Understanding and predicting volcanic activity is a complex undertaking. Volcanologists must collect and evaluate a wide range of data, such as magnetic and seismological data, and gas readings.
“Observations have already shown that variations in the concentration ratios of certain gases can be precursors of an eruption," said Maeva Doron, a research engineer at CEA-Leti. “Additionally, because gases emitted by volcanoes can be toxic for local populations, monitoring them is a matter of public health interest."
A sturdy, portable multigas sensor
Volcanologists therefore use gas sensors when taking field measurements. Unfortunately, devices that are currently on the market are not intended for use in extreme volcanic conditions. Corrosion, pressure, vibrations, and high relative humidity can all disrupt measurements and damage equipment.
However, CEA-Leti has expertise in designing and manufacturing gas sensors, which were originally developed to measure toxic gases or low-level greenhouse gases. The application to volcanology was consequently an interesting option to explore for monetizing the technology. The MUGS (MUltiGas Sensor) project arose following conversations with the Laboratoire Magmas et Volcans (LMV, Laboratory of Magma and Volcanoes) at the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC, Clermont-Ferrand Earth Physics Observatory). It aims to develop a miniature (the size of a mug) portable multigas sensor capable of resisting the extreme conditions of volcanic land and of being embedded on a drone, to safely investigate volcanic plumes. The project is funded by the Agence Nationale de la Recherche (ANR, French National Research Agency), by way of CEA-Leti's Carnot Label of Excellence.
Photoacoustic spectroscopy and quantum cascade lasers
The multigas sensor relies on the principles of photoacoustic spectroscopy detection.
“It essentially involves lighting up a gas mix using light, whereby the color (wavelength) and intensity variation can be controlled," explained Christophe Constancias, a CEA-Leti research engineer.
“The gases will then be able to absorb part of the light, which will generate a mild pressure variation in the sensor. This variation will be amplified into a small cavity, called an acoustic resonator. The pressure wave will be detected using a microphone, by adjusting the modulation frequency of the light. Additionally, every gas reacts differently depending on the color of light that is used, while the intensity of the acoustic signal that is measured depends directly on the quantity of gas that is present."
Although the research team was already proficient in the field, the MUGS project required overcoming several challenges, such as miniaturization. Thanks to CEA-Leti's expertise in silicon manufacturing, the scientists were able to design a photoacoustic cell measuring just a few millimeters which integrates microphones that can measure sound that is produced. Relying on silicon will ultimately have the advantage of embedding everything into a single component. Currently, signal collection and control rely on modular electronics made up of interconnected boards that were developed at CEA-Leti.
Finally, the assembly features four quantum cascade lasers. These make it possible to cover mid-infrared wavelengths, particularly in the 4 to 7 µm band, which is adapted to the targeted gases.
“Quantum cascade lasers are sold, but they are usually made on an InP (indium phosphide) substrate," noted Christophe Constancias. “This leads to high costs and makes mass production difficult. We therefore set about developing a quantum cascade laser on silicon, so as to use CMOS methods for a less expensive unit cost."
Conclusive early results on the La Soufrière volcano
The MUGS project has therefore resulted in creating a compact, multigas sensor that can measure greenhouse gases: CO2, CH4, N2O, CO and H2O, along with two gases that are of particular interest to volcanologists: SO2 and H2S. What is more, the device functions upon contact with air, and only requires a small volume of gas to take measurements.
“This sensor results from the collaboration between three CEA-Leti laboratories which worked in cooperatively throughout the project," added Maeva Doron.
Following several months of laboratory calibrations, the sensor was tested at La Soufrière volcano, Guadeloupe, last December 4th to 12th, 2025. This step demonstrated that the device works in a stable and repeatable way in difficult environmental conditions. Collected data was then processed and compared with measurements taken by the volcanologists. This yielded convincing results on SO2 and H2S, two of the three targeted gases. However the volcanologists' devices were unable to measure the third one, CH4. The MUGS sensor, whereas, performed this task successfully.
Researchers now wish to improve the device in order to take and transmit measurements when it is installed on a fixed location for several months at a time, or from a flying drone, to map concentrations. These prospects are being researched thanks to European funding for the GENESIS and MILADO projects.