130 research outputs found
Gridded maps of geological methane emissions and their isotopic signature
Methane (CH4) is a powerful greenhouse gas, whose
natural and anthropogenic emissions contribute ∼20 % to global
radiative forcing. Its atmospheric budget (sources and sinks), however, has
large uncertainties. Inverse modelling, using atmospheric CH4
trends, spatial gradients and isotopic source signatures, has recently
improved the major source estimates and their spatial–temporal variation.
Nevertheless, isotopic data lack CH4 source representativeness for
many sources, and their isotopic signatures are affected by incomplete
knowledge of the spatial distribution of some sources, especially those
related to fossil (radiocarbon-free) and microbial gas. This gap is
particularly wide for geological CH4 (geo-CH4) seepage,
i.e. the natural degassing of hydrocarbons from the Earth's crust. While
geological seepage is widely considered a major source of atmospheric
CH4, it has been largely neglected in 3-D inverse CH4
budget studies given the lack of detailed a priori gridded emission maps.
Here, we report for the first time global gridded maps of geological
CH4 sources, including emission and isotopic data. The 1∘×1∘ maps include the four main categories of natural
geo-CH4 emission: (a)Â onshore hydrocarbon macro-seeps, including
mud volcanoes, (b)Â submarine (offshore) seeps, (c)Â diffuse microseepage and
(d)Â geothermal manifestations. An inventory of point sources and area sources
was developed for each category, defining areal distribution (activity),
CH4 fluxes (emission factors) and its stable C isotope composition
(δ13C-CH4). These parameters were determined
considering geological factors that control methane origin and seepage (e.g.
petroleum fields, sedimentary basins, high heat flow regions, faults,
seismicity). The global geo-source map reveals that the regions with the
highest CH4 emissions are all located in the Northern Hemisphere,
in North America, in the Caspian region, in Europe and in the East Siberian
Arctic Shelf. The globally gridded CH4 emission estimate
(37 Tg yr−1 exclusively based on data and modelling specifically
targeted for gridding, and 43–50 Tg yr−1 when extrapolated to also
account for onshore and submarine seeps with no location specific
measurements available) is compatible with published ranges derived using
top-down and bottom-up procedures. Improved activity and emission factor data
allowed previously published mud volcanoes and microseepage emission
estimates to be refined. The emission-weighted global mean
δ13C-CH4 source signature of all geo-CH4
source categories is about −49 ‰. This value
is significantly lower than those attributed so far in inverse studies to
fossil fuel sources (−44 ‰) and geological seepage
(−38 ‰). It is expected that using this updated, more
13C-depleted, isotopic signature in atmospheric modelling will
increase the top-down estimate of the geological CH4 source. The
geo-CH4 emission grid maps can now be used to improve atmospheric
CH4 modelling, thereby improving the accuracy of the fossil fuel
and microbial components. Grid csv (comma-separated values) files are
available at https://doi.org/10.25925/4j3f-he27.</p
Geostatistical analysis for the assessment of rare gas soil distribution in detecting concealed faults: the Ofanto clay basin
An integrated geochemical, morphological and structural analysis was applied to a clay basin in Southern Italy (Ofanto valley) to delineate tectonic features. the resulting distribution of previous soil-gas surveys (helium and radon) and the location and orientation of
field-observed brittle deformations (faults and fractures)
were compared with air-photo interpreted morphotectonic features. The results show that the highest helium and radon values occur preferentially along elongated features
shown by mesostructural and geomorphological analyses, i.e. anti-Apennine, Apennine and, secondarily, N-S orientations. Furthermore, the application of geostatistical techniques in a testing area has enhanced the semi-quantitative evaluation of this anisotropic soil-gas distribution (linked to the gas-bearing properties of the local brittle
deformations). The correspondence between soil-gas distribution and mesostructural/geomorphological 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 created some morphological features. Geostatistical analysis of the
geochemical data combined with the other geological techniques has been shown to improve the interpretation of soil-gas results for neotectonic studies in clay basins
where tectonic discontinuities have no surface expression
Non-volcanic CO2 and CH4 degassing in an actively extending orogen, southern Apennines, Italy
The southern Apennines fold and thrust belt has been undergoing post-orogenic extension since ca. 700
kyr. Crustal extension controls active tectonics and seismogenesis in the mountain chain [1], with
seismicity being characterized by low to moderate magnitude events punctuated by strong earthquakes
[2]. Effective decoupling between deep and shallow structural levels is related to the strong rheological
contrast produced by a fluid-saturated, clay-rich mélange zone interposed between buried
autochthonous carbonates – continuous with those exposed in the Apulian foreland – and the
allochthonous units. This mélange zone also acts as a seal preventing the migration of deep-seated
aqueous fluids – as well as oil in the Basilicata region, which hosts the largest Europe’s onshore oil fields
– towards the surface.
