A metrologic method of anomaly field amplitude bottom reduction in undersampled geomagnetic marine surveys

We show the results obtained by means of a seabed reduction technique on the intensity of geomagnetic anomaly fields applied to a synthetic case and then to the real case of a geomagnetic survey of eastern Ligurian Sea (Italy). The eastern Ligurian Sea has very intense short waves anomaly fields and a sea bed that varies greatly in depth. As a result the geomagnetic space signal is characterized by a very large spectral content; in these conditions it is not possible to obtain a full sampled marine survey and vertical continuation analytic procedures and standard numerical bottom reduction based on a single vertical incremental parameter, whichever is applicable, fails to give accurate results. The present technique, which has been fine-tuned over 4 years of experimentation in environmental researchs, aims to provide a simple and efficient means to reduce the distortion of geomagnetic anomalies field caused by the variation of distance between survey plane and magnetic outcrop source position. The compensation procedure is based on evaluation, by comparison of two measurements carried out at different altitudes, of the mean vertical increment typical of each anomaly field principal frequency component bands. The component anomaly fields are then corrected by application of the corresponding vertical increments and lastly, the anomaly geomagnetic field reduced to the sea-bed is computed as Inverse Fourier Transform of a spectrum built as synthesis of the component anomaly fields' spectra. The results obtained have shown a notable increase in definition of anomaly field intensity without the production of appreciable distortions or false geomagnetic echoes. This revised version was published online in November 2006 with corrections to the Cover Date.     

Geothermal energy release at the Solfatara of Pozzuoli (Phlegraean Fields): Phreatic and phreatomagmatic explosion risk implications

The H₂O, CO₂ and H₂S outputs at the Solfatara of Pozzuoli have been measured and a map of the exhaling areas has also been made. The energy released at the surface by the fluids has been estimated to be 10¹⁹ ergs/day.The presence of aquifers at Phlegraean Fields increases the phreatic and phreatomagmatic explosion risk.Our results suggest that even if an uprising magma may interact with water at depth, an explosion could occur only at the shallow levels of a few hundred meters. Since the transfer of energy toward the surface is favoured by the presence of fractures, a detailed analysis of the deep fracture network would help to evaluate the risk levels of the various areas of Phlegraean Fields.     

Geochemistry of fluids discharged over the seismic area of the Southern Apennines (Calabria region,Southern Italy): Implications for Fluid-Fault relationships

The first comprehensive geochemical data-set of the fluids circulating over a 14,000 km²-wide seismic-prone area of the Southern Apennines, Calabria Region (Italy), is presented here. The geochemical investigations were carried out with the twofold aim of constraining the origin and interactions of the circulating fluids and to investigate possible relationships with local faults. Sixty samples of both thermal and cold waters were collected, from which the dissolved gases were extracted. The geochemical features of the water samples display different types and degrees of water–rock interactions, irrespective of the outlet temperature. The calculated equilibrium temperatures of the thermal waters (60–160 °C) and the low heat flow of the whole study area, are consistent with a heating process due to deep water circulation and rapid upflow through lithospheric structures. The composition of the dissolved gases reveals that crustal-originating gases (N₂ and CO₂-dominated) feed all the groundwaters. The ³He/⁴He ratios of the dissolved He, in the range of 0.03–0.22Ra_c for the thermal waters and 0.05–0.63Ra_c for the cold waters (Ra_c = He isotope ratio corrected for atmospheric contamination), are mainly the result of a two-component (radiogenic and atmospheric) mixing, although indications of mantle-derived He are found in some cold waters. As the study area had been hit by 18 of the most destructive earthquakes (magnitude ranging from 5.9 to 7.2) occurring over a 280-a time span (1626–1908) in the Southern Apennines, the reported results on the circulating fluids may represent the reference for a better inside knowledge of the fault-fluid relationships and for the development of long-term geochemical monitoring strategies for the area.     

