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Methods to measure gas and water vary depending upon the level of volcanic activity
Gas concentrations and emission rates (the rate at which gas is released) are measured using airborne or ground-based techniques, both during periods of relative calm and during periods of high volcanic activity. Large gas plumes can even be measured by instruments mounted on satellites orbiting the earth.
Direct measurements on the ground must be made daily
One method of sampling volcanic gases is to collect them directly from fumaroles in vacuum flasks or solution-filled bottles and then analyse the mixtures in the laboratory. This type of “direct sampling” of fumarole gases is currently the only way to fully characterise the composition of gases released from volcanoes and to collect the isotopic data needed to determine the origin of certain gases. This allows us to have a complete understanding of the state of the volcano and its evolution.
Gas collection from a bubbling source within Pelican Creek, Yellowstone. Inverted funnel placed over gas source, gas travels through tubing into evacuated/vacuum glas flask to be analyzed in lab.
Dozens of measurements are usually required to map areas of high gas concentration.
MultiGAS instruments can also be used for direct sampling and effectively monitor multiple gases at once. Using a combination of optical and electrochemical sensors, these instrument sets can measure CO2, SO2 and H2S concentrations as well as environmental parameters such as temperature and pressure. They are usually transported to fumaroles or outgassing vents during measurement campaigns, or even permanently installed at locations downwind of these vents. Instead of collecting the gases for later analysis, the data collected can be transmitted by radio to a volcanic observatory, allowing minute-by-minute recording of changes in gas concentration.
Emission rates of some gases can be measured remotely
Direct gas sampling can measure the relative amount of gas (concentration), but it does not provide information on emission rates – this is where remote sensing techniques excel. A COSPEC or DOAS spectrometer measures the amount of sulphur dioxide emitted into the atmosphere every second, hour or day. DOAS instruments attached to satellites can be used to detect very large SO2 plumes.
MultiGAS monitoring instrument installed on the dome in Mount St. Helens crater. Gas haze to the left of enclosure travels into round/black sensor (bottom of gray box) and is analyzed internally.
USGS geologist uses a UV spectrometer to detect gases from fumerole at Crater Rock, Mount Hood.
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