Advances in Surface-Groundwater Modelling in Lagoon Environment with Airborne Electromagnetics and High Resolution Seismic: Example from the Venice Lagoon

Lagoon environments are very important for groundwater modeling in costal areas, they are delicate and in rapid evolution due to global climatic changes. Airborne electromagnetics (AEM)is a very valuable methodology that can provide high density, high quality data to produce 3D hydrogeological models to depths in excess of hundred meters below surface water column. We present the results from the SkyTEM Venice lagoon survey of 2009, integrated with data from very high resolution seismic survey. The AEM data results enhance greatly the understanding of the hydrogeology and surface-groundwater interactions in the lagoon area, where indirect measurements abound but wells are missing. For example, there is clear evidence of fresh water aquifers underneath the central part of the lagoon, at depth of about 40 m. The near surface part of the AEM data compare well with seismic data, showing that main reflectors come from the interface between the superficial Late Pleistocene looser, saline water saturated sediments and the deeper, more compact and fresher Holocene sediments. There is also clear evidence of submarine groundwater discharge in the lagoon, of paleorivers, and a possible indication of gas seepage trough shallow sediments. Seismic and AEM provide complimentary datasets to discriminate between pore water salinity, lithology and gas. Seismic horizons can actually be included during inversion of AEM data, producing more robust output. AEM data from the southern part of the survey that crosses the shore line and continued also onshore allow a clear mapping of the saline water intrusion inland, and highlight the relationship between pore water salinity of the lagoon sediments and spatial distribution of salt marshes. The latter seem to act like salt sinks, increasing sediments electrical conductivity

Saltwater contamination in the lowlying coastland of the Venice Lagoon, Italy

TIle southem portion ofthe Venice coastland includes a very precarious environment. Due to an elevation down to 4 m below msl, ilie Venice Lagoon atld Adriatic Sea proximity, and the encroachment of seawater from ilie mouth of the river network up to 20 km inland, salt contamination of land atld groundwater is a severe problem that is seriously affecting the farmland productivity. An interdisciplinary multi-scale research is ongoing with the aim of understanding ilie contamination process, quantifying the effect of ilie saltwater intrusion of the crop production, and proposing possible mitigation strategies. A 21-ha representative basin has been selected and deeply monitored from the hydrogeological atld agricultural points of view. It has been clearly outlined that in the upper 5 to 10 m mainly the lowpermeable soils are contaminated by salt. Conversely, fresh to brackish waters are located in the sandy elongated paleo-channels. This is likely due to the origin of the area which was a salty marshlatld since one century ago. The freshwater supplied for almost 100 years by the rainfall and leakage from the river and chatlnel beds has been able to reduce ilie salt concentration only in the highly permeable deposits

Peatland Volume Mapping Over Resistive Substrates With Airborne Electromagnetic Technology

open6siDespite the importance of peatlands as carbon reservoirs, a reliable methodology for the detection of peat volumes at regional scale is still missing. In this study we explore for the first time the use of airborne electromagnetic (AEM) to detect and quantify peat thickness and extension of two bogs located in Norway, where peat lays over resistive bedrock. Our results show that when calibrated using a small amount of field measurements, AEM can successfully detect peat volume even in less ideal conditions, that is, relatively resistive peat over resistive substrata. We expect the performance of AEM to increase significantly in presence of a conductive substratum without need of calibration with field data. The organic carbon content retrieved from field surveys and laboratory analyses combined with the 3-D model of the peat extracted from AEM allowed us to quantify the total organic carbon of the selected bogs, hence assessing the carbon pool.openSilvestri S.; Christensen C.W.; Lysdahl A.O.K.; Anschutz H.; Pfaffhuber A.A.; Viezzoli A.Silvestri S.; Christensen C.W.; Lysdahl A.O.K.; Anschutz H.; Pfaffhuber A.A.; Viezzoli A

Glacial, \ufb02uvioglacial and \ufb02uvial sedimentary discharge in the northwestern coastal sectorof the Ross Sea continental margin since upper Miocene to LGM

