GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 661 during the Pleistocene, integrated all the consequent rivers of the basin and formed one of the major tributaries of the English Channel (British Geological Survey, 2017). The shallow zone is characterized by several river terrace order. River terrace sequences are important frameworks for archeological and geotechnical engineering purposes. Our area named ‘Chilling’ is located near to Fareham town and it presents only terrace of 2 order (Briant et al. , 2012). The 2 order terrace outcrops the Bracklsham Group. The Bracklesham Group consists of four geological formations: Wittering, Earnley, Marsh Farm and Selsey (Curry et al. , 1977). The sediments are mostly sands and silts and are rapidly eroded when exposed to tidal action. We carried out N.6 electrical resistivity / IP survey acquired with sea-land geometry and so we processed the coupled dataset. Methodology and field measurement. Electrical imaging surveys are widely used in many environmental and engineering problems. Geoelectrical surveys scope is the reconstruction of buried lithologies, by measuring the distribution of the electrical potential difference in the subsoil induced by the introduction of a known electric intensity current and recording the subsequent apparent resistivity and chargeability. Recently, several studies have also been carried out in water-covered areas where the acquisition it is more difficult. Our land surveys include conventional technique with multi-electrode resistivity meter systems. In the land survey lines the electrodes start from the land side arrived up to at the area more low tide. In the surveys that are located entirely within a water-covered environment, the electrodes are installed mounted on a streamer that is towed behind a boat. The streamer is dragged along with the help of the floats on the water surface. Marine cable consist of the graphite electrodes for transmitting (electrode distance variable from 5m to 10m) and for simultaneous measuring of ten potential channels. To correctly position, the cable the georesistivimeter was connected to a GPS receiver. Near-shore survey was carried out along the same alignments performed during the on-shore area. Once those longitudinal lines are completed to survey the data are checked and then it is possible to proceed with the transversal lines starting from the outer limit to the shore boundary. The smoothness-constrained least-squares inversion method, that is used to interpret the data from land surveys, is adapted for underwater surveys. For the inversion model, a distorted finite-element grid is used to calculate the apparent resistivity values to accommodate the underwater topography. The first few rows of elements are used to model the water layer, while the lower part of the grid is used for the sub-bottom resistivity distribution (Loke et al. , 2004). Tab. 1 reports the data sheet of the electric survey system. The sea-land data were merged taking into account of different sampling apparent resistivity and for an accurate inversion, the water resistivity as well as the depth to the bottom surface must be accurately known since a large proportion of the current flows through the water layer. The water resistivity in the model is fixed, and the inversion program attempts to determine the resistivity of the cells that would most accurately reproduced the observed measurements. To invert the data we used the RES2DINV which is a computer program that will automatically determine a two-dimensional resistivity model (2D) for the subsurface for the data obtained from electrical imaging surveys (Griffiths and Barker, 1993). We acquired several sea-land line and merged the data considering at least an overlapping of 10%. Overlapping data were fixed by a smoothing operator that takes into account the local data weight of the two dataset. 2D sea-land model used by the inversion program, which consists of a number of rectangular blocks and his arrangement is loosely tied to the distribution of the data points in the pseudosection. The distribution and size of the blocks is automatically generated by the program using the distribution of the data points as a rough guide. However, to handle data sets with a non-uniform electrode spacing we used an appropriate electrode spacing. A forward modelling subroutine is used to calculate the apparent resistivity values, and a non- linear least-squares optimisation technique is used for the inversion routine (deGroot-Hedlin and Constable, 1990; Loke and Barker, 1996). We utilized the finite-difference for forward modelling techniques.