The attention was focussed in those areas surroundings the remnants of the Apollo Temple basement, (L. Quilici, 2004), above underground tanks and along the Appia route. Results of acquisition surveys are illustrated by 2D maps with 3D features of the subsoil volume analyzed in the case of GPR and ERT.

High-resolution Ground Penetrating Radar (GPR – georadar)

The Ground penetrating Radar (GPR), which is commonly known as Georadar, is a methodology based on ground propagation of electromagnetic pulses having frequencies within the range of 15 to 1500 MHz and the recording of reflected/diffracted signals originated by geometric discontinuities or variations in the subsoil electric characteristics. By now, it has become among the geophysical methods more frequently used to investigate buried archaeological structures. Indeed, if depth and dimensions of searched bodies are compatible with pulses penetration and propagation, the high-resolution power which characterize GPR (it) respect to other geophysical methods makes possible to detect archaeological structures with a great detail. During a Georadar field survey finalized to archaeological research, the area where structures are supposed to be found must be investigated by means of parallel profiles. After completing field acquisition and elaboration of acquired profiles, anomalies detected along single radar sections (on the vertical plane for each profile direction) are reported on a map layout enclosing the study area and then correlated with those from adjacent profiles.Georadar traces were acquired with a SIR System 3000 (GSSI) equipped with two two bistatic antennas having a constant offset and nominal frequency of 400 and 900 MHz, respectively (Fig. 1); a line scan procedure was adopting, which consists in continuously moving the antennas along a predefined direction (profile).

• Fig.1 High resolution georadar Fig.1 High resolution georadar
• Fig.2 Electrical resistivity tomography Fig.2 Electrical resistivity tomography
• Fig.3 Electrical resistivity tomography Fig.3 Electrical resistivity tomography
• Fig.4 Electrical resistivity tomography Fig.4 Electrical resistivity tomography
• Fig.5 Magnetometry Fig.5 Magnetometry
•  

Electrical resistivity tomography (ERT)

The Electric Resistivity method allows to characterize the subsoil by reconstructing variations of the apparent resistivity at depth. Ground electrical parameters are in close connection with soil solid and fluid components. The starting point of this methodology consist in a ground energy input and the subsequent detection of subsoil response to such energization (potential difference). The current intensity, potential and geometric features of instrumentation give a contribute to determine apparent resistivities. Electrical resistivity tomography is a combination of vertical and horizontal electrical soundings which consents to elaborate a great amount of apparent resistivity values belonging to the portion of subsoil under investigation, and to project them on 2D and 3D sections according to type of acquired data. Its advantage consists in defining "a priori" the length of the electrodes array as a function of depth to be investigated. In addition, by varying distances among electrodes it is possible to intensify measurements to certain depths depending on the demanded resolution. During filed acquisition for this project a SYSCAL Junior Switch-72 georesistivimeter by IRIS Instruments was used (Figs. 2,3,4). Once recorded data were transferred on a PC and then elaborated by means of the following softwares: RES2DINV (by Geotomo) to obtain 2D pseudo-sections relatively to acquired profiles; RES3DINV (by Geotomo) to achieve a 3D subsoil resistivity model. Through these softwares, it was possible to perform inversion for a large amount of data collected with a multi-electrode system and, therefore, to pass from apparent to real resistivity.

Differential magnetometry

By measuring the vertical component of the terrestrial magnetic field (expressed as nT, i.e. nanoTesla), differential magnetometry prospection allow to detect magnetic anomalies due to the susceptibility contrast between target bodies and hosting material. The magnetometric method is based on the use of a magnetometer able to measure intensity variations of the Earth magnetic field and/or its vectorial components. A gradiometer is the most commonly used instrument to pursue an investigation of the shallowest subsoil levels and in particular to detect low-depth source anomalies, as is the case of archaeological structures. In order to guarantee optimal conditions for geomagnetic prospection, measures were completed at nodes of a regular grid set on the ground, adopting a regular predefined step and using the FM256 gradiometer (by Geoscan) (Fig. 5).