Geological and geophysical investigation in the North Stelae Park of Aksum (Ethiopia) as contribution for the landscaping assessment and re-erection of the Roma Stela

The present work reports the main results of geological and geophysical field surveys carried out in the Northern Stelae Park of Aksum (Ethiopia). This research is part of the environmental impact assessment for the re-erection of the Stela n. 2 (or Roma Stela), returned from Italy in April 2005. The Aksum World Heritage site is characterized by impressive pre-Christian monuments, monolithic obelisks or stelae erected as mortuary structures. The project has been funded by the Italian Government after agreements between Italy and Ethiopia taken in 1947, 1956, 1997 and 2004. The Obelisk returned to Aksum between 19 and 23 April 2005 and a project for its re-erection in the original place has been designed and approved. In order to better characterize the construction site different topics have been investigated. The composition of the ground (including geological, geomorphological, hydrogeological characteristics), soil types and potential seismicity are important aspects in the assessment of short and long - term impacts of the construction site on the landscape, considered as an integral between environmental features and the historical-archaeological context.
Along with available information, a field survey was carried out to improve field data and mapping. Geophysical surveys with georadar acquisition and electrical tomography have been performed with the aim to investigate a 3-4 m depth of the underground where the Stela 2 will be re-erected and explore the sacred archaeological area in order to detect and safeguard the unknown archaeological structures between Stela 2 and Stela 3 during the works for the re-erection. The georadar data were acquired with a IDS Corporation instrument, endowed with two channels. This configuration has allowed to acquire data simultaneously along the same profile with 200 and 600 MHz antenna frequencies. Electrical tomography has been performed by an instrument, equipped with 48 electrodes.
Along the same profile data with dipole-dipole, Schlumberger and Wenner spreads have been acquired. The analysis of data showed a good correlation and the joint interpretation allowed to better characterize the detected anomalies. In the parking area, a depth of about 1-2 m with the georadar and about 4m with the tomography have been investigated. The superficial layer consists of very inhomogeneous materials probably due to recent human activity. A sewer and the rest of the modern circular fountain are well detected. The same anomalies need to be taken carefully in consideration due to its unclear origin; it likely can be associated to a collector or a tank. The Stelae 2 and 3 area, consists of 1.5 -2.0m of blocks and debris. Same anomalies resulted very important since they are probably referred to structures. The joint interpretation of geophysical data, calibrated with geological field surveys, allowed to perform a potential map of risk which will be taken into account during the re-erection of the Stela 2.