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L’acquifero nelle ghiaie di base del Tevere: una risorsa geotermica per la città di Roma.
No full text
Le risorse geotermiche di alta, media e bassa entalpia
No full textIl pianeta Terra rilascia continuamente calore verso la superficie. Questo flusso di calore è particolarmente elevato nelle zone vulcaniche (dorsali oceaniche, cordigliere, archi di isole, punti caldi) dove il magma fuso, con una temperatura di 800-1100°C, può accumularsi in serbatoi a piccola profondità (<10 km).
Nel nucleo interno della Terra a 6400 km di profondità, la temperatura è superiore a 5000°C. Nello strato di mantello fuso, chiamato astenosfera, i cui moti convettivi sono la causa degli spostamenti delle zolle litosferiche che producono le dorsali medio-oceaniche, le grandi fosse tettoniche e le catene di montagne (Fig. 1), la temperatura è superiore a 1200°C. La presenza di queste zone calde profonde implica che in ogni punto della Terra la temperatura aumenta con la profondità. Il gradiente geotermico medio è di circa 33 °C/km, ma può arrivare anche a valori dieci volte superiori nelle zone “calde” al di sopra di serbatoi magmatici. Il gradiente geotermico scende invece a valori di 10-20 °C/km nelle zone “fredde” situate nelle aree geologicamente stabili, lontane dalle frontiere sia divergenti che convergenti tra zolle litosferiche. Il contributo termico del decadimento degli elementi radioattivi (es. uranio, torio) è invece trascurabile, perché questi elementi sono presenti con una certa abbondanza solo nelle rocce della parte più alta della crosta terrestre.
Il calore interno della Terra può essere utilizzato a fini energetici quando l’acqua meteorica si infiltra in profondità lungo sistemi permeabili, viene riscaldata dalla circolazione all’interno di rocce calde profonde, si accumula in serbatoi di rocce permeabili e può venire estratta alla superficie mediante la perforazione di pozzi.
L’entalpia esprime la quantità di energia termica di una data massa di fluido e a seconda della temperatura (T) del fluido estratto le risorse geotermiche vengono classificate in risorse di alta (T>150°C), media (T=90-150°C) e bassa (T<90°C) entalpia e hanno una molteplicità di usi energetici. (Fig. 2)
Carta della profondità del tetto, spessore e caratteristiche chimico-fisiche dell’acquifero nelle ghiaie di base del Tevere
No full textActive degassing structures of Stromboli and variations in diffuse CO 2 output related to the volcanic activity
No full textThe main CO2 diffuse degassing structures (DDS) of Stromboli were identified through extensive CO2 soil flux investigations, with 3600 measurements by an accumulation chamber. These DDS extend from the nearby crater area of Pizzo sopra la Fossa (Pizzo) to the coastal area of Pizzillo and are all associated with NE–SW deep fractures, corresponding to the main volcano-tectonic axis of the island, some of which produced flank eruptions in prehistoric times. In each of the four main DDS, a target area was defined covering the zone with the highest CO2 soil flux, where periodic CO2 flux surveys were carried out. The highest CO2 release was observed during the 2007 eruption and high flux values were recorded at both Pizzo and Pizzillo also in moments of high prolonged Strombolian activity (high number of daily explosions observed from the craters and/or high frequency of VLP seismic events). In order to better investigate the rate of diffuse CO2 degassing in relation to volcanic activity, an automatic station hourly measuring CO2 soil flux and environmental parameters (atmospheric T, P and humidity, soil moisture and T, wind speed and direction) was installed in March 2007 at Nel Cannestrà and Rina Grande DDS. Unusual positive correlations were found at Nel Cannestrà between gas flux and SE wind speed and at Rina Grande between gas flux and soil moisture, which are explained by the local conditions, which favour respectively a Venturi effect and the increase in gas flux toward the station during rains. Ten months of continuous recording confirmed the strong influence of environmental conditions on the CO2 soil flux, but statistical data processing made it possible to recognize clear positive anomalies expressing high rates of deep magmatic CO2 degassing. Comparison with seismic data indicates that high CO2 fluxes are apparently correlated with increases in volcanic activity, such as higher explosion frequency and VLP amplitude. Particularly promising is the temporal coincidence of highest recorded flux anomaly with a major explosion that occurred during the observation period. Data confirm that the two continuously monitored DDS are preferentially deep degassing sites, where anomalous increases of CO2 release could represent a geochemical precursor for either high energy explosions from the craters or the opening of flank eruptive fissures that might threaten the village of Stromboli
Volcanic hazard of the Colli Albani
No full textAlthough controversy exists about the age of its most recent eruption (either 36 ka or
,23 ka), Colli Albani volcano is unanimously considered to be quiescent and not exinct.
During the Holocene, several lahars were generated by overflows from Albano crater lake up to
the fourth century BCE, when the Romans excavated a drainage tunnel to keep the lake level
below the crater rim. Such recent activity, together with the frequent occurrence of seismic
swarms underneath the crater zone, the ongoing uplift of the volcanic edifice and the magmatic affinity
of the emitted gas, indicate the presence of an active magma chamber. The most likely site for a
new eruption is the deep crater hosted in the southern part of the Lake Albano, where the last eruptive
events occurred. Any eruption would have a strong explosive character enhanced by the interaction
of magma with the water of the lake and would endanger a densely inhabited area up to the
outskirts of Rome. The hazard of a new overflow from Lake Albano is very low because of the
present low level of the lake. There is instead a potential for CO2 release from the deep lake
water following the occurrence of rollovers, which would threaten the lake shore, a site where
thousands of people spend their vacations in the summer. However, the content of dissolved
CO2 is presently far from saturation and no Nyos-type events will occur today. Presently, the
main hazard is related to strong gas emissions (CO2, H2S and Rn) from fractured zones and gas
blowouts from wells reaching shallow gas-pressurized aquifers
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