection, several three-dimensional (3D) ERT measurements, composed of dense parallel two-dimensional (2D) tomographies with dipole-dipole configuration, were carried out. To reconstruct the resistivity distribution of the subsurface, we used the smoothness-constrained inversion method, especially as the substructure was made of mudbrick and thus resistivity values of these anomalies were close to the background anomaly. Therefore, with smoothness-constrained inversion, we could detect the slight resistivity differences, which were partly due to density differences, and hence this method was capable of detecting these anomalies. In area 1, for the first time with ERT method, we were successful in discovering an extensive mudbrick (partly burnt) substructure at a depth of 40 cm to 1 m. After analyzing the magnetic data and a 3D model of this substructure based on the ERT data, a large excavation was conducted and a mudbrick substructure with six in situ bell-shaped column bases was unearthed, which was destroyed by conflagration. The size and the architectural techniques used for this building prove that this excavated structure was an administrative building and served for representative purposes. Further presumed mudbrick structures were detected in the other two areas with the ERT method, but they have not yet been excavated.

Automated summary

The Achaemenid Empire had a vast territory and a significant population. Recent studies have shifted focus from its core region to the investigation of imperial rule in dependent regions. This paper reports the results of an innovative study that used geophysical methods, specifically Electrical Resistivity Tomography (ERT), to detect archaeological sites in Saaklemo, Georgia. The use of ERT successfully discovered a mudbrick substructure and provided insights into its architectural features and purpose.