Continuous monitoring of CO2 soil diffuse degassing at Phlegraean Fields (Italy): influence of environmental and volcanic parameters

Abstract

Carbon dioxide soil flux was continuously measured during 4 years (1998–2002) inside the crater of Solfatara by using the ‘time 0, depth 0’ accumulation chamber method. The CO2 soil flux (ϕCO2) is strongly influenced by external factors, such as the barometric pressure, the air and soil temperature and humidity, the wind speed, the amount of rain, and so on. Here, we apply a two-step filtering technique to remove the contribution of these external factors from the raw data and to highlight variations in gas flow from depth. In the first step we apply multiple regression and a best-subset search procedure to determine the minimal number of parameters to insert in the regression model. In the second step we apply time filtering on the residuals of the previous analysis through an ARIMA (integrated autoregressive moving average) model which allows us to quantify long-term trends and short-term periodicities. The statistical analysis showed that (1) the highest frequency fluctuations are due to variations of environmental parameters (particularly soil humidity and air temperature) and (2) the long-term trend of the filtered data is correlated with the ground deformation. This correlation is enhanced by back-shifting the CO2 flux signal by 3 months. These observations, along with the likelihood that the ground deformation at Phlegraean Fields is controlled by fluid pressure within the hydrothermal system, indicate that the long-term trend in soil CO2 flux is related to fluid pressure changes at depth. The delay between the soil CO2 flux and the ground deformation is most probably due to the inertia of the gas moving in the subsoil.