GNGTS 2018 - 37° Convegno Nazionale

680 GNGTS 2018 S essione 3.2 Method and Results. The ERT surveys were performed by means of a Syscal Junior (Iris Instruments) resistivity meter, coupled with a multielectrode acquisition system (48 electrodes). A constant spacing “a” (2.5 or 10 m) between adjacent electrodes was used. Along each profile, Wenner (W) and/or Wenner-Schlumberger (WS) configurations were adopted with different combinations of dipole length (1a, 2a and 3a) and number of depth levels “n” (n ≤ 6). The investigation depths were about 15 m (for a = 2.5 m) and 80 m (for a = 10 m). The apparent resistivity data were inverted using the RES2DINV software (Loke, 2001) to obtain the 2D resistivity images of the subsurface. The inversion routine is based on the smoothness-constrained least-squares inversion method implemented by using a quasi-Newton optimisation technique (Sasaki, 1992; Loke and Barker, 1996). The optimisation method adjusts the 2D resistivity model trying to iteratively reduce the difference between the calculated and measured apparent resistivity values. The root-mean-squared (RMS) error provides a measurement of this difference. The ERT interpretation has been almost based on the scarce geological and stratigraphic data found in literature and on the information of local people. In particular, from a comparison between the ERT and borehole data, reported in the work of Da Costa et al. , 2012, it has been possible to approximately associate resistivity values/patterns to different litho-stratigraphic setting. Apu Kedji. The first ERT was carried out at Apu Kedji site, a rural village located about 15 km north-est of Lomé, capital of Togo, where the no-profit organization “Il Pozzo della Farfalla” realized the first well for water supply in 2010 (Fig. 2). In this site, the main aim of the survey was to test and calibrate the results obtained from the electrical image of the subsurface with the presence of groundwater at a depth of about 5-6 m below ground level (bgl) as roughly estimated through the water well. The electrical images were performed with an electrode spacing of 2.5 m and 48 electrodes, reaching a total length of the profile of 117.5 m and an investigation depth of about 15 m. Fig. 2 shows the 2D resistivity model related to the ERT profile centred on the existing water well. For the achievement of best fit between the calculated and measured apparent resistivity values, three iterations were performed, obtaining a resistivity model with a relatively low RMS error of 4.2%.The electrical image is characterized by a relatively low resistivity range, varying from 5 to over 127 Ωm, and by an almost piano-parallel electrostratigraphic pattern. The shallow irregular and thin resistive layer can be associated to sediments (clayey sands?) with low water content, whereas the underlying conductive layer can be associated to the same saturated sediments, as testified by the presence of water into the existing well. Kevè. The second site is a populated zone, near to the village of Kevè, located about 45-50 km north-west of Lomé, where the no-profit organization “Il pozzo della Farfalla” aimed to realize the second well for water supply. In this site, two ERT were acquired close to the catholic parish of “Sacré-Coeur de Jésus”: the first one, centred on Rectory, at relatively low resolution, with an electrode spacing of 10 m and 48 electrodes, reaches a total length of the profile of 470 m and an investigation depth of about 80 m; the second one at relatively high resolution, with an electrode spacing of 2.5 m and 48 electrodes, reaches a total length of the profile of 117.5 m and an investigation depth of about 15 m. The first deeper ERT was performed to obtain information about the depth of the crystalline bedrock, whose presence was supposed on the base of geological data, whereas the second shallow ERT was carried out in order to characterize the near surface lithological, stratigraphical and hydrogeological setting. The 2D resistivity models were showed to the local drill company, that searched for an appropriate area to drill under the approval of the population. The ERT gave important guidelines to the company in preparatory and operational phases. The aquifer was founded at about 55 m