On the other hand, the mountain belt is characterized by substantial gas flow, recorded as both
distributed soil gas emissions and vigorous gas vents, associated with active faults at the surface. We
measured a CO2 flux up to 34000 g/m-2 per day at a gas vent, as well as large amounts of He (up to 52
ppm), Rn (up to 228 kBq/m3) and CH4 (up to 5000 ppm).
Overpressured CO2, which has been proposed as triggering normal fault earthquakes in the Apennines,
has been interpreted as mostly of mantle origin. However, our new results from isotope analyses carried
out on the carbon contained in both CO2 and CH4 indicate a dominant thermogenic origin for these
gases, probably associated with the emplacement of magmatic sills within the lower section of the thick
carbonate platform succession occurring at the base of the sedimentary cover in the southern
Apennines. Our results bear major implication concerning the postulated occurrence of crustal faults
allowing fluids to migrate directly from mantle depths to the surface
The behavior of rare soil gases in a seismically active area: the Fucino basin (central Italy)
Soil-gas (He, Rn) concentrations were performed to test their sensitivity for locating fault or fracture systems also when masked by non-cohesive lithologies, and to investigate about the gas-bearing properties of seismogenic faults. The Fucino basin (central Italy) was chosen as test site because it displays a network of surface and
shallow-buried active faults within the valley floor that were partially reactivated during the 1915 Avezzano earthquake (Ms47.0). The highest radon values were found aligned along the most important faults bordering the eastern and the north-western sides of the plain. Moderately anomalous values of radon activity occur along the faults located in the depression of the historical lake. Highest helium values prevail in the western part of the plain, in correspondence of a horst structure inferred to be as the prolongation of the Vallelonga-Trasacco ridge. The study provides constraints on the spatial influence of tectonics and geology on deep-seated gas migration toward the surface
Groundwater of Rome
This paper describes the contents of the new Hydrogeological Map of the City of Rome (1:50,000 scale). The map extends to the entire municipality (1285 km2) and is based on both the most recent scientific studies on the groundwater field and new survey activities carried out in order to fill the data gaps in several areas of the examined territory. The map is the result of a combination of different urban groundwater expertise and Geographic Information System (GIS)-based mapping performed using the most recent available data and has been produced with the intention of furnishing the City of Rome with the most recent and updated information regarding groundwater
Continuous monitoring of natural CO2 emissions near Rome: lessons for low-level CO2 leakage detection
Continuous monitoring has been carried out at a fluvial flood-plain site near Rome for over a year. There is a mix of biogenic CO2 and deep geogenic CO2 at the site at relatively low concentrations and fluxes compared with other natural CO2 seepage sites studied previously. Factors such as temperature and soil moisture clearly affect the CO2 concentration and flux and seasonal and diurnal influences are apparent. Statistical approaches are being used to try to define these relationships and separate out the two gas components, which would be necessary in any quantification of leakage from CO2 storage
Early stage sinkhole formation in the acque albule basin of central Italy from geophysical and geochemical observations
Sinkhole occurrence along the Tyrrhenian margin of the Central Apennines is of great importance
for applied research, land management and civil protection. This study reports on GPS-altimetry
magnetic, gravity, geoelectric, seismic, and soil gas measurements of a rapidly developing sinkhole
near the Guidonia military airport. The measurements revealed an elliptical 2-m depression
elongated 220 m in the NNE-SSW direction with the minor axis of 110 m. In spring of 2013, two
vertical cavities formed in the eastern and northeastern flanks of the depression whose depths and
shapes are rapidly evolving with the formation of widespread fracturing along the same side. The
geophysical observations image the developing sinkhole to a depth of some 50 m, the presence of
the Travertino lithotype around the depression (down to at least 40 m), and the lack of this lithotype
below the lowered area. The sinkhole's evolution appears to be structurally controlled by local and
regional faulting. These results are useful for designing further geophysical, geotechnical and
geochemical studies to monitor the sinkhole's evolution and to assess the hazard it presents in
densely urbanized area.Published36-477A. Geofisica di esplorazioneJCR Journalreserve
Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China
The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity
Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK
Fluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to ~1 km depth. Within such lithified successions, dissolution associated with the circulation of meteoric water results in increased permeability (K~10−1–100 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic conductivity is lower, decreasing from K~10−2 m/day at 150–400 m BGL to 10−3 m/day down-dip at ~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7–1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields
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