Brancolini, et al., (2007) reported that the project characterized of the Late Pleistocene and Holocene depositional sequences with a detail, that has never been reached in the past. A boat, particularly suitable for very high resolution seismic survey in shallow water (less than 1 m deep) has been fully equipped and tested. It allowed to carried out surveys of specific areas of the Lagoon that have never been investigated in the past. Moreover a seismo-stratigraphic model of the lagoon and gulf of Venice subsoil has been performed. The seismic acquisition system allowed to investigate the subsoil down to 30-40 m deep. The new data provided significant geological features related to the Late Pleistocene and Holocene evolution of the investigated areas: the southern lagoon, the Lido and Chioggia inlets and part of the shelf of the Gulf of Venice.
Most of the information on the Venetian subsoil come from the study of cores taken in the lagoon outer part, in the channels, in the islands, along the littoral strips or in other sites where sediments are not completely representative of the Holocene sedimentation. In particular, the upper layers do not represent the “true” stratigraphic succession because of the occurrence of bottom erosion, the dredging activity, the sediment rework and others effects, reflecting human activities. In addition, the cores provide information of a very restricted area.
Recently, seismic surveys were carried out along the lagoon channels [Tosi et al., 2007a,b] that normally are 4-6 m deep. However, considering that most of the Venice Lagoon consists of very shallow water, 0,5-1 m deep, the obtained data lack many information (Fig. 1). The authors considered the subsoil below the lagoon’s very shallow water a window on the recent most evolution of the lagoon environment because in a number of cases it preserves the whole Holocene history. All the previous seismic surveys in the Venice lagoon have been carried out along the ship canals [Tosi et al., 2007a,b]. Nevertheless the ship canals are scarcely representative of the recent sedimentation because they represent a very small portion of the entire lagoon and they are frequently dredged. An ad hoc boat particularly suitable for very shallow water was acquired, i.e the CORILA Aretusa. The criteria followed to choose the boat have been the draught (less than 50 cm), the facility to install the seismic gear, the level of
noise produced during the low speed steaming, the capability to operate also during poor weather conditions and finally the budget. Based on these criteria, the authors have chosen a Saver Fisher 22, 6.75m length and 2.48m breadth, with a carry capability of 7 passengers and with a 140 Hp, four stroke engine.
The Aretusa was implemented with advanced instrumentation for very high resolution seismic survey capable to distinguish layers of a few centimeter thick, in 0.5-1 m water depth and able to investigate subsoil down to 30-40 m deep. The seismic tools adopted for the survey consisted in a two channel, 24 bit acquisition system integrated with the GPS receiver and an impulsive seismic source (boomer) with a band amplitude between 200 and 9000 Hz. The system successfully operated up to 50 cm of water depth. After a careful evaluation of the lagoon setting, the new surveys were carried out in four areas, representative of specific themes in the recent evolution of the lagoon: the southern lagoon, the Lido and Chioggia inlets, the shelf off the Lido
inlet.
About 400 Km of new seismic lines, about 300 km of seismic data collected in the framework of previous surveys and a number of selected data and information regarding multidisciplinary analyses of samples from cores [Tosi et al., 2007a,b] allowed the characterization of the seismic-stratigraphic and sedimentary setting that pointed out the reconstruction of the Late Pleistocene and Holocene (about last 30,000 years) sequences with a detail that has never been reached in the past. The southern lagoon. In this area the thickness of the Holocene sediments is the highest of the entire lagoon (up to 22 m). Based on the new seismic data, it has been possible, for the first time, to image and map the three main phases that characterize the formation and the evolution of the lagoon: the marine ingression between 10,000 and 6,000 years B.P. that produce the submersion by the Adriatic sea of the previous alluvial plane, the following high stand of the sea level, that records the predominance of the sediment supply form rivers and the progressive advance of the coastline toward the sea; and finally the predominance of the erosion and exportation of sediment from the lagoon, consequence of the rivers diversion operated by humans since historical time.
These distinct phases are associated to sedimentary deposits with different geotechnical, sedimentological and geochemical characteristics. They play different roles in the erosion process of the sea floor and in the hydrogeological regime, like for example the sub-merged littoral belts, already recognized form the bathymetric surveys, but much better defined in the new seismic images or the paleo-channel network, filled by more recent sediments.
An important observation is that relevant volumes of gas seeps have been detected by the seismic images that may have important impacts in the ecology of the entire lagoon. The Lido and Chioggia inlets. These two areas underwent important human interventions in the most recent years. The thickness of the recent sediments in the Lido inlet shows a relevant asymmetric distribution, with more than 12 m near the northern side of the inlet and absence of sediment in the southern side. The setting of the Chioggia inlet is exactly the opposite: the higher thickness has been measured in the southern side (up to 7 m) while the sediment are eroded or not deposited on the northern side of the inlet.
The shelf off the Lido inlet. Seismic grid off the Lido inlet detected and characterized a wedge-shaped deposit that has been interpreted as an ebb-tidal delta, produced by the erosion of sediment in the lagoon and their exportation across the inlet by the tide fluxes. The internal geometry and the elongate shape of the ebb-tidal delta testify the interplay between the tide and the North Adriatic along-shore current. Based on the comparison between maps of the ebb-tidal delta produced by the seismic survey and some historical bathymetric maps, we calculated a depositional rate of 5 cm/year for the last 120 year.
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