G. Ciotoli

The detection of concealed faults in the Ofanto Basin using the correlation between soil-gas fracture surveys

Abstract An integrated geochemical, morphological and structural analysis was applied to a basin filled with clayey sediments in southern Italy (Ofanto Valley) to delineate tectonic features. More than 100 soil-gas samples were collected and analysed for CO2, Rn and He, and the resulting distribution was compared with the location and orientation of field-observed brittle deformations (faults and fractures), and air-photo interpreted morphotectonic features. The results show that the highest helium, radon and CO2 values occur preferentially along elongated zones similar to the most representative trends obtained by geomorphological and mesostructural analyses, i.e. anti-Apennine, Apennine and, secondarily, N–S orientations. Furthermore, the development of geostatistical techniques has allowed the semi-quantitative evaluation of the anisotropic soil-gas distribution. The gas-distribution pattern is considered to result from the combination of the anisotropic distribution of fracture traces and the randomly distributed background field. The correspondence between soil-gas distribution and geomorphological/mesostructural features, as well as the results from the geostatistical analysis, suggest that gas leakage towards the surface is controlled by the same structural pattern which also created some morphological features. This technique has been shown to be a useful tool for neotectonic studies; this is especially true in basins filled with clayey sediments, as soil gas is even able to define the leakage of deep-seated gases along tectonic discontinuities which have no surface expression.

Soil gas survey for tracing seismogenic faults: A case study in the Fucino Basin, central Italy

Soil-gas (He, Rn, CO2, and CH4) surveys were performed to test their sensitivity for locating fault or fracture systems when masked by unconsolidated lithologies and to investigate the gas-bearing properties of seismogenic faults. The Fucino basin (Central Italy) was chosen as a test site because it displays a network of surface and shallow-buried active faults within the valley floor which were partially reactivated during the 1915 Avezzano earthquake (Ms 7.0). The highest radon values were found aligned along the most important faults, especially along the San Benedetto-Gioia dei Marsi fault (SBGMF) to the east and in the northwestern sector where the Avezzano-Celano fault (ACF) meets the northwestern prolongation of the 1915 coseismic rupture. Moderately anomalous values of radon occur along the faults located in the depression of the historical lake (Trasacco fault, TF, and Ortucchio fault, OF). Anomalous values of CO2, Rn, and He were detected north of Borgo Ottomila in a zone where coseismic scarps and liquefaction phenomena were responsible for a pervasive brittle deformation during the 1915 earthquake. Highest helium values prevail in the western part of the plain, in correspondence with a horst structure bounded by the TF and considered to be the extension of the Vallelonga-Trasacco ridge. Results of this research confirm the possible usefulness of soil gas methods for locating some tectonic faults when covered by unconsolidated deposits. The study provides some clues on the spatial influence of tectonics and geology on deeply-seated gas migration toward the surface.

Carbon dioxide and radon gas hazard in the Alban Hills area (central Italy)

Abstract The sudden and catastrophic, or slow and continuous, release at surface of naturally occurring toxic gases like CO 2 , H 2 S and Rn poses a serious health risk to people living in geologically active regions. In general this problem receives little attention from local governments, although public concern is raised periodically when anomalous toxic-gas concentrations suddenly kill humans or livestock. For example, elevated CO 2 concentrations have been linked to the death of at least 10 people in the central Italian region of Lazio over the last 20 years, while it was the CO 2 asphyxiation of 30 cows in a heavily populated area near Rome in 1999 which prompted the present soil-gas study into the distribution of the local health risk. A detailed geochemical survey was carried out in an area of about 4 km 2 in the Ciampino and Marino districts, whereby a total of 274 soil-gas samples were collected and analysed for more than 10 major and trace gas species. Data were then processed using both statistical and geostatistical methods, and the resulting maps were examined in order to highlight areas of elevated risk. General trends of elevated CO 2 and Rn concentrations imply the presence of preferential pathways (i.e. faults and fractures) along which deep gases are able to migrate towards the surface. The CO 2 and Rn anomalous trends often correspond to and are usually elongated parallel to the Apennine mountain range, the controlling structural feature in central Italy. Because of this fundamental anisotropy in the factors controlling the soil-gas distribution, it was found that a geostatistical approach using variogram analysis allowed for a better interpretation of the data. With regard to the health risk to local inhabitants, it was found that although some high risk areas had been zoned as parkland, others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil-gas CO 2 concentrations of more than 70% and radon values of more than 250 kBq m −3 . It is recommended that land-use planners incorporate soil-gas and/or gas flux measurements in environmental assessments in areas of possible risk (i.e. volcanic or structurally active areas).

