GNGTS 2013 - Atti del 32° Convegno Nazionale

the electrolytic compartments. The electrodes have proved to be sufficiently stable with a maximum drift of 0.8 mV over the entire period. Field tests. The instrumental apparatus has been tested during a scientific expedition in the north- western margin of Mida Creek (03°20’S, 40°5’E), a wide swamp about 25 km south of Malindi (Kenya) (Fig. 2a, b). Mida Creek is bordered by a mangrove belt consisting of three different species: Avicennia marina at the landward level and Rhizophora mucronata and Ceriops tagal at the seaward level. In the test area the whole belt is about 300 m wide: the area dominated by Avicennia marina is nearly the half and it is comprised between the average high water spring tide and the average high water neap tide. Within the creek the tidal range is at least 2 m, while on the surrounding coast is about 4.2 m (Vannini et al. , 2008). Sixteen electrodes have been placed to obtain a C-shaped array (Fig. 2c): this configuration was selected in order to measure the self-potential variation along the sea-land direction (NS direction, electrodes 5-12) and along two profiles perpendicular to this direction (EW direction, electrodes 1-6 the southern and electrodes 12-16 the northern). The distance between two adjacent electrodes was 10 m. The reference electrode was located at the centre of the array as shown in Fig. 2c. The acquisition run for about 10 days (245 hours). The sampling interval was 1 s. As an example, sample data corresponding to a semidiurnal cycle of the tide (12 hours duration) were selected from the whole time-series. The data have been pre-processed with a low-pass filter in order to remove the high-frequency content from the signals. Time-lapse mapping of the SP anomalies was obtained from the data at five different time instants (Fig. 3). The tide data were obtained by reconstruction from the tide heights recorded in Mombasa harbour. The data then were shifted in time to match the delayed arrival of the tide in Mida Creek, calibrated with on-site sea level measurements. The results are shown in Fig. 3. Negative SP anomalies are observed throughout the selected period and they were of the order of magnitude of hundreds millivolts with respect to the voltage at the reference electrode. The changes in the sea level clearly affect the SP response. This effect is most likely due to both the electrokinetic effect of the tidal volume flowing through the subsurface and the change in the ionic concentration and species of the pore water (Martínez-Pagán, 2010). Conclusions. The design and implementation of a low-cost setup for SP monitoring in the field was discussed. The main advantages of such setup are the portability, the ruggedness and the low power consumption while ensuring a good measurement reliability. The system is capable of measuring the electric potential from up to 16 electrodes referenced to a common ground (reference electrode). The maximum acquisition rate in 200 kS/s per channel and the analog-to-digital converters resolution in 16 bits, allowing for a sensitivity of 32 µV. The recorded data is stored in an internal non-volatile memory for standalone operation. The measurement errors due to source loading and offset currents are minimized through the use of Fig. 2 – The test area in Kenya, about 25 km south of Malindi (a) in the north-western margin of Mida Creek (b); c) the C-shaped self-potential array. 141 GNGTS 2013 S essione 3.2