Integrated geological and geophysical methods to investigate the Palatine hill archaeological area (Rome, Italy)

The Palatine hill archaeological area was investigated with the aim of assessing local geohazard levels. In particular, well-logs from continuous coring boreholes, Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) surveys were integrated in order to detect cavities, both in the geological units and in the anthropogenic layer. Continuous coring boreholes were drilled on the Palatine hilltop, along the slopes, and in the alluvial valleys bordering the hill. All the boreholes crossed the anthropic layer, which ranges in thickness between 1 and 20 meters. Some of the boreholes drilled built structures with empty voids, or filled by anthropogenic material. Several boreholes also crossed a network of tunnels dug in the tuff rocks underlying the anthropic layer. These tunnels are often filled with unconsolidated, unsorted anthropogenic material. For the GPR surveys, a GPR SIR3000 (GSSI) equipped with a 500 MHz bistatic antenna with constant offset, a 70 MHz monostatic antenna, and a 35 MHz monostatic antenna, was employed. A nominal microwave velocity of about 0.07 m ns-1 was determined from fitting hyperbolas to the raw field data and used to estimate the penetration depth from the GPR survey (up to approximately 15 meters). In order to obtain a planimetric image of all possible anomalous bodies, the time-slice representation technique was applied using all field profiles. GPR survey was than constrained mainly using well-logs and archaeological stratigraphies. Low intensity reflectors are related to fine-grained geological and anthropogenic materials, while high-intensity reflectors are referred to empty voids and to masonry or foundation remains. Resistivity field data were collected with a Syscal R2 resistivity meter coupled to a multielectrode acquisition system (48 electrodes), using different array configurations (Wenner-Schlumberger and Dipole-Dipole) and an electrode spacing ranging from 1 to 10 m, obtaining different investigation depth (from about 8 to 80 m). The electrical images were successively compared and calibrated with data coming from geological, archaeological, well-logs and GPR surveys. As regards the archaeological layer, it shows an irregular lower boundary and variable resistivity values. Relatively high resistivity values (?>400 ?m) are associated to voids and/or cemented conglomeratic walls, while low to moderate resistivity values (? Particular attention was paid to two areas of the hill, i.e., the Clivo Palatino and the so-called No Man's Land. Integration of information resulting from different prospecting techniques have allowed us to define an extremely complex subsoil setting, mainly due to the presence of cavities whose partial collapse has probably involved built structures already in historical times, as well documented by the archaeologists in this area of the hill.