Anomalous fluid emission of a deep borehole in a seismically active area of Northern Apennines (Italy)

The Miano borehole, 1047 m deep, is located close to the river Parma in the Northern Apennines, Italy. A measuring station has been installed to observe the discharge of fluids continuously since November 2004. The upwelling fluid of this artesian well is a mixture of thermal water and CH₄ as main components. In non-seismogenic areas, a relatively constant fluid emission would be expected, perhaps overlaid with long term variations from that kind of deep reservoir over time. However, the continuous record of the fluid emission, in particular the water discharge, the gas flow rate and the water temperature, show periods of stable values interrupted by anomalous periods of fluctuations in the recorded parameters. The anomalous variations of these parameters are of low amplitude in comparison to the total values but significant in their long-term trend. Meteorological effects due to rain and barometric pressure were not detected in recorded data probably due to reservoir depth and relatively high reservoir overpressure. Influences due to the ambient temperature after the discharge were evaluated by statistical analysis. Our results suggest that recorded changes in fluid emission parameters can be interpreted as a mixing process of different fluid components at depth by variations in pore pressure as a result of seismogenic stress variation. Local seismicity was analyzed in comparison to the fluid physico-chemical data. The analysis supports the idea that an influence on fluid transport conditions due to geodynamic processes exists. Water temperature data show frequent anomalies probably connected with possible precursory phenomena of local seismic events.     

Gas flux to the atmosphere from mud volcanoes in eastern Romania

Gas flux measurements have for the first time been taken from vents and soil of eastern Romania mud volcanoes, the largest geological structures in Europe releasing methane into the atmosphere. In the quiescent phase, the methane emission from single vents is up to 28 t yr^?1. Diffuse soil microseepage is of the order of 10²?10⁵ mg m^?2 day^?1. A total output of at least 1200 tonnes of CH₄ per year can be conservatively estimated over the area investigated alone (～ 2.3 km²). Helium fluxes are up to five orders of magnitude higher than the average flux in a stable continental area, pointing to a close link between mud volcanoes and crustal degassing through faults crossing the deep hydrocarbon reservoirs. These data represent a key contribution towards refining global CH₄‐emission estimates, which indicate mud volcanoes as a significant and unavoidable source of greenhouse gases for the atmosphere.     

Natural arsenic contamination in waters from the Pesariis village,NE Italy

High arsenic (As) concentrations, >900 μg/L, were measured in Ca–Mg–SO₄ waters from springs and drainages in the village of Pesariis in the Carnic Alps (NE Italy). Oxidation of the outcropping arsenian marcasite ore deposits of the area is proposed as the mechanism for As release into oxygenated waters during runoff. Nevertheless, the limited extension of the ore deposit and the relatively low As content of the mineralization suggest that sulfide weathering might not be the only process responsible for the highest As concentration in groundwaters. An additional mechanism involves As adsorption onto ferric iron particulate during oxidation, the drawdown in reducing environment at depth during water infiltration, and the release of ferrous iron and sorbed arsenic to the water columns by reductive dissolution of hydrous ferric oxides (HFO). This yields the observed Fe–As correlation. Newly formed HFO precipitates when groundwaters discharge to aerated conditions, leading to the removal of As, which strongly partitions into the iron-rich sediments, adsorbed onto the surface of amorphous Fe₂O₃·xH₂O. The calculated and measured As concentration in sediments exceeds 10% by weight. Furthermore, geochemical and isotopic data indicate that the As-rich reservoir partly mixes with shallower aquifers, commonly tapped for drinking supply, representing a natural hazard for inhabitants.     