The Borchgrevink Coast stretches from the Coulman Island northward to the Cape Adare for at least 200 km, bordering the western side of the northern Drygalski Trough. The early phase of the acquisition objectives of the PNRAProjectGlevors(GlacialEvolutioninthenorthwesternRossSea,offshoreNorthVictoriaLand,Antarctica) was accomplished in the north western coastal sector of the Ross Sea continental margin along the Borchgrevink Coast, from Coulman Island to Cape Hallett. Single channel seismic and sub bottom pro\ufb01les, swath bathymetry and gravity cores were collected during the Austral summer of 2016/2017, by the scienti\ufb01c research vessel OGS/Explora. The studied area along this segment of the Borchgrevink Coast documents, by the analysis and the interpretation of available geophysical, geological and oceanographic data, the paleo ice discharge and ice \ufb02owing patterns of the inferred Mariner-Borchgrevink and Tucker coastal glaciers, since at least the Upper Miocene until the Holocene Time. The repeated and possibly asynchronous oscillations of these valley glaciers from the North Victoria Land coast, at least about tens km offshore, and their interaction with fast \ufb02owing ice streams from the south are recorded. In particular, the analysis of the architecture and of the geometrical relationship of the interpreted seismic facies and units allows to infer past glacial and interglacial environments. Modelling of paleo environments and related climate condition is achieved despite the age constrain uncertainty of local seismic stratigraphy and of biostrathigraphic correlations to coeval sediment section in southern Ross Sea inner-shelf sector (McMurdo Sound), and besides the acoustic facies ambiguity due to not enough data resolution. The interpreted shallower and Holocene-Present glacial related features are simple or composite ice-marginal landforms, with overstepping smaller recessional deposits on top or behind. They testify the coastal glaciers grounded events and the ice retreat modality during the pre-LGM and the LGM. We infer that northern coastal glaciers, from the Tucker glacier northward, about 72\u25e6 latitude, did not advance or reach the northwestern Ross Sea shelf edge at the LGM, but possibly before. The grounding line of NVL coastal glaciers would correlate with the pre-LGM grounding lineament reconstructed for the major ice-streams \ufb02owing from the south. More ancient and buried wedge, underlying the main composite Grounding Zone Wedge (GZW) system raises the issue if it was possible the formation and the preservation as a pinning point, of a previously developed GZW or of the upper part of a \ufb02uvioglacial delta in late Miocene or Pliocene time. Moreover, further offshore from the coast, the evidence, of an ancient buried glacial/\ufb02uvioglacial or \ufb02uvial delta, embedded within the glaciomarine clinoforms of the Trough Mouth Fans (TMFs) deposits \ufb01lling the Northern Drygalski Trough, suggest climate change and an inferred preceding coastal open-sea condition

A new tool available for hydrogeologists: the Airborne EM method

The Airborne EM method has nowadays reached a great degree of progress, so that it can be considered a reliable tool for the hydrogeological modeling. It is indeed able to provide a detailed distribution of resistivity, so that it is possible to draw information of stratigraphic and, sometimes, of hydrochemical type. But, in order to achieve this outcome, it is demanded a high performance of the instrumentations, as such as an accurate data processing. We show some examples of application of the AEM method, for the solution of problems connected to the groundwater mapping, to the seawater intrusion and to the improvement of the hydrogeological models

Groundwater investigation in lagoon subsurface with airborne electromagnetics: the Venice Lagoon SkyTEM survey example

Understanding the hydrogeological processes is critical for a sound management of groundwater resources in costal areas. Here lie majority of human settlements, industrial production, 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 several 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 triggered by the sea level changes. In particular, in the Venice area numerous geomorphological features 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 preferential 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 general. 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 sections 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 groundwater 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 different hydrogeological processes are under investigation.PublishedTrieste, Italy6A. Monitoraggio ambientale, sicurezza e territorioope

Joint Inversions of AEM modelling AIP effects: Helicopter-borne, Ground IP and Fixed-Wing systems

<p>It is nowadays widely accepted that Induced Polarization (IP) effects can affect Airborne Electromagnetic (AEM) measurements. Modelling the AEM data with a dispersive- resistivity allow to properly retrieve the halfspace parameters avoiding high inversion misfits and wrong structures. Even if the Airborne IP (AIP) modelling it is a known and controlled practice, there are still some open questions regarding the complexities of this modelling approach. Most of this lie into the AIP sensitivity to geological targets, others in its capability in integrate with the ground IP and other more about the parametrical management during the inversion process. To contribute on the AEM-IP modelling field of research, with this work we performed two joint inversions on real data modelling AIP effects. For the first experiment we jointly inverted AEM-IP fixed-wing data with helicopter-borne data. For the other experiment, we jointly modelled ground DCIP and helicopter-borne AEM data, modelling AIP parameters. With these experiments we retrieved that inductive airborne IP can contribute, in term of sensitivity, to the ground IP modelling procedure and that fixed-wing airborne data have a good sensitivity to chargeable geological targets as well as helicopter-borne platforms. More in general, it has been seen that inductive IP contains complementary information for modelling IP effects .</p>Open-Access Online Publication: October 30, 202

Adaptive sampling of AEM transients

This paper focuses on the sampling of the electromagnetic transient as acquired by airborne time-domain electromagnetic (TDEM) systems. Typically, the sampling of the electromagnetic transient is done using a fixed number of gates whose width grows logarithmically (log-gating). The log-gating has two main benefits: improving the signal to noise (S/N) ratio at late times, when the electromagnetic signal has amplitudes equal or lower than the natural background noise, and ensuring a good resolution at the early times. However, as a result of fixed time gates, the conventional log-gating does not consider any geological variations in the surveyed area, nor the possibly varying characteristics of the measured signal. We show, using synthetic models, how a different, flexible sampling scheme can increase the resolution of resistivity models.We propose a new sampling method,which adapts the gating on the base of the slope variations in the electromagnetic (EM) transient. The use of such an alternative sampling scheme aims to get more accurate inverse models by extracting the geoelectrical information from the measured data in an optimal way