Experimental monitoring of carbon dioxide by low power IR-sensors: soil degassing in the Furnas Volcanic Centre, Azores

In October 1993 two experimental carbon dioxide sensors were installed at the Furnas Volcanic Centre, Sمo Miguel. The monitoring sites are tilt meter stations in Furnas Village and at Castelo Branco on the SW-caldera rim. The selection of these observation sites is meant to record changes in carbon dioxide outgassing close to an active fumarole ground (Furnas Village, 3 m deep well) and at an outer caldera wall (Castelo Branco, 6 m deep well) where geothermal processes are definitely absent. Recording of carbon dioxide was made for one year by hourly readings, from October 1993 to October 1994. The experiment revealed carbon dioxide pulses characterised by short rise times (hours) and fading on the time scale of days. Interpretation of the data in relation to meteorological conditions is preliminary and suggestive. An overall covariation of carbon dioxide outgassing and changes in atmospheric pressure and/or precipitation is observed at the Calderas fumarole grounds of the Furnas Village. A favoured degassing mechanism is based on increased out-diffusion of carbon dioxide from the soil layer at the onset of low pressure intervals due to increased boiling and carbon dioxide release from the shallow local hydrothermal aquifers. Increased soil degassing after heavy rain is attributed to closure of the porous surface layer resulting in an increased partial pressure of carbon dioxide at the base of the instrument well. At the Castelo Branco station, which is located on a narrow ridge some 20 m above the surroundings, the opposite effect of barometric pressure is observed on the very low carbon dioxide flux. This still unexplained covariation may be related to chemical conditions within the soil during high pressure (dry soil) and low pressure (wet soil) periods. After every impulsive increase in carbon dioxide flux, the normal baseline is slowly reached. A periodic effect on soil degassing, to a first approximation of tidal origin, cannot be excluded at Castelo Branco. It is concluded that the experimental sensors are adequate for detailed long-standing evaluation of monitoring sites which needed prior to permanent monitoring of soil degassing.