Long-Term Soil Gas Surveys in the Northern Part of the Modena Province Pre, During and After the 2012 Seismic Sequence

Abstract:

Three geochemical surveys of soil gas (CO₂ and CH₄ flux measurements, He, H₂, CO₂, CH₄ and C₂H₆ concentrations) and isotopic analyses (δ¹³C–CH₄, δD–CH₄, δ¹³C–CO₂) were carried out as part of a feasibility study for a natural gas storage site in the Modena Province (Northern Italy), during the 2006-2009 period. In May-June 2012, a seismic sequence (main shocks of ML 5.9 and 5.8) was occurred closely to the investigated area. Chemical and isotopic analysis were repeated in May 2012, September 2012, June 2013 and July 2014.

In the 2006-2009 period, at the pre-seismic conditions, chemical composition of soil gas showed that the southern part of the studied area is CH₄-dominated, whereas the northern part is CO₂-dominated. Relatively anomalous fluxes and concentrations were recorded with a spotted areal distribution. Anyway, CO₂ and CH₄ values are within the typical range of vegetative and of organic exhalation of the cultivated soil.

2012-2013 soil gas results show CO₂ values essentially unvaried with respect to pre-earthquake surveys, while the 2014 values highlight an increasing of CO₂ flux in the whole study area. On the contrary, CH₄ values seem to be on average higher after the seismic sequence, although with a decreasing trend in the last survey (2014).

Isotopic analysis were carried out only on samples with anomalous values. The δ¹³C-CO₂ value suggests a prevalent shallow origin of CO₂ (i.e. organic and/or soil-derived) probably related to anaerobic oxidation of heavy hydrocarbons. Methane isotopic data (δ¹³C-CH₄) indicate a typical biogenic origin (i.e. microbial hydrocarbon production) of the CH₄, as recognized elsewhere in the Po Plain and surroundings.

Obtained results highlight a different CO₂ and CH₄ behaviour before, during and after the seismic events. These variations could be produced by increasing of bacterial (e.g. peat strata) and methanogenic fermentation processes in the first meters of the soil. No hints of deep degassing can be inferred for the study area after the earthquake, as suggested by isotopic analysis.

These achieved outcomes constitute the starting point for subsequent geochemical surveys, in order to assess the temporal variations and to better understand the geochemical processes related to the seismic sequence.