Ground-based heat and CO2 flux measurements

Heat Flux Measurements
CO₂ Flux Monitoring and Diffuse Degassing Structures (DDS)

Heat Flux Measurements

The systematic measuring of the thermal energy released by active volcanoes in quiescent periods can constitute a powerful monitoring tool for volcanic activity. This is based on the relatively high thermal energy outputs estimated for many volcanic-geothermal systems (Fig. 24) and their potentially high variability as a result of magmatic and tectonic processes.

In the case of crater lakes, where mass and energy balances of the water body allowed accurate estimates of the energy output, the release of volcanic thermal energy was determined to range from 50 to 380 MW per system. In volcanoes without crater lakes as well as in many geothermal sites, thermal energy is released directly from fumarolic vents or through soil diffuse emission, together with the expulsion of volcanic-hydrothermal gases.

Based on soil diffuse degassing structures (DDS) and soil temperature gradient measurements, a thermal energy of 58 MW was estimated to be discharged from soil diffuse emission at the hydrothermal crater area of Nisyros. The heat fluxes of individual DDS are typically tens or hundreds of W /m2.

In the framework of the GEOWARN project, an industrial heat flux sensor produced by LASTEM SRL for direct measurements of the soil heat flux in a range from 1 to 1,500 W /m2 (LASTEM sensor (LS): Fig. 24 and 25), has been experimentally tested via computer numerical modeling and laboratory tests.

The LS was later used in 4 different field surveys at Stefanos, Kaminakia North, and at Polibotes Micros hydrothermal craters (Nisyros volcano, Greece), and at Solfatara crater (Campi Flegrei, Italy). The main aim of this work was to set up a method for the direct measuring of heat flux from hot and warm ground, which can be used for both quick measurement campaigns and for the setup of a continuous monitoring system, which has now been installed in the Solfatara crater at Campi Flegrei.

Fig. 24 Measurements of diffuse CO2-fluxes and LS heat flux sensor measurements. Osservatorio Vesuviano Napoli (OVNI)
(Click on image to enlarge).

Technical specification of LS heat flux sensor and computer modeling

The LS sensor is a disk (thickness 0.7 cm, diameter 5 cm) composed of two steel layers (0.2 cm thick) with an inter-bedded 0.3 cm thick epoxy layer (Fig. 25). The thermal conductivity of the sensor on the whole is 0.5 W / (K m).

The sensing part is a thermopile detector which measures the heat flux through the epoxy layer in the central 2 cm of the LS, giving an electrical output of 8 to 14 W / m2. The operating range is 1 to 1,500 W / m2and -40 to +80°C.

The measurement is done by inserting the sensor at a shallow depth (1-10 cm) into the soil and reading the electrical potential of the LS upon stabilization (typically 30-60 minutes).

Fig. 25 LS heat flux sensor measurement on the floor of Stefanos hydrothermal crater (Nisyros). Osservatorio Vesuviano Napoli (OVNI)
(Click on image to enlarge).

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