GNGTS 2017 - 36° Convegno Nazionale

662 GNGTS 2017 S essione 3.2 Results and conclusion. The geoelectrical survey, performed near-shore and on-shore domine, allowed an investigation from ground level down to a depth between 20 m and 40 m, which is bound to the length of the prospecting and could highlight both vertical contacts and lateral ones. In particular, 21 geoelectrical profiles were performed, 6 of which were longitudinally oriented north-south along the cliff, the beach and the intertidal zone, and 15 transverse oriented in the east-west direction across this longitudinal one. Full inversion of the dataset has allowed us to obtain sections most balanced and therefore easier to interpret. In particular, we report the results carried out by two sub-perpendicular lines identify with acronym T1 (coastal transversal) and P1 (coastal parallel) on the map of Fig. 2. The area is characterized by several terraces order, we find sedimentary structures associated to sand- infilled paleo-channel in both sea and land zone . The detailed pattern and sedimentological evolution of the channel fill deposits – know from both historical and geological sources – made it the ideal site to test the ERT method. Because of the salinity homogeneity in sea survey, the electrical resistivity profile reflect mainly the lithology variations while in land area, resistivity also depends on the chemistry of interstitial water. Fig. 3 shows the distribution of resistivity values of the two lines P1 and T1. Resistivity values of the land part are more high than sea zone. This is due to at the full saturation of the marine deposits (low resistivity). Local stratigraphy on the land reveals in the most superficial part the Brickearth deposits (Lewin, 1966) followed by the alluvial gravel covering the Bracklesham Group (Carry et al. , 1977). These deposits consist of sand, silt and gravel that we find in quaternary marine and continental sequences. On shore, the Brickearth deposits are at 0–2 m depths with resistivity values (>70 ohm·m). These Tab. 1 - Data sheet and survey parameters used. Georesistivimeter IRIS Instruments mod. Syscal Pro TRANSMITTER UNIT Max voltage: 800V in switch mode. Max voltage: 1 000V in manual mode. Max current: 2.5A, typ. accuracy 0.2%. Max power: 250W with internal DC/DC converter and 12V external battery; 1200W with external AC/DC and motor generator. Option 25mA max for readings on samples. Pulse duration: 0.2s, 0.5s, 1s, 2s, 4s, 8s. Internal 12V, 7Ah battery, plug for external battery RECEIVER UNIT Automatic ranging, 10 true differential input channels. Input impedance: 100 Mohm. Max input voltage :15V. Protection up to 1000V. Typ accuracy: 0.2%, resolution: 1 microV. Digital rejection better than 120 db at power lines 50 and 60 Hz. Stacking process, SP linear drift correction. Reading of current, voltage, standard dev., 20 IP windows (preset or selectable) GENERAL Memory: 44.800 readings USB & SD card link (7.000.000 readings in option). GPS input for coordinates. Fiber glass casing, weather proof. Temperature range: 20 to +70°C. CABLE GEOMETRY Electrode spacing (land) Schlumberger reciprocal array configuration: variable 3/4 m Electrode spacing (sea) Wenner array configuration: variable 5-10 m