GNGTS 2013 - Atti del 32° Convegno Nazionale

The system consists of up to 16 channels to measure the voltages between the potential electrodes and a reference electrode connected to the system ground. The potential electrodes (Fig. 1d) are connected to 4 amplifier boxes (Fig. 1c) with a 4-channel buffer amplifier within each box and then the signals are fed to the acquisition device (Fig. 1b), powered by an external battery. The data acquisition device is a NI-sbRIO-9636 by National Instruments. It features 16 single-ended (8 differential) 16-bit analog inputs at 200 kS/s, +/- 10V maximum input range, a real-time processor, a user-reconfigurable FPGA, 256 MB of DRAM for embedded operation and 512 MB of nonvolatile memory for storing programs and data logging. The 16-bit analog- to-digital converter allows to obtain a sensitivity (LSB) of 32 µV when the +/- 1V input range is selected, much higher than the minimum required of 0.1 mV (Revil and Jardani, 2013). The sbRIO-9636 is hereby used as a standalone device and it is programmed for data logging and real-time data processing in LabVIEW programming environment. The acquisition is controlled by the FPGA and a sampling frequency of 1 kHz for each channel is chosen. The signals are then filtered and downsampled to 1 Hz by the real-time processor and data is stored on the internal nonvolatile memory. Measurements of self-potential are subject to a number of errors such as the instability of the measuring electrodes and near surface noise due to anthropogenic activities (e.g. power lines, electromagnetic interference), telluric currents, meteorological effects or natural processes associated with soil electrochemical reactions or root activities (Perrier and Pant, 2005). Other common sources of error are related to the measurement of voltage sources with high internal impedance (i.e. high contact impedance between electrode and soil). In this case the measurements are subject to loading errors and offset current errors occurring at the input of Fig. 1 – a) Block diagram of the measurement system designed for the self-potential monitoring; b) the data acquisition device NI-sbRIO-9636 by National Instruments; c) the amplifier boxes with the four-channel JFET- input operational amplifier (Texas Instruments TL064); d) the graphite electrodes. 139 GNGTS 2013 S essione 3.2