GNGTS 2016 - Atti del 35° Convegno Nazionale

548 GNGTS 2016 S essione 3.2 GPR and ERT acquisitions are carried out in tandem to improve results for archaeological research (Capozzoli et al. , 2015; Perciante et al. , 2015). The presence of structures with shape and size well-known allowed us to use the site for other geophysical experiences, in particular to test the capability of GPR and ERT in water saturated soil, improve the protocol of acquisition and enhance the quality of the data. Geophysical surveys are performed with a GPR System SIR-3000 (GSSI-Instruments) and a Syscal Pro Switch 96 (Iris Instruments) georesistivimeter with the water table placed at different heights. For each position of the water table GPR and ERT acquisitions are made and results are compared and analysed. GPR data are processed with ReflexW software with linear and basic operations. ERTs are inverted with ERTLab software (Geostudi Astier) through a finite element inversion algorithm to solve the forward modelling problem (Morelli and LaBrecque, 1996). The geophysical plan of investigation was performed in according to the following steps: 1. 3D loop-shaped acquisitions with both current and potential electrodes distributed along all the site surface as showed in Fig. 2. The test site was investigated with the water table placed at the depth of respectively 1.50, 1.00 and 0.20 m; 2. 2D ERT parallel profiles along two characteristic transects characterized by the presence of the most significant structures. Surveys are made with cables realized in laboratory using brass electrodes coupled to unbending pipes and positioned under water directly on the ground surface (submerged electrodes) or in floating mode (floating electrodes); 3. 2D GPR profiles along the same lines examined with 2D ERT whit a depth of the water level above the ground surface of 0.15 m. The same conditions are adopted in the second step characterized by a similar scenario. The 400 MHz antenna was coupled to the acquisition system and mounted on a floating element. Fig. 2 – Experimental set-up for 3D loop-shaped ERTs acquisitions. Results. In the first step, 3D ERTs based on loop-shaped grid of 96 steel electrodes applied according the Wenner Schlumberger and Dipole-Dipole arrays are achieved. Generally, it seems that the better results are obtainable with Wenner Schlumberger mode allowing us to very easily individuate some archaeological remains, especially in drier conditions. While, in wetter conditions, the background electrical resistivity values are very similar to archaeological remains ones and therefore it was not possible to clearly individuate most of remains. However, promising results are obtained in the second step of the research when only bi- dimensional ERTs along two characteristic transects are acquired. As it was expected very low resistivity values are measured with all the arrays due to the conductive environment; but some electrical anomalies could be associated to the geometry of the buried structures. In Fig. 3 the graves placed at a depth greater than 50 cm show a resistive behaviour with respect of