GROUNDWATER INVESTIGATION IN LAGOON SUBSURFACE WITH AIRBORNE ELECTROMAGNETICS: THE VENICE LAGOON SKYTEM SURVEY EXAMPLE A. Viezzoli 1 , P. Teatini 2 , L. Tosi 3 1 Aarhus Geophysics, Denmark 2 Dept. Mathematical Methods and Models for Scientific Applications, University of Padova, Italy 3 Institute of Marine Sciences, CNR, Venice, Italy Introduction. Understanding the hydrogeological processes is critical for a sound management of groundwater resources in costal areas. Here lie majority of human settlements, industrial produc- tion, and fish farming. Human pressure on the coastland environment is constantly increasing, and many studies predict a rising of seawater level in the next 50 years raging from few cm up to sev- eral tens of cm, with expected threatening consequences (e.g., Carbognin et al., 2009). If these are common characteristics of most costal areas, wetlands, lagoons and estuaries also have often unique flora and fauna depending on the groundwater-surface water processes. The hydrologic setting of the transitional environments is complicated by their Late Quaternary subsoil architecture. The deposits represents the transition through the fluvial in tide-dominated depositional systems trig- gered by the sea level changes. In particular, in the Venice area numerous geomorphological fea- tures representing i.e. fluvial paleoriver beds, ancient tidal channels, and paleobeach ridges occur (Tosi et al., 2009). These features are generally filled by sandy deposits and can be considered pref- erential path for the groundwater flow, both in the horizontal and vertical directions. In order to have a better understanding of the hydrogeological setting of these areas, and also to produce more useful models, it is crucial to acquire information both inland and within the lagoon or wetland, covering both its permanent wet and tidal areas. Acquiring information that can be used to model the groundwater processes of these areas is often logistically challenging and therefore expensive and slow. This applies both to punctual, invasive and direct measurements such as depth to groundwater table and salinity from boreholes, to non invasive, area covering, indirect data such as resistivity or seismic investigations. Apart from the logistics, in many cases the quality of the data reflects the spatial and or temporal alternation of dry land and ponds-marshes-surface water in gen- eral. Airborne electromagnetics (AEM) can greatly improve the data quality and coverage in such areas, while cutting significantly the acquisition costs. Its direct output is geoelectrical cross sec- tions or maps that are then used as input for hydrogeological models. The application of AEM for groundwater monitoring and modeling has been steadily rising in the past decade, due to parallel developments of better AEM systems and processing, e.g. inversion methodologies. However, so far there have been extremely limited attempts of applying AEM to areas such as lagoons, wetlands, rivers or bays. This manuscript shows that AEM can produce quantitative results useful for ground- water modeling also in these areas, presenting the results of a survey carried out in the central and southern sectors of the Venice Lagoon, Italy, by the SkyTEM system. We present some of the inversion outcome as horizontal average resistivity maps at different depth intervals and cross sections obtained by SkyTEM application in the two areas where differ- ent hydrogeological processes are under investigation. Results of the AEM survey. The SkyTEM system is often used for groundwater mapping. Examples of other systems often applied in hydrogeophysical investigations include the fixed wing transient (i.e., time domain) Tempest system, and the frequency domain helicopter borne systems Resolve from Fugro and a DiGHEM alike from BGR. SkyTEM was chosen as its dual moment pro- vides a bandwidth, i.e., a penetration range (from shallow to intermediate depths) suitable for this particular target. The excellent signal to noise ratio at late times due to the presence of the good con- ductor allows for using a base frequency of 12.5 Hz, thus reaching deeper penetration than usual. Data are processed to eliminate artifacts and assign noise levels at late times, and stacked to increase signal to noise ratio while preserving lateral resolution. They are then inverted starting from a homogeneous half space, using the Spatially Constrained Inversion (SCI) technique (Viezzoli et al., 609 GNGTS 2009 SESSIONE 3.1 GNGTS 2009 SESSIONE 3.1