First results of non-destructive surveys at the ancient walls of Amelia (Central Italy).

The preservation and the safeguard of ancient cultural heritages in the current urban contexts would be sometimes complicated due to their rapid evolution. Nevertheless, it represents a mandatory task which must be pursued in order to transmit to the future generations monuments of ancient civilizations. Non-invasive approaches such as a combination of geophysical methods could represent an efficient way to study and investigate their inner structure without drilling of the investigated structures. In general, Ground Penetrating Radar (GPR) and Electric Resistivity Tomography are widely used for archaeometry (Conyers, 2013; Jol, 2009), and, more in detail, Tsourlos and Tsokas (2011) have applied such techniques at the South Walls of the Acropolis of Athens. In this study, we have investigated an ancient (VI and IV century BC) portion of the polygonal walls of Amelia at Terni in Central Italy, affected by a wide collapse in January 2006. We combined Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) surveys with geomatic techniques like the Laser Scanning. The main aims of the geophysical survey were the definition of the internal geometric characteristics of the walls. We collected a high resolution 2D GPR vertical and horizontal sections, plus three horizontal ERT profiles using non-destructive electrode (moisture of humid bentonite and a solution of copper sulphate). Such datasets have been integrated with ones obtained by GNSS, Total Station, Terrestrial, Laser Scanning, Digital Terrestrial Photogrammetry monitoring, with data provided by directs measurements and with some archaeological information available in literature. We interpreted the radargrams analyzing the wavelengths and amplitudes that defines a peculiar signature for each sectors. Several diffractions are clearly visible within the unmigrated profiles, whilst the migrated data better display flat reflections and the lateral discontinuities, assessing a more reliable shape to the targets. The data show the presence of blocks characterized by very different sizes and degree of organization between the internal side and the facade. The top part of the radargram shows a more uniform signature and short wavelength reflections, interpretable with the presence of small stones (visible on the façade as well) up to a depth of 1, 1.5 m meter. At this depth, a strong signal attenuation is apparent and probably interpretable with the presence of conductive ground/soil. The max thickness of the basal sector can be estimated in about 3.5 m, that define a strong attenuation interface can be interpreted with the internal termination of the calcareous stones. We think this peculiar “Meghalitic” signature could be suitable to be used as a fingerprint for further extension of the survey to other sectors. The ERT data showed shallow high resistivity areas and a deeper low resistivity unit (max 130 ohm.m). Such a discontinuity probably represents the contact between the walls and the conductive backfill and soil located behind them, at a depth of about 3.5 m, in agreement with the strong attenuation displayed by the GPR data. We successfully provided a first geophysical images of the inner structures of the Amelia walls, highlighting different signatures and the geometrical characteristics of their inner structure. We defined an efficient methodology suitable to be extended to other sectors. First results showed how the proposed non-destructive approach, encompassing geomatics and geophysical prospecting, represents an efficient way to improve the knowledge on the characteristics and on the inner structure of the such a valuable monument. We recommended further studies and analysis to better define the real need of focused actions for the preservation of such a cultural heritage.