Ground-based heat and CO2 flux measurements

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

CO₂ Flux Monitoring and Diffuse Degassing Structures (DDS)

Degassing is a continuous process, since gas is emitted from all types of magmas. Most active volcanoes release volcanic gases as a surface expression of this degassing process in the form of plumes, discrete fumarolic activity and soil diffuse degassing.

The gases released by volcanoes are a mixture of components derived from at least two different sources: a) from a magmatic source, exsolving and releasing volatiles from silicic melts into the country rock and b) from vapour separating from external fluids.

These latter fluids of either meteoric, groundwater or seawater origin (or a combination of them) are circulating around the magmatic conduits via porous media or fractures (the so-called "volcanic-hydrothermal system"). The driving force of this movement is the thermal gradient between the magmatic source ("magma reservoirs") and the surface, which is evidenced by fumarolic emissions.

Recently, the use of new techniques for the measurement of diffuse CO₂ fluxes permitted quantitative estimation of the gas-flow released in diffuse emission areas from different volcanic systems (Fig. 24).

These newly acquired data suggest that in the case of quiescent volcanoes exhibiting hydrothermal activity, the major gas emissions are discharged from relatively small areas, or Diffuse Degassing Structures (DDS). The associated energy losses, mainly through steam condensation, are by far the most important terms in the heat balance equation of the whole volcanic system.

During the 1990's, a geochemical technique for measuring diffuse soil CO₂ fluxes was developed. This method, called ”accumulation chamber time 0 method”, consists of the measurement of soil CO₂ fluxes at many points (typical several hundreds) homogeneously distributed over the area of study. The quick measuring times (1-5 minutes) allow one to cover large areas in detail in relatively short times.

CO₂ flux data analysis

The total degassing output from the surveyed area can be estimated by means of statistical or geostatistical techniques. Two generally used statistical techniques are a) arithmetic averaging and b) a graphical-statistical approach. For surveillance purposes, the definition of the 'natural variability' is essential for the recognition of anomalous states.

The new CO₂ flux data analysis of Nisyros (Fig. 26) comprises a geostatistical technique that consists of a probabilistic approach based on stochastic simulation algorithms. Specifically, we used the Sequential Gaussian Simulation algorithm (SGS) using the code of the GSLIB software.

The stochastic simulation is a process that builds alternative, equally probable, high resolution models of the spatial distribution of an attribute, assumed as the realization of a random variable.

Simulation differs mainly from kriging or interpolation algorithms because the target of a simulation is the reproduction of the global features (texture) and statistics (histogram, variogram) of the attribute, and because it allows the definition of the accuracy of both local and global estimations.

The measure of the accuracy is given by the differences between N alternative simulated values of one point (local accuracy) or by the differences between N alternative simulations of a field (global accuracy).

Fig. 26 CO₂ flux at Lakki plain (the result of 200 simulations based on field measurements). Osservatorio Vesuviano Napoli (OVNI)
(Click on image to enlarge).

During one single campaign, 1.268 data points were used and arranged in a 20m spaced grid within the hydrothermal crater area of Nisyros. The simulation was performed by computing the flux values in 27,520 cells of 5 x 5 m each. The mean total CO₂ output computed by E-estimate of 50 alternative realizations (i.e. linearly averaging all of the realizations) is 34.6 t d-1 (Ø = 3.8 t / d).

Fig. 27 The CO₂ flux (heights) / temperature (colours) correlation from the hydrothermal craters in Nisyros during the seismic crisis in November 1997 combined with the orthorectified 3D IKONOS satellite image. Swiss Federal Institute of Technology (ETHZ)
(Click on image to enlarge).

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