Geochemistry and isotope geochemistry of the Monfalcone thermal waters (northern Italy): inference on the deep geothermal reservoir

Geochemical investigations were carried out to define the origin of the low- to moderate-temperature thermal waters feeding the Monfalcone springs in northern Italy. Chemical data indicate that waters approach the composition of seawater. Mixing processes with cold low-salinity waters are highlighted. The δ¹⁸O and δD values are in the range ?5.0 to ?6.4 ‰, and ?33 to ?40 ‰, respectively, suggesting the dilution of the saline reservoir by karst-type freshwaters. A surplus of Ca²⁺ and Sr²⁺ ions with respect to a conservative mixing is ascribed to diagenetic reactions of the thermal waters with Cretaceous carbonates at depth. The measured Sr isotopic composition (⁸⁷Sr/⁸⁶Sr ratio) ranges between 0.70803 and 0.70814; after correction for the surplus Sr, a ⁸⁷Sr/⁸⁶Sr ratio indicating Miocene paleo-seawater is obtained. The dissolved gases indicate long-lasting gas–water interactions with a deep-originated gas phase of crustal origin, dominated by CO₂ and marked by a water TDIC isotopic composition in the range ?5.9 to?8.8 and helium signature with 0.08?<?R/Ra?<?0.27, which is a typical range for the crust. A possible scenario for the Monfalcone thermal reservoir consists of Miocene marine paleowaters which infiltrated through the karstic voids formed within the prevalently Cretaceous carbonates during the upper Eocene emersion of the platform, and which were entrapped by the progressive burial by terrigenous sediments.     

Geochemical investigations of submarine volcanic exhalations to the east of Panarea, Aeolian Islands, Italy

Results are presented on scubadiving investigations carried out on thermal manifestations in the area of Panarea (Aeolian Islands). The area investigated falls inside a caldera which extends from the main island to the group of islets located to the northeast. The distribution of the gaseous manifestations is regulated by the NE-SW, NW-SE and N-S regional tectonic directrices, through which the more recent basic magma intruded, giving rise to dikes and pillow lavas. f_O2-temperature relation of the gases sampled in the investigated area was calculated to be: logf_O2 = 11−24,593/T which indicates that a buffering mechanism acted on the gases as they cooled down during their ascent. The high ³He/⁴He ratio (6 × 10⁻⁶) and the δ¹³C = −3.2%. (PDB), suggest the presence of a magmatic component in the gas feeding the investigated manifestations. The above relations and the almost constant high He/N₂ ratio suggest that all the fumaroles are fed by the same deep hot fluids. On the basis of both the chemical characters of the fluids and the geothermo-barometric data, a deep geothermal body, having a temperature of about 240°C, is recognized. Two other shallower thermal aquifers, with a temperature of 170–210°C, are identified. A circulation pattern of the geothermal fluids is also proposed. On the basis of calculations regarding the convective energy released by the geothermal system of Panarea, and the magmatic mass responsible for the positive gravimetric anomaly of the area, it was estimated that the last volcanic activity took place less then 10,000 years ago.     

Volcanic steam output directly measured in fumaroles: the observed variations at Vulcano Island,Italy, between 1983 and 1987

The mass of steam emitted by the fumaroles of the crater of Vulcano (Aeolian Islands, Italy) was evaluated by using a direct measuring method. Measurements were effected with this method between 1983 and 1987 in nine field campaigns, the results of which are given in this paper. The steam flux measurements were performed by an instrument expressly designed for steam condensation. The estimates of the thermal energy transported daily by the steam gave values of up to 10E12 J. The sharp variations in the total mass outputs recorded in 1984 and 1985 are here interpreted as being a consequence of the probable accumulation of fluid pressure at depth, during the evolution of volcanic activity. The different results obtained from the various areas of the fumarolic field during the evolutionary phases have led to the hypothesis that different subsystems exist that feed the fumaroles, and that their activity is triggered by volcano-tectonics. This method can be applied in the evaluation of both fumarolic steam and thermal energy outputs, and therefore it is particularly useful in geothermal exploration, in the evaluation of the environmental impact of volcanic degassing, and in the forecasting of volcanic eruptions. The evaluation of the steam output is also useful for quantifying the release of other gaseous species and giving univocal interpretations regarding their variations in concentrations. Furthermore this direct method provides a new way of calibrating the flux estimates effected by other telemetric techniques (Lidar, Cospec, etc.).