The development and implementation of a scanning spectrometer system for measurement of volcanic gas fluxes

Large fluxes of sulphur-rich gases at the Earth’s surface can be used to assess magma movement and forecast volcanic activity.  This information can be used to make decisions about risk mitigation as well as development and commercial activity in affected areas.

COMET scientists Marie Edmonds and Clive Oppenheimer from the University of Cambridge, and Tamsin Mather and David Pyle (both now at Oxford), pioneered the development of automatic spectrometer networks for monitoring SO2 emissions.  This has significantly improved understanding of volcanic degassing and the monitoring of volatile fluxes in large volcanic eruptions. 

In the mid-2000s, they began to work on the development of methods to forecast volcanic activity, focusing on Soufrière Hills Volcano, Montserrat.  Ultimately, underpinned by several years of field measurements and the analysis of volcanic gases and erupted tephra, this led to the creation of a scanning UV spectrometer network.

Montserrat.  Credit: BGS
Soufriere Hills eruption. Credit: BGS

The eruption at Soufrière Hills began in 1995, and gas flux measurements were a mainstay of the monitoring program. Between 1997 and 2001 data on SO2 and hydrogen chloride (HCl) emissions was collected, but the SO2 flux was being measured using a bulky correlation spectrometer on board a boat beneath the gas plume.  It took 3-4 hours to collect 2-3 data points, at substantial cost. Flux measurements were collected once per week at best and there were months where no data were collected at all.  It soon became clear that a better data collection method was needed.

The development of small UV spectrometers, alongside those in differential optical absorption spectroscopy, opened up new possibilities for volcano monitoring.  In particular, the new spectrometers had low power requirements and were small and portable.  They clearly had the potential to acquire considerably more data than previously, and so the team, led by Marie, worked with Chalmers University in Sweden to develop a scanning spectrometer system geared towards monitoring volcanic gas fluxes.

Following a successful field trial in 2001, work began on designing, building and implementing a network of three spectrometers at Soufrière Hills. Between 2003 and 2012, data collected by the network produced the most detailed time series of volcanic SO2 emissions from any volcano worldwide.

Monitoring at Soufrière Hills is especially important during the 1-2 year-long pauses in activity that occur every few years.  Since July 2013, the volcano has been quiet, but knowing whether the eruption has stopped, or merely paused, is critical to planning development and approving access to volcanic areas on the island.

Massive ash cloud above Montserrat.  Credit:BGS

Analysis of melt inclusions in the tephra from Soufrière Hills confirmed that sulphur-rich gases were being generated at magma chamber depths of more than 5 km, with HCl generated at much shallower levels.  This means that the SO2 emissions were not strongly linked to magma eruption, whilst HCl emissions were; and that SO2 fluxes can be an effective proxy for deep magma supply, and HCl for eruption rate.

The end of an eruption is marked by SO2 fluxes that are consistently less than 50 t/d for more than 12 months.  So far, the SO2 flux at Soufrière Hills has remained high, indicating that gas-rich regions of the magma reservoir are being outgassed at depth and that there remains a possibility that the eruption may resume.

This assessment has allowed the Montserrat government to make informed decisions about where to build new infrastructure, focusing their efforts in the north of the island, away from volcanic areas, and whether to attempt large projects with commercial value, which might bring in revenue but need to be weighed against the possibility of volcanic activity.

Overall, the development of the scanning UV spectrometer networks has revolutionised volcanic gas monitoring.  Scientists from other volcano observatories have come to Montserrat to learn about the network, and the automated scanning spectrometer design has been implemented at 20 volcanoes worldwide.  In addition, Marie has been directly involved with advising monitoring schemes for White Island (New Zealand), Etna and Stromboli (Italy) and Tungaragua (Ecuador).