A novel volcanic gas observation technology to improve eruption prediction models

Funded under an ERC grant, the BRIDGE (Bridging the gap between Gas Emissions and geophysical observations at active volcanoes) project aims at no less than ‘prompting a major scientific and technical advance in volcanology.’ It was built upon the observation that, in spite of recent technological progress, volcanic gas observations’ contribution to volcano monitoring is still very limited.

This results in an inability to make real-time high-rate observations of volcanic gas flux and compositions: To this day, geochemists have been experiencing technical difficulties in capturing volcanic gas chemistry and flux at a high rate (1 Hz) and using permanent instrumental arrays. Moreover, the poor temporal resolution of existing observations has made it impossible to analyse fast-occurring volcanic processes such as those occurring shortly prior to eruptions.

BRIDGE technology is solving these problems with innovative instruments for 1 Hz observations of volcanic SO2 and CO2 fluxes. Information gathered from these instruments can be combined with geophysical data in order to fill in current knowledge gaps and yield improved modelling of a variety of volcanic features, including mechanisms triggering explosive volcanic eruptions.

How do you explain the fact that volcanic gas observations lag so far behind?

Prof. Alessandro Aiuppa: Volcanic gases are a relatively recent topic of study. While more established geophysical techniques have been used on volcanoes for more than a century — for example, seismicity which has been monitored since the mid/late 1800s — volcanic gases attracted scientific investigation only in the 1930-50s. Instrumental volcanic gas observations only started in the 1970s and were consolidated only in the 2000s.

This ‘late’ interest of scientists in volcanic gases explains the gap it leaves in geophysics.

What are the consequences for the accuracy and effectiveness of volcanic observations?

Although a deterministic forecast of volcanic eruption onset is still challenging, volcanologists now have a much clearer understanding of the processes that drive a volcano to erupt, and it is clear that magmatic volatiles play a key role. They are transferred from the silicate melt into a magmatic gas phase as the magma is decompressed on its way to the surface; and formation and expansion of this exsolved gas phase leads to magmatic pressure build-up that triggers an eruption. It is therefore vital to study their composition and flux.

Unfortunately, direct sampling of volcanic exhalations and analysis in the lab have so far hampered the analysis of fast-occurring volcanic processes, and have limited refinement and/or experimental validation of models of magma flow (and degassing) shortly before eruption.

How did you aim to solve this problem?

Our goal was to refine existing techniques and set up new technologies for studying volcanic gases. We designed, produced, tested and field-deployed a new generation of volcanic gas sensing instruments, and we successfully realised the first prototypes of fully automated UV camera networks. This is affording us long-term continuous volcanic SO2 flux observations at high temporal resolution (> 1 Hz, and up to 25 Hz) with a compact, robust and easy-to-use configuration.

We have also refined a novel technology called ‘Multi-component Gas Analyser System’ (Multi-GAS), a gas sensing unit that has become the reference for in situ nearly continuous monitoring of volcanic gas plume composition. Finally, we have also fully succeeded in the challenging task of developing the first DIAL-Lidar for direct, remote sensing of the volcanic CO2 flux.

Our multi-instrument, ready-to-deploy gas sensing network enables fast responses for future volcanic crises in the EU or elsewhere. The instrumental gas monitoring devices, realised within BRIDGE, are now being exported to several volcano observatories worldwide, where they are being implemented in local monitoring networks.

Can you explain how these instruments work?

Our UV cameras are CCD devices that are used to take sequences of images of a volcanic plume (the atmospheric dispersion of volcanic gases). Images are captured using optical filters, allowing us to show limited portions of the incoming solar radiation. By simultaneously exposing two cameras, selective absorption of incoming sunlight by volcanic SO2 can be quantified and converted into an SO2 flux (mass of SO2 released per unit of time by the target volcano).

Our multi-GAS instruments, then, are compact multi-sensor units combining infra-red spectrometers and electrochemical sensors. Volcanic gases/plumes are actively pumped inside the Multi-GAS, and the concentrations of different gases (H2O, CO2, SO2, H2S, H2, HCl) are measured in real time (at 1 Hz). The Multi-GAS is permanently deployed on a volcano’s summit with data being telemetered to a volcano observatory. Accurate temporal records of volcanic gas compositions are therefore obtained.

Finally, the DIAL-Lidar is essentially composed of a transmitter (laser) and the receiver (telescope). A lidar is merely an optical radar: a laser pulse is transmitted to the atmosphere, and some of its photons are backscattered to the telescope by air molecules and aerosols. The optical power corresponding to this photon flux (transformed into an electronic signal by a photodetector) is proportional to the chemico-physical properties of the atmosphere along the laser beam. Air attenuates the laser pulse due to molecules and aerosol scattering and to the specific absorption of gases: if the laser wavelength coincides with absorption lines of a target gas, the attenuation will be stronger.

DIAL takes advantage of this effect: unlike a usual lidar, two wavelengths, ON and OFF, are transmitted, with only the former being absorbed by the target gas. If the absorption line is narrow and ON and OFF wavelengths are close enough, the target gas concentration along the lidar optical path can be derived from the ratio between the OFF and ON signals. In our specific case, we developed a novel dye-laser-based DIAL-Lidar using a complex transmitter that integrates an injection seeded Nd:YAG laser with a double grating dye laser. This transmitter is used to generate laser radiation at ~2010 nm, a region of the electromagnetic spectrum absorbed by atmospheric CO2.

Are you happy with the results of your field tests?

Very happy. All the instruments developed have successfully been installed at active volcanoes and are now contributing a huge flow of data, providing key information on volcano behaviour. Our project has successfully completed a novel interpretative scheme of shallow crustal volcanic processes, based upon the combined analysis of co-acquired volcanic gas and geophysical (seismic, infrasonic, geodetic, thermal) signals.

What would you say are the most important learnings from the project?

The key lesson from BRIDGE is that understanding volcano behaviour requires a multi-disciplinary angle of attack. Achievements in BRIDGE make the case emblematic, and demonstrate that integration of volcanic gas and geophysical data is key to improved understanding of mechanisms of gas/magma ascent within magmatic conduits during quiescence, and prior to/during volcanic eruptions.

Our multi-disciplinary monitoring observations at the Stromboli and Etna volcanoes, made possible thanks to the observational networks realised in BRIDGE, reveal that more accurate eruption forecasting is possible when gas and geophysical signals are jointly analysed.

BRIDGE
Funded under FP7-IDEAS-ERC.
BRIDGE project''s CORDIS web page
project website