TIME TIME- LAPSE 2D ELECTRICAL RESISTIVITY TOMOGRAPHIES FOR INVESTIGATING T LAPSE 2D ELECTRICAL RESISTIVITY TOMOGRAPHIES FOR INVESTIGATING THE PICERNO LANDSLIDE HE PICERNO LANDSLIDE (BASILICATA REGION, SOUTHERN ITALY) (BASILICATA REGION, SOUTHERN ITALY) R. Luongo 1,2 , C. de Bari 2 , G. Calamita 1,2 , A. Loperte 1 , A. Perrone 1 , A. Satriani 1 , M. Votta 1 , V. Lapenna 1 (1) CNR, IMAA, Tito Scalo, Italy (2) Dipartimento di Ingegneria e Fisica dell’Ambiente, Università degli Studi della Basilicata, Potenza Italy SUMMARY SUMMARY. The preliminary results related to the first applications of a prototype system for time-lapse acquisition of electrical resistivity and TDR measurements are presented. The system was developed in the frame of MORFEO (Monitoraggio e Rischio da Frana mediante dati EO) project funded by the Italian Agency Space (ASI) and finalized to the activities of the Italian Civil Protection Department (DPC) in the landslide risk management. The system was planned with the aim to obtain time-lapse 2D Electrical Resistivity Tomographies (ERTs) and to verify if they could provide useful information about rainwater infiltration, groundwater table variations and water content in the first layers of the subsoil. The ERTs and the TDR measurements are acquired with time intervals selected by considering the rainfall intensity and frequency coming from a rain gauge placed in the test site. The results are opportunely integrated and validated with piezometric data. INTRODUCTION. INTRODUCTION. The rainwater infiltration into the soil and the increase of pore water pressure in the vadose zone can be considered the main causes of shallow landslides triggering. The standard techniques used to measure the water content of the soil and the water table levels in areas of potential instability are the TDR method and the piezometric measurements, respectively. These techniques, while allowing to obtain direct information of the considered parameter, provide a punctual information about the specific hydrological characteristics of the investigated soil. Recently, the literature reports many examples of applications of indirect methods for the study and the estimate of water content in the first layers of the subsoil. In particular, the 2D ERT has been applied for obtaining information about the temporal and spatial patterns of water infiltration processes (Michot et al., 2003; Cassiani et al., 2006; Werban et al., 2008). The ERT if well integrated and compared with direct data could allow to overcome the punctual character of the standard measurements. This work reports the preliminary results coming from a prototype system planned to obtain time-lapse 2D ERTs and TDR measurements in a landslide area located in Basilicata region (southern Italy). The area (fig.1), which represents one of the test sites of Morfeo project, has been frequently involved in reactivation phenomena the most recent of which occurred on March 2006. The system was placed on the lateral boundary of this landslide (fig.2) with the aim to monitor in real-time the rainwater infiltration into the soil by combining different investigation techniques. All the measurements (ERTs, TDR and piezometric) are performed along the same profile (AB in fig.2) with time intervals selected by considering the rainfall intensity and frequency coming from a rain gauge installed in the area. The next step will be to integrate all the acquired information in order to evaluate the applicability of ERT in the monitoring of rainwater infiltration and in the estimation of water content. PROTOTYPE SYSTEM PROTOTYPE SYSTEM The system consists of the following components (fig.3): - a 48-channel cable (a) connected to a georesistivimeter Syscal Pro Switch 48 of the IRIS Instruments; - n. 48 steel electrodes (b) placed at a distance of 1 m; - n. 2 holes equipped by piezometers (c) and placed along the same profile of ERT acquisition; - n. 2 TDR probes 20 cm length (d) connected to the Soil Moisture Equipment Corporation TRASE and installed at different depths (1 m and 1,5 m) in a hole located close to the n.35 steel electrode; - a rain gauge. All components of the system are connected to a pc that is managed in remote-control Fig. 2 – Location of the prototype system and acquisition profile. Fig.1 – Photo (modified by Google Earth) reporting the location of the Picerno landslide in Basilicata region (southern Italy). Fig.3 – Prototype system and its main components PRELIMINARY RESULTS PRELIMINARY RESULTS TIME TIME-LAPSE 2D LAPSE 2D ERTs ERTs. At first, just after the installation, some ERTs were performed to verify the functioning of the system. For the acquisition three different electrode configurations were chosen: the Wenner, the Wenner – Schlumberger and the Dipole – Dipole in order to better test which of the three allowed to highlight the features of the subsoil and the presence of the piezometric surface. Afterwards, using the “time-lapse” mode and a Wenner configuration, the acquisition of electrical resistivity data was started. The number of acquisitions was fixed in four per day, with a constant interval of six hours between two acquisitions in succession, starting from 1:30 am. Figure 4 shows the “time-lapse” 2D ERTs obtained using the data acquired in a single day. ERTs reach an investigation depth of about 8 m and are characterized by a large range of resistivity values between 0.81 and 652 Ω m. The water table, characterized by very low resistivity values, is assumed to be at a depth of about 2.5 m. This is also confirmed by piezometric measurements. Figure 5 reports the percentage change (c) in model resistivity between the first (a) and the fourth (b) acquisition. Although the single ERTs seem to be quite similar, the model of percentage change shows a significant variation above all in correspondence of the water table zone. Fig.4 – 2D time-lapse ERTs performed every 6 hours in the same day Fig.5 – 2D percentage change model resistivity (c) related to the first (a) and the fourth (b) acquisition performed the same day TDR MEASUREMENTS. TDR MEASUREMENTS. Since February 2010 soil moisture measurements have been acquired every 6 hours by using two TDR probes 20 cm long, vertically installed at depths of 1.00 and 1.50 meters. Figure 6 reports the acquisitions performed up to date. The percentage content of soil moisture and its evolution seem to be constant in the time. This could be due to the soil high saturation degree which is characteristic of winter season. …WORK IN PROGRESS... WORK IN PROGRESS... Taking into account the information obtained by the preliminary time-lapse 2D ERTs, also compared with TDR and piezometric measurements, the next step of the work will be to integrate all the data. It should allow to verify the reliability of time-lapse ERTs in the monitoring of the groundwater table variations and in estimate of water content. a b c d a b c Fig.6 – Soil moisture content profiles REFERENCES REFERENCES Michot D., Benderitter Y., Dorigny A., Nicollaud B., King D. and Tabbagh A. (2003). Spatial and temporal monitoring of soil water content with an irrigated corn crop cover using surface electrical resistivity tomography. Water Resources Research 39 (5), 1138; Cassiani G., Bruno V., Villa A., Fusi N. and Binley A.M. (2006). A saline trace test monitored via time-lapse surface electrical resistivity tomography. Journal of Applied Geophysics 59, 244–259; Werban U., al Hagrey S.A. and Rabbel W. (2008). Monitoring of root- zone water content in the laboratory by 2D geoelectrical tomography. J. Plant Nutr. Soil Sci. 171, 927–935.