GNGTS 2013 - Atti del 32° Convegno Nazionale

Portable low-cost measurement system development for Self- Potential (SP) monitoring in severe environmental conditions M. Masi 1 , V. Pazzi 2 1 Department of Information Engineering, University of Firenze, Italy 2 Department of Earth Sciences, University of Firenze, Italy Introduction. In the field of environmental sciences self-potential (SP) data are generally considered as promising and reliable to investigate subsurface properties, especially for their ease of acquisition and simplicity in making qualitative interpretations (Revil and Jardani, 2013). Although it is one of the oldest geophysical method, the self-potential technique is currently adopted in a broad range of both qualitative and quantitative applications such as localizing and quantifying groundwater flows (Revil et al. , 2006; Jardani et al. , 2007; Jouniaux et al. , 2009), characterizing volcanic areas (Fournier, 1989; Di Maio et al. , 1996; Zhang and Aubert, 2003), monitoring contaminant plumes (Weigel, 1979; Naudet et al. , 2003), studying landslides (Perrone et al. , 2004; Naudet et al. , 2008) and geothermal exploration (Corwin and Hoover, 1976). Self-potential is a passive method consisting in the measurement of the electric potential at a set of measurements stations. SP anomalies usually indicate the presence of a source of current in the ground due to subsurface disturbances (e.g. ground waters, geochemical phenomena). Three main contributions are widely recognized as a source of occurrence of SP signal (Revil and Jardani, 2013): the reduction-oxidation (redox) potentials, the diffusion potentials and the electrokinetic effect. The presence of an electronically conductive body creates an oxidizing area acting like an anode, and a reducing area acts like a cathode. This source of electrical potentials is referred to as the redox potential. Besides, the diffusion potential is related to the concentration gradients of ionic species in pore water. The electrokinetic effect (or streaming potential) is associated with the drag of excessive charge by the flow of water in porous rocks, causing a net source current density. The SP method can also be applied as a monitoring method (Perrier et al. , 1998; Trique et al. , 2002) where a multi-electrode array is used to track the changes of the subsurface variables with time. Monitoring strategies are generally required to evaluate the performance of engineered environmental control systems (e.g. remediation systems, leachate in landfills), to assess potential environmental impacts and public health risks from contaminant releases or to characterize environmental processes including chemical transformations, diffusion and advection phenomena or biological reactions, occurring in natural or engineered systems. Measuring the temporal variations of the electric potential is an effective tool to characterize and monitor these phenomena especially with regard to hydrogeological fluxes and electrochemical processes in the subsoil. In several occasions the monitoring instrumentation is required to operate unattended or is exposed to environmental condition in which a degradation/oxidation of electromechanical parts is expected. In such situations the investment for the equipment could be economically unfeasible. In order to fulfill such requirements we developed a measurement setup with the main advantages of being portable, low cost, rugged, sufficiently accurate and optimized for low power consumption. The measurement setup was first checked in laboratory for reliability and accuracy then was evaluated during a data acquisition campaign in Kenya to measure the SP variation linked to the fluctuations of the sea level during tidal events. Apparatus. Measurement system. The measurement system (Fig. 1a) was designed to monitor electrical potential in soil and sediments in locations where rugged instrumentation is required because of the harsh environmental conditions or when instrumentation security is critical. 138 GNGTS 2013 S essione 3.2