Short- and long-term gas hazard: the release of toxic gases in the Alban Hills volcanic area (central Italy)

Abstract In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur (Tivoli, Cava dei Selci, Solforata, Tor Caldara). The gas comprises a large proportion of CO 2 , with minor CH 4 , H 2 S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). His release is triggered when the total pressure of the fluid phase exceeds the hydrostatic pressure, thus forming a free gas phase. Furthermore, both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO 2 , H 2 S and Rn) poses a serious health risk to people living in this geologically active area. This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH 4 and H 2 S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ. In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO 2 concentrations of over 70% and a CO 2 flux of about 0.7 kg m −2 day −1 , as well as radon values of more than 250 kBq/m 3 . In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m 3 ) and very high values in the cellars (up to 250.000 Bq/m 3 ). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas).

Morphological and geochemical evidence of neotectonics in the volcanic area of Monti Vulsini (Latium, Italy)

Interdisciplinary research has been carried out to contribute to the definition of the neotectonic arrangement of the eastern area of Monti Vulsini, affected by Upper Pleistocene volcanic activity. Particularly, geochemical and geomorphological inquiries have been performed to identify tectonic elements whose recent activity could be related to the volcanic history of this area. The morphological field evidence of tectonics, as well as the analysis of azimuthal distribution of stream channels and the statistical analysis of the amplitude of relief (Ar), revealed the eastern Monti Vulsini volcanic area to be controlled by different tectonic alignments. At the same time the geochemical contribution, focused on geostatistical elaboration of soil–gas data, has attempted to process unbiased criteria to define soil–gas anomalies and to infer correlation with their fault-related linear geometry. In particular, rose diagrams of anomalous gas concentration alignments and azimuthal distribution of stream channels, performed for each hierarchical order, were compared to obtain information about the location and orientation of tectonic elements, as well as about the possible reconstruction of their activity in recent times, by means of relative chronology.

Characterization of a CO2 gas vent using various geophysical and geochemical methods

An understanding of gas migration along faults is important in many geologic research fields, such as geothermal exploration, risk assessment, and, more recently, the geologic storage of man-made carbon dioxide (C O2 ) . If these gases reach the surface, they typically are discharged to the atmosphere from small areas known as gas vents. In a study of an individual gas vent located in the extinct Latera caldera, central Italy, near-surface geochemical and geophysical surveys were conducted to define the spatial distribution of gas-induced effects in the first few meters of the soil and, by inference, the 3D structure and geometry of the associated gas-permeable fault. Grid surveys and detailed profiles were performed across this vent using time-domain reflectometry (TDR), ground-penetrating radar (GPR), frequency-domain electromagnetics (FDEM), electrical resistivity tomography (ERT), and gas geochemistry measurements. Detailed profilesurveys indicate that the leaking C O2 has changed the physical, chemic...

Seismic interpretation of the Laga basin; constraints on the structural setting and kinematics of the Central Apennines

Abstract: The Messinian Laga basin is the largest foreland basin within the Central Apennines fold and thrust belt (Italy). This area, actively investigated in the 1980s and 1990s for hydrocarbon resources, is considered a valuable analogue for clastic reservoirs developed in confined structural settings. Furthermore, it represents a key area for understanding the evolution of the Apennines, as it links the internal, structurally uplifted Early Miocene fold and thrust belt of the western Central Apennines with the more external and recent belt to the east. Despite several papers published on this area, the only reconstruction of the substratum structure is an internal and classified industry report. During the present study, we had access to a seismic database comprising 200 km of seismic profiles that were collected between 1983 and 1990. These data allowed us to reconstruct the structural setting of the Laga basin substratum, define the lateral continuity of the main compressional structures within the basin, construct a balanced cross-section, and define the shortening values.

