GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 17 Tab. 1 - Main statistical parameters of the soil gas and flux data. CO 2 is expressed in %, v/v; He is expressed in ppm; CO 2 flux is expressed in g m-2 d-1. GM, geometric Mean; Min, minimium value; Max, Maximum value; LQ, Lower Quartile; UQ, Upper Quartile; 10%, 10th percentile; 90th percentile; Std.Dev, Standard Deviation. N Mean GM Min Max LQ UQ 10% 90% Std.Dev. CO2 229 6.81 2.75 0.14 76.00 1.60 4.40 0.80 8.70 14.47 He 82 7.07 6.22 4.99 33.30 5.22 5.47 5.12 12.0 5.11 CO2 flux 181 103.60 32.90 2.87 2309.26 16.63 46.34 11.24 113.00 317.89 The calculated mean maximum values of all gases are very high for an intra-Apennine basin, and comparable with values generally measured in the volcanic areas of the Tyrrhenian margin. ���� ��������� ��� ���� �� ���� ��������� �� ��� �������� �� ���� ��������� ��������� This important gas leak is also confirmed by the presence of more permeable migration pathways linked to a fractured zone (i.e., gas vents), as well as other gas emissions and mineral springs occurring to the south (i.e., Pratella and Ailano). The analysis of the statistical distribution by using normal probability plots indicates that values above 10% of CO 2 , 5.7 ppm of He and 70 gm -2 d -1 for the flux of CO 2 can be considered anomalous for the study area. Fig. 3a-c shows the maps of the distribution of measured gas species in the soil. Maps of the CO 2 and He distribution highlight the same spatial pattern of the gas concentration in the soil, and indicate the presence of five major gas vents with diameters ranging from about 45 to 90 m, aligned along the N-S trending Colle Sponeta fault scarp (Fig. 2b); two further minor gas vents occur in the eastern border of the study area. The map of the CO 2 flux clearly highlights the same gas vents along the Colle Sponeta fault scarp, just a little bit shifted, and with more outlined anomalies. This behaviour is caused by the fact that soil gas CO 2 has, in general, a wider area of anomalous values around the gas vent centre due to lateral diffusion effects induced by its higher density (Beaubien et al. , 2014), and this feature allows tracing the underlying gas-permeable fault. The CO 2 flux distribution shows less smooth anomalies with very high values in the core of the gas-vents relative to the surrounding area, resulting in a spatial distribution of anomalous values that decreases rapidly outside the gas-vents (Fig. 3c). This implies that leakage occurs in a restricted area around the core of the gas-vent where sediments geotechnical properties have been locally compromised. Conversely out of the leaking area the diffusive flux is strongly affected by soil type, moisture, etc. (Annunziatellis et al., 2008). Preliminary data obtained by the geomorphological and soil gas surveys indicate that the Colle Sponeta area is characterised by an advective and localised gas leakage associated with gas vents aligned along the N-S trending Colle Sponeta fault scarp. The presence of aligned gas vents clearly indicates the occurrence of a fractured zone that can be probably associated with the presence of a main fault, as suggested by other shallow evidences of gas migration towards the south (e.g., the sinkhole in Fig. 2c). Worthy to note is the occurrence of abundant He emissions along the fault trace, a rare phenomenon in an area outside the volcanic districts. These evidences suggest the presence of a wider degassing area linked to a regional, deep- rooted active fault zone in the southern Matese area. References Al-Hilal, M. and Al-Ali, A.; 2010: The role of soil gas radon survey in exploring unknown subsurface faults at Afamia B dam, Syria . Radiat. Meas., 45 , 219–224. Amoroso O., Ascione A., Mazzoli S., Virieux J. and Zollo A.; 2014: Seismic imaging of a fluid storage in the actively extending Apennine mountain belt, southern Italy . Geophys. Res. Lett., 41 , 3802–3809. Annunziatellis A., Beaubien, S.E., Bigi, S., Ciotoli, G., Coltella, M., Lombardi, S.; 2008: Gas migration along fault systems and through the vadose zone in the Latera caldera (central Italy): Implications for CO2 geological storage. International Journal of Greenhouse Gas Control, 2 , 353-372, doi: 10.1016/j.ijggc.2008.02.003 Ascione A., Mazzoli S., Petrosino P. and Valente E.; 2013: A decoupled kinematic model for active normal faults: Insights from the 1980, M S = 6.9 Irpinia earthquake, southern Italy . Geol. Soc. Am. Bull., 125 (7-8), 1239-1259.