Werner, Charles

The scope of the project “VENEZIA - Subsidence monitoring service in the Lagoon of Venice for regional administrative and water authorities” was to define and implement a land subsidence monitoring service in the Lagoon of Venice for regional and administrative authorities. In order to provide the best knowledge of the land subsidence process around the Lagoon of Venice, SAR-based monitoring techniques (differential SAR interferometry and interferometric point target analysis) were integrated with levelling and GPS surveys into an overall information system. Land subsidence due to natural and anthropogenic causes has represented one of the most serious environmental problems for the Lagoon of Venice and its catchment (Tosi et al., 2002, Carbognin and Tosi, 2003, Carbognin et al., in press). Land subsidence has increased the vulnerability and the geological hazard (i.e. river flooding, riverbank stability, intrusion of seawater in the aquifer system, deteriorating of the littoral sectors with a general coastline regression and an increment of the sea bottom slope close to the shoreline) of these areas, a large portion of which lies below the mean sea level. After the regulation of groundwater exploitation from the Venetian aquifer-aquitard system, a remarkable slowing down of the induced subsidence in Marghera (industrial zone), in the historical center of Venice and along the littorals was ascertained in the 1970’s. Recent studies (Carbognin and Tosi, 2003) have shown that land subsidence is still in progress in the southern and northern coastal areas and in the nearby mainland, where groundwater is extracted from artesian wells, thicker and more compressible Holocene sediments are present, and organic soil oxidation takes place in reclaimed areas. Until 1999, levelling of the Venice region was carried out only along the coast and the lagoon edges and the monitoring network was composed by benchmarks along single levelling lines; a fine grid network was established only in the city of Venice. In recent years, the levelling network has been updated to cover all the southern part of the Lagoon, and plans exist to cover also the northern sector. The same network used for the levelling surveys is also considered for differential GPS measurements. In addition to these ground-based methods, differential SAR interferometry using long series of SAR data (Wegmüller and Strozzi, 1998; Strozzi et al, 2001) and interferometric point target analysis (IPTA) (Wegmüller et al., 2003, Werner et al., 2003) have emerged as very promising tools for the monitoring of land subsidence at high spatial resolution. The VENEZIA project was organized along a service definition phase, a service implementation phase and a service quality assessment phase. Important elements of the project were the definition, implementation and validation of the service for interferometric point target analysis and the integration of the SAR-based monitoring techniques with levelling and GPS surveys into an overall information system capable to provide the best knowledge of the subsidence process to the authorities that manage the Po Plain area around the Lagoon of Venice.

We present a reliable methodology to estimate the energy associated with the subaerial diffuse degassing of volcanic-hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO2 fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil-gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (Kc). For each area, we computed the values of Kc which range from 0.4 to 2.3 W m 1 C 1. Using the CO2 soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW.

In the Interferometric Point Target Analysis (IPTA) point-like targets that remain phase coherent over time are identified in a sequence of satellite Synthetic Aperture Radar (SAR) images and used to estimate the progressive deformation ofthe terrain with millimetric accuracy. Building upon conventional interferometric SAR techniques, IPT A overcomes atrnospheric delay anomalies and tempora! and geometrie decorrelation by exploring the tempora! and spatial characteristics of radar interferometric signatures collected from point targets widely available over urban areas but that can be also found scattered outside cities and villages. In this contribution the application of IPT A to the monitoring of land' subsidence in the urban and littoral environments of the Venice Lagoon is described. The results achieved using ali the available ERS SAR images acquired between 1992 and 2000 are very significant due to the achieved target spatial and tempora! coverage.

The Venice Lagoon, Italy, is a unique worldwide environment which is presently vulnerable due to loss in surface elevation as a result of land subsidence referred to the mean sea level. Land displacements in the Venice coastland have been determined over time by traditional monitoring techniques (i.e., spirit leveling and GPS). Recently, SAR-based analyses have been used to complement the ground-based methods. Interferometric analysis on persistent point targets has been proved to be very effective in detecting land displacement in the coastal environment. ERS SAR and ENVISAT ASAR images spanning the time period 1992-2005 and 2003- 2006, respectively, have been processed at regional and local scale and on “natural” as well as “artificial” reflectors.

The first measurements of volcanic/hydrothermal water vapor and heat flux using eddy covariance (EC) were made at Solfatara crater, Italy, June 8–25, 2001. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes of water vapor varied between 680 and 11200g H2O m−2 d−1. Heat fluxes varied diurnally with the solar input, and the volcanic component of sensible heat ranged from ∼25 to 238W m−2. The highest measurements of both sensible and latent heat flux were made downwind of hot soil regions and degassing pools and during mid-day. The ratio of average volcanic heat (both latent and sensible) to CO2 flux resulted in an equivalent H2O/CO2 flux ratio of 2.2 by weight, which reflects the deep source H2O/CO2 gas ratio. The amount latent heat flux/evaporation was determined to be consistent both with what would be expected from the magnitude of CO2 fluxes and the fumarolic H2O/CO2 ratio, as well as with observed surface temperatures and wind speeds given a moist soil. This suggests that the water vapor that condenses in the shallow subsurface is remobilized at the soil–atmosphere interface through variable evaporation dependent on the deep heat flux and surface temperature. The results suggest that EC provides a quick and easy method to monitor average H2O/CO2 ratios continuously in volcanic regions, providing another important tool for volcanic hazards monitoring.

We present a comparative study of soil CO2 flux ( ) measured by five groups (Groups 1–5) at the IAVCEI-CCVG Eighth Workshop on Volcanic Gases on Masaya volcano, Nicaragua. Groups 1–5 measured using the accumulation chamber method at 5-m spacing within a 900 m2 grid during a morning (AM) period. These measurements were repeated by Groups 1–3 during an afternoon (PM) period. Measured ranged from 218 to 14,719 g m–2 day–1. The variability of the five measurements made at each grid point ranged from ±5 to 167%. However, the arithmetic means of fluxes measured over the entire grid and associated total CO2 emission rate estimates varied between groups by only ±22%. All three groups that made PM measurements reported an 8–19% increase in total emissions over the AM results. Based on a comparison of measurements made during AM and PM times, we argue that this change is due in large part to natural temporal variability of gas flow, rather than to measurement error. In order to estimate the mean and associated CO2 emission rate of one data set and to map the spatial distribution, we compared six geostatistical methods: arithmetic and minimum variance unbiased estimator means of uninterpolated data, and arithmetic means of data interpolated by the multiquadric radial basis function, ordinary kriging, multi-Gaussian kriging, and sequential Gaussian simulation methods. While the total CO2 emission rates estimated using the different techniques only varied by ±4.4%, the maps showed important differences. We suggest that the sequential Gaussian simulation method yields the most realistic representation of the spatial distribution of , but a variety of geostatistical methods are appropriate to estimate the total CO2 emission rate from a study area, which is a primary goal in volcano monitoring research.