GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.2 203 addition, the rising salt water also occurs along the bed of the river Volturno as the bottom of the river is located at the mouth, about 3.5 m below the sea level (Corniello et al. , 2010). Geophysical methods and data processing. The Electrical Resistivity Tomography (ERT) consists of the experimental determination of the apparent resistivity ρ of a given material, by joint measurements of electric current intensity and voltage introduced into the subsoil through separate couples of electrodes, driven in the ground surface. All natural rocks can conduct electricity when subjected to an electric field. The measure with which the rocks are crossed through the current depends on the type of resistivity that they present. The resistivity parameter is influenced by: texture and porosity, degree of cementation, the temperature of the rock, clay content, water content and its temperature and salinity. Furthermore, under equal lithological conditions, there are some geological processes that cause an immediate variation of resistivity because they change the porosity. In general, many of these processes lead to a reduction of the resistivity as: clay alteration, dissolution, billing rock, saltwater intrusion. The instrumentation used for the measurement of the resistivity consists of two parts: one for the measurement of the current intensity I injected into the ground through the electrodes A and B and one for the measurement of the potential difference ΔV between the electrodes M and N . In the experimental surveys reported hereafter, the ERT data have been gathered through electrodes of length equal to 40 cm. The electrodes were then connected through multichannel cables, adopting the Wenner- Schlumberger array configuration. This type of arrangement is hybrid between the Wenner and Schlumberger arrays (Pazdirek and Blaha, 1996): during the acquisition, the wiring is continuously changed so that the spacing a between the ‘potential electrodes’ remains constant, while that between the ‘current electrodes’ increases as a multiple n of a . The value of n , in this case is given by the ratio between the distance of the electrodes A-M (or N-B ) and the spacing between the electrodes of potential M-N . For this array the distribution of the measurements is comparable with the Wenner array, but the horizontal coverage is better. The choice of such arrangement was due to the necessity to study areas in which both lateral and vertical variations of resistivity are present. The resulting horizontal distribution of the underground data points in the pseudo-section, in fact, is comparable with that typical of the Wenner array, but their vertical resolution is better. Moreover, this type of array is a fair compromise between the device Wenner and the dipole-dipole. The intensity of the signal is smaller than the Wenner but is higher than the dipole-dipole axial. At constant distance between the current electrodes, the depth of investigation that can be achieved with the device Wenner-Schlumberger is 10% higher than the Wenner device. The geoelectric measurements of resistivity were executed with the georesistivimeter “SYSCAL Pro” of Iris Instrument. Within the chosen area for 4D monitoring of the salt wedge, geometrically similar to a rectangle of about 4600 square meters (length 115m and width 40 m), the geoelectric surveys performed by acquiring 9 geoelectric profiles. These profiles are arranged parallel to each other and with a spacing of 5 m and the multi-electrode resistivity measurements used 24 electrodes, for a total of 216 electrodes. Data acquired have been processed using 3D inversion technique performed with ERTlabplus software. Data inversion started from a discretized model of the investigated area, constructed starting from average apparent resistivities on measured pseudosection. The inversion procedure uses a smoothness-constrained least-squares routine implemented into Occam’s optimization algorithm (La Brecque et al. , 1996b), which allows determining iteratively a 3D resistivity model for the subsoil. Result and discussion. The results of the inversion procedure are three high resolution ERT 3D models in different periods, more specifically in May and October 2013 and in May 2014. These acquisitions have allowed to obtain a 4D monitoring of saline intrusion in a specific subsurface “volume”. Resistivity data processing within that volume has therefore defined the electrical characteristics and geometry of the subsurface going to spatially delimit the intrusion