Tiber delta CO2-CH4 degassing: A possible hybrid, tectonically active Sediment-Hosted Geothermal System near Rome

Fiumicino town in the Tiber River delta, near Rome International Airport (Italy), is historically affected by large amounts of carbon dioxide (CO2) in the ground and gas eruptions triggered by shallow drilling. While it is known that CO2 originates from carbonate thermometamorphism and/or mantle degassing, the origin of methane (CH4) associated with CO2 is uncertain and the outgassing spatial distribution is unknown. Combining isotope gas geochemistry, soil gas, and structural-stratigraphic analyses, we provide evidence for a hybrid fluid source system, classifiable as Sediment-Hosted Geothermal System (SHGS), where biotic CH4 from sedimentary rocks is carried by deep geothermic CO2 through active segments of a half-graben. Molecular and isotopic composition of CH4 and concentration of heavier alkanes (ethane and propane), obtained from gas vents and soil gas throughout the delta area, reveal that thermogenic CH4 (up to 3.7 vol% in soil gas; δ13CCH4: −37 to −40‰ VPDB-Vienna Peedee Belemnite, and δ2HCH4: −162 to −203‰ VSMOW - Vienna Standard Mean Ocean Water in gas vents) prevails over possible microbial and abiotic components. The hydrocarbons likely result from known Meso-Cenozoic petroleum systems of the Latium Tyrrhenian coast. Overmaturation of source rocks or molecular fractionation induced by gas migration are likely responsible for increased C1/C2+ ratios. CO2 and CH4 soil gas anomalies are scattered along NW-SE and W-E alignments, which, based on borehole, geomorphologic, and structural-stratigraphic analyses, coincide with active faults of a half-graben that seems to have controlled the recent evolution of the Tiber delta. This SHGS can be a source of considerable greenhouse gas emissions to the atmosphere and hazards for humans and buildings.

Soil gas distribution in the main coseismic surface rupture zone of the 1980, Ms = 6.9, Irpinia earthquake (southern Italy)

Soil gas measurements of different gas species with different geochemical behaviors were performed in the area of the Pecore Plain, a 200 m × 300 m sized, fault-bounded extensional basin located in the northern Mount Marzano massif, in the axial belt of the southern Apennine chain. The Pecore Plain area was affected by coseismic surface faulting during the Ms = 6.9, 1980 Irpinia earthquake, the strongest and most destructive seismic event of the last 30 years in southern Italy. The collected data and their geostatistical analysis provide new insights into the control exerted by active fault segments on deep-seated gas migration toward the surface. The results define anomalies that are aligned with the NW-SE trending coseismic rupture of the 1980 earthquake along the western border of the plain, as well as along the southern border of the plain where a hidden, E-W striking fault is inferred. Geospatial analysis highlights an anisotropic spatial behavior of 222Rn along the main NW-SE trend and of CO2 along the E-W trend. This feature suggests a correlation between the shape and orientation of the anomalies and the barrier/conduit behavior of fault zones in the area. Furthermore, our results show that gas migration through brittle deformation zones occurs by advective processes, as suggested by the relatively high migration rate needed to obtain anomalies of short-lived 222Rn in the soil pores.

Potential hazards of CO2 leakage in storage systems : learning from natural systems

Publisher Summary The aim of this chapter is to focus on the effects of increasing global atmospheric CO2 concentrations on plants and marine ecosystems, less is known about the effects of high, but very localized, CO2 concentrations originating from depth. It summarizes some of the findings from the EC-funded natural analogues for the storage of CO2 in the geological environment project (NASCENT) project, which studied a number of natural CO2 seeps throughout Europe. Of these, four sites were chosen for this chapter which, allowed different leakage processes and impacts to be assessed. One site was located in northern Greece, around the commercially producing florina CO2 gas field. The other three sites were in central Italy, including: a selected area of the latera geothermal complex, where natural deep CO2 migrates upwards along faults and is emitted to the atmosphere, the San Vittorino intermontane basin where CO2-charged ground waters cause extensive dissolution of limestone to form large sinkholes, and the Ciampino area to the southeast of Rome, where CO2 derived from deep-seated volcanism within the Alban hills complex migrates along faults in a residential area. Results indicate modifications in groundwater chemistry, sinkhole formation, and elevated toxic gas exposure risks in these areas caused by the occurrence of numerous active CO2 vents, however data also show that effects can be spatially restricted, and that health risks can be minimized with simple and inexpensive approaches and regulations.